CA2112244A1 - Work-modulated pacing rate deceleration - Google Patents

Abstract

A rate-responsive cardiac pacemaker in which deceleration of the pacing rate is modulated according to recent patient activity or "work". Based on signals provided from an activity sensor in the pacemaker, the pacemaker maintains and periodically recomputes a Work value corresponding to the amount of patient activity detected over time. In response to a decrease in or cessation of detected patient activity, the pacemaker reduces the pacing rate, first at a more rapid deceleration rate, then at a lower deceleration rate, and then once again at a more rapid deceleration rate. The length of time during which the pacing rate is reduced at the lower deceleration rate is modulated according to the Work value maintained by the pacemaker. The work-modulated pacing rate deceleration occurs only if certain achievement criteria are met, the achievement criteria being specified in terms of the amount of time the pacemaker's target pacing rate exceeds a predetermined rest value, and the difference between the target rate and the rest rate.

Description

W,O 93t23115 2112 2 4 ~ Pcrtus93/o289l WORK-MODULATED PACING RATE DECELER~TlON

ElELD OF THE INVI~TION
This invcndon relates to the field of implantablc medical dcvices, and more pardcularly rdates to p#~naks' which respond to a padent's metabol~c demand andS vanes the daay ~ates of the pacing ~ate in subst~ntial similarity to thc heart's normal bd~avior.

BACKGROUND OF THE INVENTION
Early cardiac pacemal~rs provided a fixed-rate sdmuladon puise genera,t,or that could bc r,eset, on dcm~nd, by sensed atrial and/or ventricuhr depolarizations.
Modern pacemakers include complex sdmuladon pulse generators, sense amplifiers, and lcads which can be configured or prog~ammed to operate in single- or dual-chamber m~des of operation, ddivering pacing stimuli to thc atrium and/or ventricle at fixed tates or ~ates that vary bctween an upper rate limit and a lower rate limit.
bl rooent years, single- and dual-chamber pacemalcers h~ve been devdoped lS which m~e ~_s which are directly or indirectly related to the patient's me~bolic Fequirements (i.e., demand for oxygenated blood) and vary the pacing rate in ~nse to such paNne~s. Such measured p uametiers include, for example, physicd acdvi~ of the patient, right ventricular blood oemperature, vcnous bloodo~ygen S~WI, e~ion, minute ventihtion, and various pre- and post-systolic time in~als _ied by implance or prcssure sensors within the right ventncle of ~e heart. Such sensior-driven pacana~s have boen developod for the purpose of ~st:oring ~ate re~onse to exercise in patients lacldng the ability to increase rate adoquately by ertion.
In general, a rate-responsive pacemaker includes a sensor which produces an output that varies between a maximum sensor output level and a minimum sensor output levd ~nsensor output"). The pacemaker delivers pacing stimuli at apacing ~te (~pacing rate~) which varies as a linear or monotonic function ("r) of the sensor output, between a selectable lower pacing rate ("lower ra~e limitn) and upper W093r23115 21122~4: . 2 Pcl/us93/o28sl-pacing ratc ("upper rate limit"). Function f has a selectable slope, where the slope of f corresponds to the ratio of pacing rate change to sensor output change. That is, the slope of f rcflects the amount of change -- increase or decrease in the pacing rate rc~lting from an incremental change in scnsor output. The slope of f is adjustable by means of an extcrnal programmer along with the lower and upper rate ~Imit values.
Thus, thc pacing rate typically provided is equal to the programmed lowcr rate limit plus an increment which is a funcdon of the measured sensor output, as follows:

pa~ing rate = lower rate ~ f(sensor output).

Whilc this rate response techniquc provides a useful and workable system betwecn programmed parametcrs, thc behavior of the pacemaker is complex and often not readily apprehensible. Paccmakers that measure the physical acdvity of the pati0t by means of a piezoelectric transducer have become popular among rate-responsivc paccmakers. Such a rate~responsive pacemaker employing a piczoelectric transducer is disclosed in U.S. Patcnt No. 4,48S,813 to Anderson ct al. and assigned IS to thc assignee of the prcsent invendon, which patent is in~rpola~d herein by refer~noe in its entirety.
Somc tcmperature sensing pacemakers have employed reladvdy more complex functions to take into account an initial dip in temperature duc to the onsct ofcxcrcise. One such pacemaker is described in U.S. Patent No. 4,719,920 to Alt et al Furthermore, the decay slopc of conventional activity-based rate-responsive p~n~rs do not approximate the heart's normal behavior, in that ~ey are progwnmcd to follow a curve bascd on a single time oonstant. This discrepancy betwoen thc narmal heart deccleration function at the cnd of physiologic strcsses due to accumulatcd metabolic debt, and the conventional pacemaker decay function hasnot bcen recdfied by any pacemaker presently available on the market and known to thc inventors.

WO 93~23115 2 ~12~2 ~ ~ PCl/US93/02891 , _ .

Thus, the inventors believc that it would be desirable to provide a cardiac paccmalccr of the rate-re~onsive type which varies its attack and/or decay pacing ~ates in hannony with the heart's normal behavior.
In U.S. Patent Application Serial No. 071567,204 filed August 14, l990, by S Bennett et al. entitled ~Rate Responsive Pacemaker and Pacing Method" (hereinafter referrcd to as the Bennett et al. reference) there is disclosed a rate-responsive paccmaker having a modificd pacing rate decay curve after a period of increased acdvity. Thc mcthod disclosed in the Bennett et al. reference includes the stcps of sclecting a set of prcdetermincd achievement criteria such as an achievement rate and an achievement duration or time interval. The achievement rate is initially selected between an upper pacing rate and a first pacing rate switch threshold. The pacing mcthod then detamines whetha the achievement criteria have been met. If the achievement criteria have been met, then the decay time constant of the pacing rate decay curve changcs from a first value to a second value when the pacing rate drops lS bdow the first pacing ratc s~itch threshold.
Fur~cr in aooordancc with thc Bennett et al. reference, a second pacing rate switch th~eshold lower than the first pacing rate switch threshold is seloctcd, and if thc achicvemcnt criteria have been met, then the pacing rate dccay time constant is modificd f~m the second valuc to a thW value when the pacing rate drops below the socond pacing rabe switch threshold. This third value of the pacing rate dccay time constant may be e~ual to the first value.
Acoording to Bennen et al., the dccay rate time constant is not modified in the above~sibed manncr if ~c achicvcment critena have not first been sadsfied.
Thc Bennctt ct al. p~mal~ also penodically calculates a new activity pacing rate, and calc~latcs a ncw activity target rate based upon the activity sensor output.
Thc achicvcment rate is calculated as~ follows:

Achievement Rate = Lower Rate + A(Upper Rate - Lower Rate) 2 112 2 g 4 PCr/US93/028g1 whae "A" is a percentile value. Thus, if the programmed upper and lower rate settings define a range of possible pacing values, tne achievement rate is defined as some percentage of that range. Similarly, the first pacing rate switch threshold is calculated as:

S First Pacing Rate Switch Threshold = Lower Rate + U(Uppe~ Rate - Lower Rste) where ~U~ is a pcrcentile ~ralue. The second pacing rate switch threshold is defined as:

Sccond Pacing Rate Switch Threshold = Lower Rate ~ 10% of Lower Rate.

Thc Bennett et al. reference is hereby incorporated herein by reference in its entircty. As would be apparent to one of ordinary skill in the pacing art, the modified pacing rate decay curve disclosed by Bennen et al. comprises first and third docay phases defined by the programmed decay rate time constant, and a second dccay phasc, inte~posed betwoen the first and third phases, in which the programmed dccay Iate time constant is temporarily replaced with a modified time constant. The ~buy points between the first and second decay phases, and the second and third dccay phases, are defined in terms of pacing rate. The transition fr~m the first phase to thc seoond phase occurs at a point where the pacing rate drops to the first pacing sa~te switch th~e~old, and the transition from the second phase to the third phase oo~rs at a point where the pacing rate drops to the second pacing rate switch ~ue~old. Although the first and second pacing rate switch thresholds may be pro~amm~le values, the decay rate will only change at these programmed values.
Moroover, while the p~ammed and modified time constants are programmable value, the pacing rate decay will occur at one or the other of these programmed vahes unless the decay pa~neters are re-programmed. Thus, the Bennett et al.
pacanal~ could be generally characterized as modifying decay cunre by changing the 21122~4 WO 93/23115 5 PCr/US93t028gl time constant of the pacing rate decay, assuming the achievement eriteria have been reaehed, dwing an interval determined by the eurrent pacing rate.
By way of eomparison, a paeemaker in aceordance with one embodiment of the present invention ean be generally eharacterized as modifying the decay eurve by S ehanging the time eonstant of the pacing rate decay, assuming the achieven~ent criteria have boen reached, dwing an interval determined not by the eurrent paeing rate, but by a measwe of the amount of work reeently performed by the patient.
It is bdieved by the inventors that it would be advantageous to provide a p~er in whieh the deeay time eonst~nt is modified bascd not strictly upon the eurrent paeing rate, but also upon a measure of the patient's recent levds of exertion, the pacanai er will more effeetivdy mimie the deeeleration behavior of a healthyheart.

SU~RY OF THE INVENTION
Aceord;ingly, the present invention provides a rate-~onsive paeemaker in IS wb~eh ~c decay or docd~ion of the paeing ~ate in response to decleass in detoeted patient aetivity is controlled aeoording to a metrie of recent work performed by the padent.
Spocifically, a paeemaker in aeeordance with the present invention maintains a numenc ~Work~ value that is periodieally reeomputed in eonjunetion with p riodiccaleulations of a ral~responsive Target Rate. When detected patient acdvity is such that the ra~rc~onsive Target Ratc atceeds the ~ng rate, tbe pacing rate is in a s~e of aocele~ation and thc Work ~ralue is paiodically incrcased by a varying inclemental amount pr~tional to the differa ce between the Target Rate and the pacing DtC. A "tciling~ or upper lirnit on thc Work value may be imposod.
When padent activity dec eascs or ceases, the rate~Jesponsive Target Rate will also docrease. When the Target Rate is below the current pacing rate, the pacing rate is in a state of dece~e~ation. ln this case, the Work value is periodically and incrctnentaDy decla~ by an incrcmcntal atnount tefened to as thc Work Da:ay i WO 93~23115 6 PCr/US93/0289 value. In the preferred embodiment, recomputation of the Target Rate and Work value occurs periodically, for example every two seconds.
The desderation of the pacing rate occurs in several distinct phases. ln the case that the decelerating pacing rate exceeds a preselected Switch Rate, below the programmed Upper Rate Limit, a first, Initial Deceleradon Phase occurs in which the pacing daduation rate is govcrned by a first, shorter time constant. The InitialDeceleradon Phase continues until the pacing rate re~ches thc Switch Rate. When a deselerating pacing rate is less than or equal to the Switch Rate, a intermediate, Work-Modulated Deceleradon Phase occurs, in which the pacing deseleration rate is governed by a second, longer dme constant. The duration of the Work-Modulated Deceleration Phase is determined according to the Work value during that phase. By definition, the Target Rate will be less than the pacing rate during the Work-Modulated Decdaadon Phase, so that during this phase the Work value will itself be decaying. The cnd of the Work-Moduhted Deceleradon Phase occurs when the Work lS valuc has docayed to zao.
After the Work-Modulated Deceleradon Phase, a final, Latent Deccleration Phasc brings thc pacing rate down to the programmed Lower Rate Limit. During thel~nt Deoderadon Phasc, the pacing rate decderadon is governed by a shorter dme constant than during the Work-Modulated Phase. The Latent Deceleration Phase dmeoons~nt may or may not be the same as the tdme constant for the lnitial Deceleradon Phase.
In a preferred-embodiment of the invendon, a Work-Modulated Deceleration Phase will only oocur after certain Achievement Criteria have been reached. The Achievement Critena arc a~lnessed in terms of a ~ange of Achievcment Duradons and a Dnge of Achievcment Times. Only when the Target Rate has been greater than a predetermined rest Iate f0 a sufficient time to sadsfy the Achicvement Criteria will Work-Modulated pacing rate decele~on occur. Once the pacing rate decelerates to ~e programmed lower ~ate, the Achievement Criteria must again be fulfilled before Work-Modulated pacing rate deceleradon will occur.

~ro 93/23115 ~ 7 Pcr/US93/O289l .
BRlEF DESCRlPrlON OF THE DRAWINGS
The foregoing and other aspeets of the present invention will be best ~aled with reference to the detailed deseription of a spee.ifie embodiment of the invention, whieh follows, when read in eonjunetion with the aceompanying drawings, S wherein:
Figure 1 is an illustration showing placement of a paeemaker in aceordance with the disebsed embodiment of the invention in a patient;
Figure 2 is a g~aph of paeing rate versus time showing a prior-art paeemaker's pacing rate rc~onse following eessation of patient activity, compared to a normal heart's rc~onse thereto;
Figure 3 is a graph of paeing rate versus time showing.a prior-art pacemaker's paeing rate re~onse to several episodes of padent aedvity;
Figure 4 is a flow diag~am depieting the ope~ation of a prior-art rate-re~onsive pacemaker with a modified paeing deeay funetion;
lS Figure S is a bloelc diag.ram of a pacemaker in aceordance with the diselosed anbodiment of Ihe invendon;
Figure 6 is a graph illustrating combinations of Target Rate and time whieh will s~dsfy the Aebievement Critaia for the pacemaker of Figure 5;
Figme 7 is a graph of pacing rate versus dme and Target Rate versus time showing the pacing ~e re~onse to patient aetivity of the pacemaker of Figure 5;
Fig~ 8 is a flow diag~am depicting the ope~ation of the paeemaker of Figure 5;
Figure 9 is a g~h of pacing ~ate vews ~time showing seve~al different pacing Jatc lespones to patient ~ctivity of the ~can~ of Figure 5; and 2S ~ Figure 10 is a g~aph of pacing rate vsus time showing several different . pacing Iatc re~onses to patient a¢tiyity of the pacemaker of Figure 5.

Wo 93/23115 2112 2 4 ~ 8 PCr/US93~02891 INVI~ITION
Rcfe~ring to Figure 1, there is illust~ated the placement of a pacemaker 10 in accordance with one embodiment of the prcsent invention. Pacemaker 10 is shown in S Pigure 1 as it would be implanted in a patient 11. The preferred embodiment of the invention indudes at least one activity sensor 12, which may be, for example, a piczoelectric clement disposod on a can or housing 14 of ~kN 10. Pace~er 10 may additionally include otha sensors, such as a pressure sensor or the lilce implanted within heart 16 or disposed on the distal end of pacemi~er lead 18. A pacemake~ which mcasures the physical activity of a paticnt by means of a piezoclectric transducer is disclosed in thc above-rcfcrenced U.S. Patent No. 4,48S,813 to Anderson et al. It is to be undastood that the prcsent invention is not limited in scope to either single-sensor or dual-sensor pacemakers, and that other sensors besides activity and pressure sensors could be used in p~cticing thc present invention. Nor is the praent invention li nited lS in its scope to single chamber pacemal~rs. A multiple-chamber (e.g., dual~wnber) ~ can ~so be used in practicing thc present invention. lt sho~d also be unds~od that while the plesent invention will be described herein with referenoe to the decay curve of the p~er's pacing rate in the cont~ext of an activity-basod rate-re~onsi~e paounsl~er, the invendve conccpt herein can be implemented for modifying thc attack cur~e of the pacing rate, and may be employed in pacemakers using pressure and/or o~er types of sensors.
It is believed-that a description of prior an rate-responsive pæema~as, and in pardc~ar, the ~na~r described in the Bennett et al. reference will facilitate a better unders~uling of a pacemal~cr in accordance with the present invendon.
2S In Figure 2, a ~esentative pacing rate curve for a convendonal acdvity-sensing ~te-re~onsive pacemaker is shown in comparison with a normal heart rate decay curve.
In Figure 2, the verdcal axis represents the pacing rate in pulses per minute (PPM), and the ho~izontal axis represents time.
In Figure 2, the padent is initially at rest, and thus no acdvity is sensed. During this period of rest, pacemaker 10 delivers pacing pulses at its p~og~ammed base or lower 211224~
~VO 93/23115 g pcr/us93/o289l rate (LR), as indicated by the line 20. When the patient begins exercising so that activity scnsor I2 begins detecdng activity, the pacing rate begins to increase, beginning at deflection point22. The attack or accderation curve 24 shows the pacemaker responding to such increased levels of detected activit,v.
S When attack curve 24 reaches a plateau 26, the pacing rate ger~eraily stabilizes at an activiq~mined rate or an upper rate for the duration of the exercise or physical acti~rity. A deflocdon point 28 corresponds in time to the cessation or substantial redwdon in the padent's acdvity level. The pacing rate is said to be decelerating beginning at point 28.
Two docay or deceleration curves 30 and 32 descend from deflecdon point 28 and indicate a decrease in the patient's detected acdvity level. In the absence of intervening instances of hdghtened acdvitv, curves 30 and 32 tend to approach a predetermined lower pacing rate.
Decay curve 30 represents the deceleration curve in a convendonal pacemaker, IS as c~lified in U.S. Patent No. 4,722,342 to Amundson. The decay curve 32, on the o~er hand, represents the heart's normal doceleration rate, as illustrated in a te%tbook by Mynnn H. Ellestad, M.D., entitled ~Stress Testing Principles and Practice", pages 489 - 492.
It is apparent that deceleration curves 30 and 32 do not match completely, in that conventional pacemakers pace at an elevated rate, i.e., curve 30, with respect to the qpical human response, i.e., curve 36, and thereafter return to the resting or lower rate sooner ~an the Mical human ~onse 38. This elevated pacing rate in conventional ~ana~ers may cause a sensation of the heart rate Uracing~ or beating too fast at the end of activity, pahaps even provoking undesi~able side-effects. Additionally, convendonal ~ers may pace t~o slowly for seve~al minutes after the end of activity.
Cu~ve 32 in Figure 2 comprises two decay portions, an initial porlion 36 and a latent portion 38, each having a different decay time constant. As will be hereinafter e~pl;lined in more detail, the selection of switch point 34 and the ~ime constants of the ini~al and latent decay portions 36 and 38 are important subjects of the BenneU et al.
invention.

WO 93/23115 10 PCr~US93/0289L
211224~
D~3;FlNlTlONS OF TERMS
The following definitions of terms used herein will assist in a better understanding of the prescnt invention:

Achievcment Criteria: Values supplied by the clinician which set an attainment threshold S for the pacing rate. This threshold comprises a rate component (Achievement Rate) and a time component (Achievement Du~ation). Achievement Rate is a programmable perccntage of the difference between the programmed Lower Rate (LR) and the programmed Upper Rate (IJR). Achievement Duration is a minimum time interval ova which the rate-responsive target pacing rate must e~ceed the Achievcment Rate. In the Bennett et al. patent, the Achievement Rate is spocified by the dinician as an absolute pacing rate in the range from 7~PPM
to 175-PPM in l^PPM intervals, and the Achievement Du~tion is fixed at four s~oonds. An alternative embodiment in Bennett a1bwed the criterion to be a pcrantagc of thc prog~nmed upper rate. In accordance with the prcsently l.S discloscd embodiment of the invention, however, the Achievement Criteria are e~#ed in a n~malized form, so that various combinations of pacing ratcs and dluations will satisfy the Achievement Critena. Moreover, thc normalizcd a~ession of the Achievanent Critcria allows the decelcration responsc in acoordanoe with the praent invention to be r~alizcd for any allowable combination of programmed UR and LR values. This normalization will be hdnafter desaibcd in greater det~il with reference to Figure 6.
, A~ivity Count: Aaivity Count is a measure of thc output of the activity sensor over a _o int~val of time. In the presently disclosed embodiment of the invention, each event during a~ two-second period in which the amplitude of the 2S scnsor output excecds a predetermined activity threshold is counted and retained.
The Activity Count is updated every two seconds, and its aggregate value comprising the count value accumulated at the end of two two-second cycles (i.e., after f s conos~ is used in the calculaoon of the sensor Targ Pac7 rate for 1, 21122~
~0 93~23115 ~ 1 I Pcr/US93/O289l aedvity. A pacemaker employing a piezoelectric signal and maintaining the n~o-seeond aedvity count as just described is disclosed in pending U.S. Patent No.
5,052,388 to Sivula et al. entitled ~Method and Apparatus for lmpbmenting Aetivity Sensing in a Pulse Generator", which reference is hereby incorporated S by reference in its endrety.

Aetivitv Rate Res~onse Gain: This setting eorre~onds to the slope of the function eorreladng the acdvity-based Target Rate to the Acdvity Count value which eorresponds to the aedvity sensor output. The setdng for the Acdvity Rate Response Gain, somedmes alternadvely referred to as the "acdvity sensor gain", eorresponds to a pardcular rate-response eurve (RR). With rate-response, the allowed programmable values for the Activity Rate Response Gain range from 1 to 10 at setting intervals of 1.

This value restriets the rate at whieh the activity~ased scnsor paeing rate ean inerease, sueh that an aedvity ~attack" eurve lS provides for a more gradual and physiologically appropriate ehange in paeing rate. In ~e presently diselosed embodiment of the invention, these dme values ~escnt the dme required to reach 90% of the difference betwoen a first steady-state aedvity-driven paeing period (i.e., constant acdvity signal for at least a six-seo~nd interval) and a second, shorter, steady-state, aetivity driven paeing period when a step inereasc in aedvity oceurs. With rate-response, the allowed pro~nmable values for the Acdvity Response Time Acceleradon Constant are 0.25-minutes, 0.5-minutes, and 1.2-minutes.

Aetivity ResDonse Time Deceleration Constant: This value ~s~icts the rate at which the aedvity-based sensor Pacing Rate can decrease, such that an activity "docay"
2S eurve provides for a more gradual and physiologieally appropriate change in pacing rate. In the presently disclosed embodiment of the invention, the Activity Response Time Deceleradon Constant represents the dme required to reach 90%

WO 93/23115 2 1 1 2 2 4 ~ 12 Pcr/us93/o289~ _ of the difference between a first steady-state activity-driven paeing period (constant aetivity signal input for at least six seconds), and a second, longer,steady-state, activity-driven pacing period when a step decrease in activity level oecurs. With rat~response, the allowed programmable values for the Activity S Response Time Doceleration Constant are 2.5-minutes, S-minutes; or l~rninutes.

Aetivitv Threshold: This is a minimum value which the amplitude of the activity sensor output must e%ceed in order to serve as input to the rate determination algorithm.
The higha the threshold, the greater the amplitude necessary to beeome an event counted in the Aetivity Count. With rate-response, the allowed prog~ammable values for the Activity Threshold are LOW, MEDlUM LOW, MEDIUM, M~IUM HlGH, and HIGH.

~: This is a value supplied by the clinician which establishes a lower limit on the pacing ~ate. If the sensor is disabled, or its sensor output is not high enough to re~ster an activity count that would increase the ~ate, the pacemalcerlS will ddiver stimulating pulses at the programmed Lower Rate. Allowed pro~ammable _e~r values for Lower Rate may range f~m 30 to 180 PPM
in S-PPM inten als.

U~er Rate OJR): Thc Upper Rate is a ~ralue supplied by the clinician which limits the maximum stimu~tion ~tc in sa~responsive mode, such that the sensor~riven pacing rate does not bocome hcmodynamically exccssive. Allowcd p~ammable values for the Uppcr Ratc rangc from 30-PPM to 180 PPM at S-PPM intervals, provided that UR must always be greater than or equalt to thc programmod LR.

~acin~ Rate: This value is calculated by pacemaker 10 in conjuncdon with the acdvity scnsor, based upon its re~ective Target Rate and the contribudon thereto based upon its respctive acceleration and deceleration function. That is, the actual pacing ratc may at any time differ from the activity-based Target Rate if the ~Iyo93~2311s 2 1 1 2 2 4 4 Pcr/us93/o2891 pacing rate is prevented from immediately increasing or decreasing to the TargetRate due to the limitations imposed by the Acceleration and Deceleration functions described herein.

1~: The Targct Rate is calculated by pacemaker 10 in conj~mction with the activity ssnsor, based upon progIammed settings and the respective sensor output.

Opaadon of the Bennett et al. pacemalcer will now be briefly described with refaence to Figure 3. Figure 3 illustrates an exemplary activiq attack and decay curve 40 for the Bennett d al. pacemalcer. The venical axis of Figure 3 represents th~e pacing ~ate in PPM and the horizontal axis reprcsenS time in seconds. Pive threshold levels are illus~ated as honzontal lines 42, 44, 46, 48, and 50. ln parlicular, line 42 represents the p~ogrammed UR, line 44 represents the Achievement Rate, line 46 represents an Upper Switch Rate, line 48 represents a Lowa Switch Rate, and line 50 represents the pro~ammed LR.
As w~d above, the p~ogrammed UR 42 is a ~alue supplied by the physician ~S which l;mits the ma~imum stimulation rate when activiq reaches or excoeds a prodaani~ bvd. Pacan~ 10 is not allowod to pacc abovc UR 42. In the Bennett d al. pacan~, the Achievement Rate 44 is a value that c~ bc set by the physician~n~ a predetermined p rcen~age of the diffaence bctween UR 42 and LR 50.
That is, aocording to Bamett et al., Achievement Rate is defined as follows:
.

Achic~n~t Rate = Lowet Ratc ~ A(Upper Rate - Lower Rate) .
whcre ~A~ is a perccntile value preferably ranging between 50% - 100%. Achievement Rate 44 may vary from onc patient to another. However, an exemplary achievement rate disclosed in ~e Bennett et al. reference is 125-PPM.

WO 93/23115 21 12 2 ~ ~ 14 Pcr/us93/o28~

The Upper Switch Rate 46 is a value that can be selected by the physician and similarly represents a predetermined percentage of the difference between the programmed Lla and UR. In particular, Upper Switch Rate 46 is defined as:

Upper Switch Rate = Lower Rate + U~Upper Rate - Lower Rate) S where ~U" is a percentile value. The Upper Switch Rate vanes from one patient to another. However, a preferred range of 20% - 50% is disclosed in the ~ennett et al.
reference. The Upper Switch Rate phys an important role in the operation of the Bennett et al. device, in that it determines the location of an upper switch point 52 which is g~aphically representedi in Pigure 3 as the intersection point between activity curve 40 and the line 46 representing the Upper Switch Rate. Upper switch point 52 corresponds to switch point 34 in Figure 2.
The Lower Switch Rate 48 also plays an important role in the operation of the Bamett et al. device, in ~at it determines a lower switch point 54, glaphically re~esented in Figure 3 as the inter~ction point between acdvity curve 40 and the line 48 representing the Lower Switch Rate.
The programmed Lower Rate 5() is a i~alue supplied by the physician which limitsthe minimum sdmulation level when activity decreases to, or is, below a certain p~n~ LR level. Pacemaker 10 is not allowedi to pace below Lower Rate 50.
Por illust~don pu~oses in the B~nnett et al. reference, Lower Rate 50 is chosen as 7 PPM. While the Upper Rabe 42, Achievement Rate 44; Upper Switch Rate 46, Lower Switch R~te 48, and Lower Rate 50 can be individually selocted, it is to be understood ~at these ~alues can also be set to nominal default values to simplify programming procedures.
Operation of the Bennett et al. pacemaker as depicted in Figure 3 begins with the patient in an initial resting condition and the pacemaker pacing at the programmed Lower Rate. When the patient is stressed by exercise, the Bennett et al. pacemaker responds by increasing the pacing rate, as illustrated by attack curve 58, until it reaches a maximum pacing rate or plateau 60, at which ime the pacing rate stabilizes for the W. O 93/23115 2 1 1 2 2 ~ 4 Pcr/usg3/028g1 duration of the strcss. The pacing rate 60 may or may no~ be limited by uppeir rate 42, depending upon the level of detected activity.
If the paticnt maintains a hdghtened exercise level, and the Bennett et al.
paceimaker has paced above the Achievement Rate 44 for a predetermined interval of time (e.g., four seconds is disclosed in the Bennett et al. reference), theh the Bennett et al. pa~ma~er automatically triggers a modified decay feature. After the modified decay fea~re is tr'iggered, when the pacing rate is decreased, it is decreased according to a decay curve 62 which is deflectcd at the upper and lower switch points such as 52 and 54, as thc pacing rate falls first to the Upper Switch Rate 46 and then to the Lower Switch Rate 48, respectively. lt is stated in the Bennett et al. reference that the four-second Achievement Duration value substantially minimizes false triggering by artifacts.
With continuing reference to Figure 3, upon decrease of the activity level the Bennett et al. paccmaker begins reducing its pacing rate, with the reduction initially occ~ing at the programmed Activity Response Time Deceleration constant, for lS a~;unple, 2.5-minutes.However, once the pacing rate reaches the Upper Switch Rate th~shold 46, the docay time constant is increased in orda to slow the rate at which the pacing ~ate is reduced. The Benneu et al. modified decay feature simulates the heart's wrmal behavior under the circumstances, and causes the Benneu et al. pacemaker to respond optimally to thc individual padent's cardiovascular needs.

The modified decay curve 56 generally ~Iresponds to the latent decay pordon 38 in Figure 2. If, prior to reaching the Low Switch Rate 48, the padent rcsumes asuddcn hcightencd stress or excrcise lcvel, then, as-indicated ~y deflecdon point 64, the pacing rate inc~eases cosrespondingly, as indicated by attack curve 66. It should be notod ~at at this stage, since the modified decay curve 330 has not reached the Lower Switch Rate 48, the modified decay feature of the Bennett et al. pacemaker is still enabled and has not been turned off. The modified decay feature will be disabled (turned off) only afur the pacing rate d~ops below the Lower Switch Rate 48, along curve 68, at which time the modified decay feature will not be re-enabled undl the Achievement Crileria have again be n n~.

WO 93/23115 2 112 2 4 ~ 16 PCr/USs3/02891 ~

Therefore, as the pacing rate decay curve 70 reaches the Upper Switch Rate 46, a corresponding switch point 72 causes a change in the deceleration time constant. In this manner, the pacing rate is allowed to decay along the decay curve 337 at the time constant of 45-seconds, and upon rcaching the Upper Switch Point 72, the pacing rate S follows the modified decay curve 74.
The decay timc constant of modified decay curves 56 and 74 ate substantially similar, and their time constants can be selected from a range specified in the Bennett et al. refetence bctween 90 to 18~ seconds. As a person of ordinary skill in the art would apprcciate, decay curves 56 and 74 can have different time constants, depending upon the desired bchavior of the pacemaker.
Since, in Figure 3, attack curve 66 has reached and exceeded the achievement rate thrcshold 44, it might be dcsirable to set the time constant of decay curve 74 at an intermediate valuc between the decay time constant of decay curve 62 (i.e., 45-scconds) and the modified time constant of dccay cutve 56. By analogy, the decay time constant of cluve 70 could also be seloc~d to differ from the conventional 45-second decay time con~nt of curvc 62.
As curve 74 reaches Lower Switch Rate 48, its decay time constant changes to a faster titne constant of cutvc 68, similar to the conventional 45-second time constant.
A diffcrent dme constant can be sdected.
Upon reaching lower switch point 54, the modified decay achievement criteria aremet, such that as long as the paticnt's exercise and stress levels do not cause the pacing ate to reach or excecd Achicvement Rate 44 for a predctermined length of time, then ~c pacing rate is allowed to decay at a nominal 45-second dme constant.
This feature is i11ustrated by attack curve 76 which falls short of reaching Achievement Rate 44. Docay curve 78 is followed, even though the pacing ~ate decays bdow Upper Switch Rate 46 and Lowa Switch Rate 4~. Hence, as illustrated, curve 78 is allowed to docay with a single, uninterrupted time constant, since it is presumed ~at under such circumstances the patiant does not require additional time to recuperate from increased sudden stress. Therefore, no deflection is effected at the intersection points 80 and 82.

WO 93/23115 17 Pcr/us93/o28sl It is noted in the Benneu et al. reference that intersection points 80 and 82 can become switch points similar to the upper and lower switch points 72 and 54. In the altemative, curve 78 can decay at a time constant different from that of decay curve 62.
It is also noted in the Bennett et al. reference that one or more additional upper and S lower switch levels can be defined betwoen Achievement Rate 44 and Upper Switch Rate 46, as well as between Upper Switch Rate 46 and Lower Sw~itch Rate 48, in order to galate a more gradual deflection of the overall docay curve.
Turning now to Figure 4, the operation of the Bennett et al. pacemaker will be des~ibed in somewhat greater detail in connection with flow chart 100. The software prog~am aul/or hardware starts at 110, and then determines, at 112, a new rate-response Targd Rate according to the following equation:

l'R = (AC~count~D) x (32768x60) where TR is the Target Rate as defined above, calculated in re~onse to the activity scnsor output; and C and D ate programmable values that generate the shape of the rate ~esponse curves. The values of C and D are generated by the Bennett et al. pacemaker or by an e~ttemal programmer ~not shown in the Figures) as a function of the selec~d Upper Rate, Lower Rate, and Rate Rcsponse values. C and D are programmed into a memo¢y or storage register of the Bennett et al. paccmalcer using conventional pro~ammi~g mc~ods. Thc Bennctt ct al. paccmaker includcs an arithmetic and logicuoit (ALU) capable of malting ~e necessary calculations and controlling the sate of the p~r based upon thc calculated Target Rate ( I~).
An atamplc of a rate re~onsivc pacemaker in which the target pacing rate is calcula~ed in a manner similar to that of the Bennett ct al. reference is the above-~efaenced Sivula et al. device. Such a pacemaker is also disclosed in pending U.S.
Patent Application Scrial No. 07/794,766 filed on November I5, 1991 in the name of Paul Marc Stein and entitled ~Method and Apparatus for Implementing Activity Sensing in a Pulse Generator", which is now inco~porated by reference hcrein in its entirety.

W O 93/23115 2112 2 4 l 18 P(~r/US93/02891 While there are numerous well-known paoemakers which calculate a target pacing rate in such a manner as described in the Stein, Sivuh et al. and Bennett et al. references, it is to be understood that the present invention is not limited in scope or applicability to only those pacemakers. It is bdieved by the inventors that the present invention may be S effoctively and advantageously implemented in various ways in conjuncti~n with various different pacemalcers.
Returning to the description of the Bennett et al. pacemaker, each fime the physician alters the selected Upper Rate, Lower Rate, or Rate Response settings, the programmcr generates a new set of C-term, ~term, s vitch rate, and achievement rate values, and loads them into memory or program registers in the pacen~a~er, so that the ALU may calculate the Target Rate thereafter, based upon the updated values.
Regardlcss of which of the selected parameters has changed, the resulting function rda~ng the paGing rate to the sçnsor output will take the same basic form, extending from the Lower Rate at minimal activity sensor output to the Upper Rate at an achievabk lS sensor output, with a sensor output required to achieve the upper rate decreasing as the rate-re~onse scning is increased.
As indicated in block 112 of Figure 4, the Bennett et al. pacemaker periodicallycalcuhtes the activity Target Rate at two-second intervals, along the curve 40 of Figure 3.
Ncxt, the sofn~are dctermines at 114 whether the modified decay featurc in ~cco~dance ~vith the Bennctt ct al. disclosure has been activated or programmably enabled ~ria a programmer such as the Mcdtronic Model 9760 or the like. If the modified docay feature has not bocn activatcd, thcn the software sets the docay rate to be equal to the p¢o~ammed decay rate, e.g., 45-second. This is indicated at block 115 in Figure 4.
2S - Thc Bennett et al. pacemal~r then calculates the next activity pacing rate at 116!
and saves lhe activity related data at 117, for use in calculating the new activity target ate at 112. Thc above routine is repeated until the modified decay feature is activated.
If the modified decay feature is found to be enabled at block 114, then, as indicated by block 120 the so~e de~ermines whether the achievemcnt criteria haveboen met. That is, the software dctermines whether the pacing rate has been greater than WO 93~23115 2 1 1 2 2 4 4 Pcr/usg3/02891 .
or equal to the Achievement Rate for four seconds or n~ore. If the Achievement Criteria have not boen met, then the software, at block 115, sets the docay rate to be equal to the programmed decay rate, ealculates the activity pacing rate at 116, saves the activiq data at 117, and then calculates the new activiq target rate at 412.
If, on the other hand, the Achievement Cdteda have been met; then, at block 133, the software determines whether the eurrent pacing rate is greater than the Upper Switeh Rate 46. If it is, then, onee again, the software sets the decay rate to be equal to the programmed decay rate at block 115, ealeulates the new aetiviq paeing rate at bbck 116, saves the aetivity data at bloclc 117, and ealeulates the new activity Target Rate at block 412. . .
If the pacing rate is less than or equal to the Upper Switeh Rate 46, then the software determines, at 137, whether the pacing rate is between the Upper Switch Rate 46 and the Lower Switch Rate 48. If it is, then, as indieated by bloek 439, the software cbanges the deeay rate to the modifiod Ot slower deeay rate, as illustrated by decay lS ewves 56 and 74 in Figure 3. The aetivity pacang rate is then ealeulated at 116, the ~tiviq dala is s~ed at bloelc 117, and a new aetiviq Target Rate is ealeulated at block 112.
If the paeing rate is less than the Lower Switeh Rate 48, then as indieated by bloek 115, the software ehanga the decay rate to the programmed value.
Tun~ing now to Figure 5, a bloek diagram illustradng the eonsdtuent eomponents of a ~er 10 in acoordanee with the presently diselosed embodiment of the invtn60n is providcd: ~Ithough the present invention w,n be described herein in conjunc~on wi~ a ~na~ 10 having a micwpm~or-based arehiteeture, it will be uul~ood ~at paoemalcer 10 may be implemented in any logic based, custom integ~ted 2S cucuit atcbitecn~e, if desired. lt will also be understood ~at the present invendon may be utilized in conjunction with other i~mplantable medical devices, such as cardioverters, defibIilhtors, cardiac assist systems, and the like.
In thc illustradve embodiment shown in Figure 5, pacemaker 10 includes an activity sensor 12, which as previously noted may be, for example, a piezoelectric dement bonded to the inside of the pacemaker's shield. Such a pacemalcerlactivity W093/23115 211224~1 20 Pcr/us93/o289l sensor configuration is the subject of the above-referenced patent to Anderson et al., which is hcreby incorpo~ated by reference in its entirety. Piezoelectric sensor 12 provides a sensor output which varies as a function of a measured parameter that relates to the mctabolic roquirements of patient 11.
Pacemal~er 10 of Figure S is programmable by means of an exten~l prograrnming unit (not shown in the Pigures). One such programmer suitable for the puIposes of the present invention is Ihe Medtronic Model 9710 programmer which has boen commercially available for several years and is intended to be used with all Medtronic paa~s. The pro~amnier is a rnicroproccssor device which provides a-series of ~0 encoded signals to pacemaker 10 by means of a programming head which transmits radio-froquency (R~:) encodcd signals to pacemaker 10 according to the telemetry system laid out, for e3~ample, in U.S. Patent Application having Serial No. 07/765,475, filed on September 25, 1991 by Wyborny et al., which application is hereby incorporated by re~erencc in its entirety. It is to be understood, however, that the progwnmingmethodology discloscd in Wybomy et al. patent is identified herein for the purposes of illustration only, and that any programming mcthodology may bc employcd so long as ~e desiret information is transmitted to the paccmaker. It is bclicved that one of sldll in ~# art would bc able to choosc from any of a number of available prog~ ming techniques to acoomplish this task.
The programmer facilitatcs the selecdon by a physician of the desircd paramcter to bc programmed and the entry of a particuhr sctdng for the desirod parameter. For p~poses of tbe prescnt invention, the SpeCiflCS of op~ation of the programmer are not Wieved to bc impcnant with the excepdon that whatever programmer is used must iDcludc m~ns for sdccting an upper rate (IJR), a lower rate (LR), and one of a plurality 2S of ~atc rqc (RR) sefflngs to bc hcreinafter described in g~eater dct~il.
In the illush~ativc embodimcnt, the lower rate may be programmable, for cxample from 40 to 90 pulses per minute (PPM) in increments of 10-PPM, the upper rate may bc prog~amn~able between 100 and 175-PPM in 25-PPM increments, and thcre may be ten rate response funcdons, numbe~ed one tnrough ten, available.

21122~4 WO 93/23115 21 PCr/US93/02891 ln addition, the programmer may include means for selcction of acceleration and decdcradon para~ neters which limit the rate of change of the pacing rate. Thcseparametcrs may be variously referred to in the ratc responsive pacemaker context as acceleration and dccdcration settings or attack and dccay scttings. These may beS e~essed in tcrms of the timc interval requircd for the pacemakcr to chahge bctwecn the current pacing ratc and 90% of the desircd pacing intcrval, assuming that the activity level cone~onding to the desired paang rate remains constant. Appropriatc selectable values for the programmed accderation time would be, for example, 0.25 minutes, 0.5 minutcs, and 1 minute. Appropriatc selectable values for the programmed deceleration time would be, for example, 2.5 minutes, 5 minutes, and 10 minutes.
Pacemaker 10 is schematically shown in Figure S to be electrically coupled via a pacing lead 18 to a patient's heart 16. Lead 18 includes an intracardiac electrode located near its distal end and positioned within thc right ventricuhr (RV) or right atrial (RA) chamber of heart 16. L~ad 18 can carry either unipohr or bipolar electrodes as lS is wdl b~own in the an. Although an application of the present invention in the context of a single chamber pacnaller will be disclosed herein for illustrative purposes, it is to be understood that the present invention is equally applicable in dual-chamber L~ad 18 is coupled to a node 150 in the circuitry of pacemaker 10 through input capacitor 152. In the presently disclosed embodiment, activity sensor 12 is bonded to the inside of thc pacemaker's outcr protective shield or can 14 (not shown in Figure 5).
as noted with reference to Figure 1 and in accordance with common practice in the an.
As shown in Pigure 5, the output from acdvity sensor 12 is coupled to an input/output c~rcuit 154.
Input/output circuit 154 contains the analog circuits for interface to heart 16,activity sensor 12, an antenna lS6, as well as circuits for the application of stimulating pulses to heart 16 to control its rate as a function thereof under control of the software-implemented algorithms in a microcomputer circuit 158.
Microcomputer circuit 158 comprises an on-board circuit 160 and an off-board circuit 162. On-board circuit 160 includes a microprocessor 164, a system dock circuit wo 93/23115 2 1 1 2 2 ~ 4 22 PCI-/US93/028 r ~

166, and on-board RAM 168 and ROM 170. In the presently disclosed embodiment of thc invcntion, off-board circuit 162 comprises a RAM/ROM unit. On-board circuit 160 and off-board circuit 162 arc each coupled by a data communication bus 172 to a digital controllcr/timer circuit 174. Microcomputer circuit 158 may bc fabricatedi of a custom S integrated circuit device augmcnted by standard RAM/ROM components.
lt will be understood that tne electrical components represented in Figure S arepowerod by an appropriate imphntable battery power source 176, in accordance with common practicc in the art. For the sake of clarity, the coup1ing of battery power to the various componcnts of pa~mal~r 10 has not becn shown in the Figures.
Antenna 156 is connected to inputloutput circuit 154 for purposes of uplink/downlinlc telcmetry through RF transmitter and receiver unit 178. Unit 178 may correspond to the telemetry and program logic employed in U.S. Patent No. 4,566,063 issued to Thompson et alL on Dccember 3, 1985 or in the above-referenced Wyborny et alL patent, both of which are incorporated herein by reference in thdr endrety. The lS particular prog~amming and tclemetry scheme chosen is not believed to be important for the pulposes of the p~esent invendon so long as it p~ovides for entry and storage of values of ~re~onse pa~ame~s discussed herein.
A crys~al oscillator circuit 180, typically a 32,768-Hz crys~l~ontrolled oscillator, provides main timing clock signals to digital controller/timer circuit 174. A V,~EF and Bias circuit 182 generates stable voltage reference and bias currents for the analog circuits of input/output circuit 154. An analog-to-digital convener (ADC) and multiple~ter unit 184 digitizes analog signals and voltages to provide ~real-time~ telemetry inuacardiac signals and batt~y end~f-life (EOL) rephcement function. A power-on-rcset (POR) circuit 186 functions as a means to reset circuitry and related functions to a default condition upon detection of a low battery condidon, which will occur upon initial de~ice power-up or will transiently occur in the presence of elec~omagnetic interference, for example.
The op~ng commands for controlling the timing of pacemakcr 10 are coupled by bus 172 to digital controller/timer circuit 174 wherein digital dmers and counters are employed to establish the overall escape interval of the pacemalcer, as wdl as various YV~O 93~23115 ~12 2 ~ ~ pcr/uss3/o28g1 refractory, bhnking, and other timing windows for controlling the operation of the peripheral components within input/output circuit 154.
Digital controllerltimer circuit 174 is coupled to sensing circuitry including asense amplifier 188, a pcak sense and thrcshold measurement unit 190, and a S comparator/threshold detcctor 192. Circuit 174 is further coupled to an electrogram (EGM) amplifier 194 for receiving amplified and processed signals picked up by the electrode disposed on lead 18 which signals are rcpresentative of the electrical activity of the patient's heart 16. Sense amplifier 188 amplifies sensed electrical cardiac signals andprovides thisamplificd signal to pealc senseand threshold measurement circuitry 190, which provides an indication of pealc sensed voltages and the measured sense amplifier th~eshold voltage on muldple conductor signal path 67 to digital controller/timer circuit 174. The amplified sense amplifia signal is then provided to comparatorlthreshold detoctor 192. Sense amplifier 188 may correspond, for cxample, to that disclosed in U.S. Patent No. 4,379,459 issued to Stein on April 12, 1983, incorporated by rcference lS herein in its entirety. The electrogram signal developed by EGM amplifier 194 is used on those occasions when the implanted dcvice is being interrogated by an external pro~ammer, not shown, to transmit by uplink telemetry a representation of the analog d~am of the patient's dectrical heart activity, such as describcd in U.S. Patent No.
4,556,063, issucd to Thompson ct al., assigned to the assignee of the present invention and inco~porated herein by reference. An output pulse generator 196 provides pacing stimuli to the padent's heart 16 through coupling capacitor 198 in response to a pacing triggcr signal developed by digital controller/dmer circuit 174 each time the escape interval times out, or an externally t~ansmitted pacing command has been received, or in rc~onse to other stored commands as is well known in the pacing art. Output 2S amplifia 196 may amespond gendly to the output amplifia disclosed in U.S. Patent No. 4,476,868 issued to Thompson~ on October 16, 1984 also incolporated herein by rcferencc in its entirety.
While specific embodiments of input amplifier 188, output amplifier 196, and EGM amplifier 194 have been identified herein, this is done for the purposes of illus~ation only. It is belicved by the inventor that the specific embodiments of such circuits are not critical to the present invention so long as they provide means for generating a sdmulating pulse and provide digital controller/timer circuit 174 with signals indicadve of natural and/or stimulated contractions of the heart.
Specification of the Achievement Criteria is one area of distinction bctween theS Bennett et al. reference and a pacemaker in accordance with the pr~sent invention.
Whereas in the Bennett et al. reference the Achievement Criteria are specified in terms of a single Achievcment Rate value (e.g., 125-PPM) and a single Achievement Duration value (e.g., four seconds), a pacemaker in accordance with the presently disclosed cmbodiment of the invention instead recognizes a family of Achievement Rate /
Achievement Duration pairs. In particular, with reference to Figure 6, a Work Duration Identity curve describing the normalized Achievement Criteria for the presently disclosed embodiment of the present invention is shown. In Figure 6, the honzontal axis reprcsents time units. and the vertical axis represents percentage achievement of the rate Iange.
lS The curve shown in Figure 6 defines the relationship between equivalent Target Time Units, with all points along the curve of Pigure 6 corresponding to equal Target Rale Target Time Unit products. For example, the point designated 2û0 along the curvc of Figure 6 co~onds to a Target Time Unit of one time unit and a Target Rate that is at 100% of Achicvement Range, where the Achievement Range is some range of pacing ~cs between the programmed Upper and Lower Rates. The point designated 202 ~long the curve of Figure 6 corresponds to a Target Time Unit of two time units at a Target Rate that is at 50% of the Achievement. The points 200 and 202 have equivalent Target Rate Ta~get Time Unit products; indeed, all points along the curve of Figure 6 have e~uivalent Target Rate Target Time Unit products.
The Achievement Range is normalized so that it may correspond to any range of rates. However, in the presently preferred embodiment of the invention, 100% of Achievement Range is selected to correspond to the programmed Upper Rate. 0% of the Achievement Range may be selected to be at or above the programmed L~wer Rate.
As an example of a specific application of the Work Duration Identity curve of Figure 6, assume that pacemaker 10 is programmed with an Upper Rate of UR = 150-~Og3/2311s 25 2 1 1 2 2`4 ~ PCr/US93/028g1 PPM and a Lowcr Rate of LR = S0-PPM. The Rate Response Range is thus lS~PPM
minus S0-PPM equals lO~PPM. The Achievement Range may therefore be specified any range within the UR and the LR. As previously noted, the highest (100%) rate in the Achievement Range is preferably chosen to be the programmed UR, in this case lS~
S PPM. The lowest (0%) rate in the Achievement Range may be selected to be any rate bctween S0-PPM and lS~PPM. Assume, for the sake of illustration, that the lowest rate in the Achicvcment Range is selected to be 50-PPM, so that tne Rate Range and the Achicvcmcnt Range arc co~xtensive, each spanning the lO~PPM range from 50-PPM
to lS0-PPM. Also assume that thc time unit for the horizontal a~is is selected to be one minute. Then, from Figure 6, it can be seen that the Achievement Cnteria i$ deemed to be met when the Target Rate is at 1û0% of the Rate Range (i.e., 150-PPM, computed as 100% of 100 PPM (equals lO~PPM) plus 5~PPM (the starting point of the Rate Range) for one time unit (i.e., one minute); or when the Target Rate is at 7S% of the Rate Range (i.e., 12S-PPM, computed as 75% of lO~PPM (equals 75-PPM) plus S~
lS PPM (the ss~rting point of the Rate Range)) for 1.3 time units (i.e., 1.3-minutes); or when thcTarget Rate is at 50% of the Rate Range (i.e., lO~PPM, computed as 50% of 100 PPM (oquals 5~PPM) plus S~PPM (the starting point of the Rate Range)) for two time units (i.e., two minutcs). These computations are summa~ized in the following Tabls 1:

TIME AT TARGET
PEROENT OF, PERCENT OF CORRESPONDS TO IU~TE TO MEET
ACHIEVEMENT R~TE A TARGET AC:HIEYEMENT
R~NGE R~NGE R~TE OF CRll~RlA
100% 100% 150-PPM 1 TIME UNIT
7S% 7SX 125-PPM 1.3 TIME UNlTS
S0% S0% 10~PPM 2 TIME UNITS
2SX 25% 75-PPM 4 TIME UNITS
OX 0% ` 50-PPM N/P

Note that for 0% of Achievement Range, the entry in the "Time at Taryet Rate to Meet Achievement Criteria" column of Table 1 is "N/P" (Not Possible), since the WO 93/23115 112 2 4 4 26 PCI/U593/~2891.

Achievement Critena cannot be met if the Target Rate stays at 0% of the Rate Range (50-PPM).
It is to be understood that the example discussed above and summarized in Table 1 is but one possible combination of parameters which may be selecte~ for pacemaker S 10 in accordance with the present invention. A second possible combination would be, for example, (again assuming a programmed UR of 15~PPM and a programmed LR of S0-PPM), an Achicvemcnt Rangc of 60 PPM, spanning from 90 PPM to lS~PPM. In this case, the Achievemcnt Criteria would be met by a Target Ratc at 100% of ffle Rate Range (i.e., lS~PPM, as before) for one ~me unit (again, assume one time unit equals onc minute); or by a Target Rate at 85% of the Rate Range (i.e., 135-PPM, computed as 85% of lO0 PPM (oquals 85-PPM) plus S~PPM (the starting point of the Rate Range) for 1.3 timc units); or by a Targct Ratc at 70% of the Rate Range (I.e., 120-PPM, computod as 709~o of lO~PPM (cquals 7~PPM) plus S~PPM (the starting point of theRate Rangc) for two timc units), and so on. The Achievement Criteria computations for l.S this sooond cxample are summarizcd in the following Table 2:
- T~BLE 2 TIME AT TARGET
PEROENS OP PEROENT OF CORRESPONDS TO RATE SO MEET
ACHIEVEMENT RASE ATARGET ACHIEVEMENT
R4~NGE R~NGE R~TE OF CRITERI~
1009G 100% 150-PPM I TIME UNIT
7sæ 859S 135-PPM 1.3 SIME UNITS
50% 70% ~ 120-PPM 2 SIME UNlTS
25% SS% 105-PPM 4 TIME UNITS
0% 40% 90-PPM N/P

21122~
~0 93/23115 27 ;~ . . . PCI/US93/02891 As from Table 1, from Table 2, it is apparent that the Achievement Criteria in this second illust~ative examplc cannot be met unless the Target Ratc `cxceeds the lowest Achicvemlt Range ~te of 90 PPM.
A third illustrative example of a combination of parameters that mày be selectedS for a ~n~er 10 in accordancc with thc p~esGntly disclosod cmbodimcnt of the invention is summarized in the following Table 3. In this third atample, an Achievement Range of 40 PPM, spanning from 1 l~PPM to lS~PPM is seloctcd.

TIME AT T~RGET
PERCENT OF PERCENT OF CORRESPONDS TO RATE TO MEET
ACHIEVEMENT R~TE A T~RGET ACHIEVEMENT
RANGE R/~GE RATE OF CRITERIA
100% 100% IS0PPM I nME UNIT
IS 7S% 90g 14~PPM 1.3 TIME UNITS

2SX 70g 120-PPM 4 nME UNITS
0X 60g 110-PPM NIP

In the abovc ex;unpies, the normalized time unit was assumed to be one minute;
however, different ~ralues may be chosen, for exarnple one and a third minutes, two minuta, four ~nu~es, and so on. The l/x nature of the Work Identity Curve of Figure 6 providcs clean, inD~itive values for the Achievnent Criteria, so that, for example~ if timc units of two minuu:s ins~ead of one~ minute were specified, the values in the fourth column of each of Tables 1, 2, and 3 above would be doubled.
H;lving dcfined the Achievement Cnteria as described above with reference to Figure 6 and to Tables 1. 2, and 3, it remains to be shown how pacemaker 10 WO93/2311S 21122~4 28 PCl/US93/02891~.

responds when the Achievement Criteria are met. ln accordance with the presentlydisclosod anbodimcnt of thc invcndon, thc decay curve modificadon performed by pacemalcer 10 is defined in tcrms of several rate deceleradon phases, specifically an Initial Decclcration Phase, an intermediate Modified Deceleration Phase, and a final S l~tent Deccleration Phase. ln accordance with thc prcsently disclosed cmbodiment of thc invention, it is the intcrmediate Modificd Deceleradon Phase which is work-moduhted, as shall be hc~einaftcr dcscribed in some detail. By ~work-modulatedn, it is meant that the duradon of the Modified Deceleration Phase is a funcdon of the -amount of work rcccntly performed by the patient, where the amount of work performed by the patient is measured by thc output of the acdvity sensor.
Figure 7 shows a solid linc 212 corresponding to thc p~cing rate curve for paoem~r 10 in accordance with the p~esently disclosed embodiment of the invention.
Also shown in Figure 7 is a dashed line 210 rcprcsenting the rate-responsive Target Rate hmction, computed by p canaber 10 as dcscribed in thc Bennett ct al. and Sivula et al.
lS rciferences. Pacing rate curve 212 reprcsents the actual pacing rate of paccmaker 10, daived from ~e Targct Rate as subjected to the Activity Rcsponse Time Acceleration fwlction and thc Activity Rate Gain Funcuon, dcscribed in the foregoing definitions of ~ns, and the Activity Response Deceleration funcdon of the present invendon.
In Figure ?. the Target Rate inc~cases from 5~PPM (thc programmed LR) to 130 PPM (thc programmed UR) at time T = 0. The actual pacing rate funcdon 212, of course, does not make an immediatc transition in respc~nse to the abrupt change in Target Ratc, but instead gradually increases from 5~PPM to 13~PPM as mandated by the Accele~ation function. As previously noted, the programmed Acceleration Time Constant ~093/23115 29 2112;2 ~ ~ PCI/US93/02891 defines how long it will take to achieve an acceleration from thc present pacing rate to 90% of the Target Rate. In the presently disclosed embodiment of thc invention, the clinician can sdect, by means of the e~ctemal programmer, one of three differentAccderadon Time Constants, 15-Sec, 30-Sec, or l-min.
S At time T z l-min, the Target Rate computed by pacemaker 10 falls from 13 PPM to 5~PPM. Tne decderation of the actual pacing rate is detcrmined in accordance with the work-modulated decay rate function of the presendy disclosed cmbodiment of thc invention. Work-modulated deceleration is intended ~o provide a more physiologic pacing rate deceleration oncc a patient terminates exercise at a higher heart rate. The Inidal Docele~ation Phase occurs immediately after a period of intense exercise, followed by the intcrmediate Work-Modulated Decderadon Phase.
In Figurc 7, the ~itial Decelcradon Phæ bcgins at dcfloction point 214, and continucs to deflection point 216. T;he Initial Dccelcradon Phase has a 2.5-min Dooclaation Timc Constant in the presently prcfcrred embodimcnt.
lS In the Bcnnctt ct al. dcvice, the physician selects an Upper Switch Ratc and a L~wer Switch Rate, as previous1y dcscribed with reference to Pigure 3. Por the work-modulatod pacing rate de~ad~ funcdon of the present invendon, thc physician selects on1y a single Swi~ Rate, gennally conesponding to the Upper Switch Rate of the B0nctt et al. ~efcrence. In thc cxample illustrated in Figure 7, it is assumed that the Switch Rate is programmed to a valùe of approximately 100 PPM. In Figure 7, the pacing rate cunre 212 reachcs the Switch Rate at point 216. In Figure 7, the Target Rate curve 210 is shown to be at 10û% of the Rate Range for one minute. Thus, at time T
= l-min, the Achievement Criteria defined by the Work Identity Curve of Figure 6 are W O 93/23115 : 30 P(~r/US93/02891~.

sadsfied (Achievement Duration of l-min and Achievement Rate of 100% of the Target Rate Range corresponds to point 200 on the Work ldentity Curve). Since the Achievement Criteria are satisfied, the work-modulated rate deceleradon function will be enabled, so that at the Switch Rate, the programmed Deceleradon Time Constant will S be temporarily rephced with a Work-Modulated Deceleration Time Constant selected by the physician.
As noted above, in the presently preferred embodimcnt of the invention the P~grammed Switch Rate is defined and implementod as is the Upper Switch Ratë in the BenneU et al. device, namely as a selected percentage of the difference between the prog~ammed LR and UR. However, it is also contemphted by the inventors that the Programmablc Switch Rate could be defined and implemented as a funcdon of the current work ~alue. In particular, it is contcmplated by the inventors that the Programmable Switch Rate could be increased in proportion to increases in the current Work value, as in thc following equadon:

Switch Rate = f(Work) --(WorklX) x (Upper Rate - Lower Rate) + Lower Rate where X is a programmable or predetermined constant.

In the disclosed embodiment, the Work-Modulated Deceleration Time Constant is 2~min. Thus, beginning at point 216 in Figure 7, the pacing rate will begin to decelerate along a 2~min deceleration curve.

211224~
~0 93/2311S 31 PCI /US93/028gl In the Bennett et al. devicc, the pacing rate decays according to the modified dooelation timc constant until the pacing rate reaches the Lower Switch Rate; that is, the modified docay curve terminates when the pacing rate reaches the Lower Switch Rate. In thc prcsently disclosed embodiment of the invention, on the-other hand, the S tamination of thc In~medi~ Docelesation Phase is not determined according to the pa~ng Iatc, but is instcad d~nnin~ by thc amount of work recently performed by the pa~ent.
Pacemalcer 10 in accordance with the presently disclosed embodiment of the invention defines ~work~ as the differcnce between the Target Rate and a preselected Rest Rate over ~ne, wherc thc Rest Rate is dcfined as follows:

Rest R~te z L~wer Rale ~ 0.1 x (Upper Rate - Lower Rate) When the Wo¢lc-Modulatod Ratc Response Deccleration Punction is enabled, the p~ maintains a n~nning Work value ~at is updated cvy two seconds in much ~c same way as ~ Targct Ra~e is computed every two seconds. The Work value is LS ann~ eve~y two seoonds aoco¢ding to the following recursive formula:
If ~arget Rate 2 Pacing Ratc), then WorkN = WorkN., + (Target Rate - Rest Rate);

WO93/23115 32 PCr/US~3/02#91 ) If 211224~

~arget Rate ~ Pacing Ratc), WorlcN = WorkN, - Work Decrement Value S where Worl4, is thc Work vatue for the cwrent two-second interval, Worl~t,., is the Work vatuc f~m the prcvious two-second intemt, and Work Docremcnt Value is a value ~ding to the amount to subtract from the Work vatue when the current pacing rate a~ceods thc current Tar8et Rate. The Work Decrement Value may be a pro~ammable value; however, in the presently preferred embodiment of the invention, ~e Work Decrement Vatue is defined as (Maldmum Exercise Value 120).
~n t~pcr Dmit on ~c Work value may be imposed, dther as a prog ammable function, or as a built-in func~ion. To this end, a Maximum E~ise Timc value, which may d~er be pn~ammably selectable by the physician or p~eset at the time of ~. nulScnlle, is defined. ln cach two second interval, if the new Work ~taluc excceds the Ma~limum ~E~e Time value, the Work value is forced to the Mal~imum E~ercise Time.
1 t is to be no~d that in defining Work in the manner set forth above, selection of , the Rest Rate~is equivallt to sdec~ing the Achicvement Range to e~ttcnd from a lowcr bound of (Lowcr Rate + (0.1 x (Upper Rate - Lower Rate)~) to an upper bound of UR.
With Wollc definod in this m~ner, for constant work by thc paticnt a largcr Work ::
Dec~ l V~lluc (or Small r Maximum Exe~cise Time) will sl~ten the duration of the Work-Modt laled por~ion of the rate ~lo~ function.

~0 93/23115 33 pcr/us93lo2891 The rate deceleration of pacemaker 10 is subjected to the Work-Modulated Doceleradon function whenever thc current Work value is grcater than zero; othenvise, the programmcd Decderation Time Constant controls rate deceleration.
While the pacing rate is greater than the programmed S~fitch Rate, the S prog~ammed Decele ation Time Constant will be used in calculating the pacing rate.
When the pacing rate decelerates to the Switch Rate, a Modified Deceleration Time Constan~ of 2~min is used. The Modified Deccleration Timc Constant controls the pacing late decderation undl thc Work value reaches zero, at which time the ratedeceleration will be govcrned either by the original, programmed Deceleration Time Constant, or by another programmed decelcradon value (although in the prcfcrred embodiment, the programmed Deceleradon Time Constant is used).
In Figu~e 8, a flow diag~am 300 il1ustratdng the operation of the Work-ModulatedPacing Rate Decele~ation function in accordance with the presently disclosed embodiment of ~c invention is shown. Pacemaker 10 slarts at 310, and then determines, at 312, a ne~v ~ate-re~onse Target Rate according to the Target Rate equadon set forth above with reference to Figurc 4.
At 314, thc Work valuc is calculated according to the formuh set fonh above.
At 316, pa~ 10 detcnnines whetha the Work-Modulated Pacing Rate Deceleration Function has been enabled, such as by an external programmer or the like. If the Work-Modula~d Deceleradon Funcdon has not been enabled, pacemaker 10 sets tne deceleradon time constant to the programmed value, at block 324. Pacemaker 10 then calculates a new pacing rate based upon the current pacing rate, Target Rate, and Dcceleration or Acceleration Time Constants, at 326, and saves tne activity-related data W O 93/23115 2112 2 4 ~ PC~r/US93/02891.

at 328 for use in calculating the new Target Rate during the next two-second interval.
The above routine is repeated until the Work-Modulated Rate Deceleration Function is enabled.
If the Work-Modulated Rate Deceleration Punction is found to be enabled at S decision block 316, pacemalcer 10 then determines whether the Achievement Criteria have been met. In the presently disclosed embodiment of the invention, this determination is made by determining whether the current Work value is greater than zero. lf Work is greater than zero at block 318, pacemaka 10 determines whether the current Pacing Rate is greater ~an the programmed Switch Rate. If so, the programmed Doccl~ation Time Constant is used (block 324); if not the Work-Modulated Deceleration Time Constant is used (block 322).
Then, paccmaker 10 computes the new pacing rate, ~asod upon the current pacing ~ate, Target Rate, and Accderation or Deceleradon Time Constants, at block 326.
Acti~ity salsor data is accumulated at block 328, and the process is repeated beginning with block 312.
To fwther illustrate the effect of the Work-Modulated Deceleration Function on ~e pacing rate, scve~l pacing rate curves shown in Figure 9. In Figu~e 9, fivepacing ~atc curves designaled 400, 402, 404, 406, and 408 are shown. Each of thepacing ~atc curves share an identical accele~adon phase, designated in Figure 9 as 410, but differ from one another in the amount of time each one spends at a maximum pacing ~e of approximately 90 PPM. In the case of curve 400, the length of time in which ~he Target Rate exceeds the pacing rate is so short that very little Work is accumulated; as a result, the oeceleration phase of curve 400 is hrgely controUeo by the programmeo 21122~
~yo 93/23115 ~35 ' Pcr/uss3/o289l Deceleration Time Constant. Por curve 402, however, the pacing rate remains at 90-PPM for a longer penod of time, allowing the Work value to inease more.
In Figure 9, it is assumed that the Switch Rate is programmed to some value abave 9~PPM, so that none of the curves shown in Figure 9 will undergo an initial S Rapid Da~tion Phase at thc programmod Doceleration Time Constant. Por curve 40Q, when the Target Rate falls substantially at point 414, the rate deceleration will be govanod by the Work-Modulated Docel~ation function, since the Work value at point 414 is greatcr than ze~o. Spccifically, the Work-Modulated Deceleration Time ~:onstan~
(e.g., 2~min) will govern the deceleration from point 414 to point 416. During tne time between points 414 and 416, the Work Value will decay as described in the Work value foQmula set forth hereinabove. At point 416, the Work value bas decayed to zero, so that the docde~ation of the pacing ~ate is then governed by the prog~a~mmed Deceleration r~ Cons~ant of 2.5-min. This decelesation occurs until the pacing rate has returned to the progr~nmed LR.
For curve 404, the pacing ~ate accelerates from 5~PPM to 90 PPM at the same time as cwves 400 and 402, but ~ ins at the 90 PPM level for a longer period of time, un~l point 418, at which time it is assumed that the Target Ra~e drops to 50-PPM. Since curve 404 r~nainod at thc 90-PPM levd for a longer time than curve 402, more Work is a~owcd to accumulate for curve 4û4 than for curve 402. Since more Work has accumulla~d, the Work-Modula~l Deceleration phase of curve 4û4 hsts longer than for cun~e 402. As can be seen from Figur 9, the Work-Modulated Deceleration phase for cu~rve 404 lasts from point 418 to point 420, when the programmed Deceletation Time Constant is engaged.

WO 93/231lS 2112 2 i 4 36 rcr/uss3/0289!

Similarly, curve 406 undergoes an acccleration phase identical to that for curves 400, 402 and 404, but remains at the 90 PPM level for a longer time, until point 422.
Since more Work accumulatcs for curve 406 than for any of the curves 400, 402 or 404, thc Worlc-Modulatcd Dadcration phase for cun~e 406 lasts longer than for those cunres, from point 422 to point 424. At point 424, deceleration resumes with the programmed Doccleration Timc Constant.
Finally, curve 408 undergoes the same acceleration phase as curves 400, 402, 404, and 406, but remains at the 90 PPM level for considerably longer, until point 426, allowing proportionally more work to accumulate. When the Target Rate returns to 5~
PPM at point 426, the Work-Modulated Deceleration Phase begins, lasting until the Work valuc was docayed to zero at point 428. Thereafter, deceleration of cur~e 408 orcurs at thc prog~nmod Deccleration Time Constant.
Tu~ning now to Figu~e 10, another group of pacing ~ate curves is dcpictod. In Figurc 10, pacing atc curvcs 430, 432, 434, and 436 are shown, with each curve undcrgoing an idsntical accderation phase from 5~PPM to 130-PPM. In Figure 10, howwa, the accele~adon phases of the four cunres begin at different dmes, whereas in Figure 9, thc accde~ation pbases of the curvcs began at the same time ~T = 0). In Figurc 10, thc docdaa~tion phases bcgin at the same time CT = 0), wheseas in Pigure 9 thc d~on phascs began at different times. In Figure 10, the programmed Switch Ratc of approximately 112-PPM is indicated by dashed line 438.
Figure 10 illustrates the ~elationship between the deceleration curves resultingfrom thc pacn~r's re~onx to varying amounts of accumuhted Work. Since cach of thc ratc curva of Figure 10 begins its deceleration phase from a pacing rate above the W~O 93~23115 2112 2 4 ~ PCr/US93/02891 prog~ammcd Switch Rate 438, the decelcration phases of each curve each begin with a rapid Initial Docelcration Phase at the programmod Deceleration Time Constant of 5-min., until the respective pacing rates have been reduced to the Switch Rate. Since the Initial Deoeleration Pllase is, in accordance with the p~esently disclosed embodiment of S thc hvcndon, not work-modulated, the duration of the Initial Doceleration Phase for each of the curves in Figure 10 is the same, lasting from point 440 to point 442.
Curve 430, which pdor to time T = O rcmuns at the upper rate of 130 PPM for the longest dme of any of the curves of Figure 10, has a Work-Modulated Deceleradon Phase ~at bcgins at point 442 and lasts until point 444 (dme T = 2~min.) Curve 432, which remains at the 130-PPM level for a shorter period of dme, also has a Work-Mod~atod Dooeleration Phase th~t begins at point 442; however, for curve 432, the Wo~c-Modulatcd Deceleradon Phase only lasts until point 446, since less Work ~muh~od dunng the shortcr time that curve 432 rcmained at the 13~PPM lcvd.
Sinil~ly, ~e Worlc-Modulatod Doccleration Phase of curve 434 e~ctends only from point lS 442 to po~nt 448, and that of curve 436 from point 442 to point 450.
Figurc 10 empha~zes the fact that the time constant governing the Initial Phase of differcnt rate curves is the same, regardless of how much Work has accumula~ed for thc different rate curves; ii~ewise, the time constant governing the Wo~c-Modulated Docdcration Phase of differcnt rate curves is the same, regardless of aocumulaled Work. It is the du~ation of the Work-Modulated Deceleration Phase that wies in accordance with accumulated Work.
From the foregoing detailed description of a specific embodiment of the present inv~, it ~ould be a~ent that a pacemal~ having a Wo~Modulated Pacml~ Rate WO 93/23115 PCr/US93/02891-21122~ll 38 Doceleration funcdon has been disclosed which more closely mimics the natural physiologic ~esponse of a human heart to inc~eases and decreases in padent acdvity.
Although a specific embodiment of the inventdon has been set forth herein in some det~il, it is to be unders~od that this has been done for the purposes of illustratdon only, S and is not to be taken as a limitadon on the scopc of the invendon as defined in the appcnded claims. It is to be understood that various alterations, subsdtutions, and modificadons may be made to the embodiment descnbed herein without departing from the spirit and scope of the appended claims.

Claims (7)

WHAT IS CLAIMED IS:

1. A cardiac pacemaker responsive to detection of an increase in a patient's work level over time to increase the rate of delivery of pacing pulses, and responsive to detection of a decrease in a patient's work to decrease the rate of delivery of pacing pulses, wherein said decrease in the the rate of delivery of pacing pulses is modulated by a quantification of said detected increase in the patient's work.

2. A method of operating a rate-responsive cardiac pacemaker comprising the steps of:
(a) detecting when a patient's work level over time exceeds a predetermined rest level;
(b) quantifying said excess of detected patient work level;
(c) increasing the rate of delivery of pacing pulses in response to said detection of work exceeding said rest level;
(d) detecting when said patient's work level response to said rest level;
(e) decreasing the rate of delivery of pacing pulses along a deceleration curve that is modulated according to said quantification of excess patient work.

3. An implantable cardiac pacemaker, comprising:
a pulse generator, coupled to a patient's heart via a pacing lead, said pulse generator responsive to a pacing trigger signal to deliver pacing pulses to said heart;

an activity sensor, coupled to said patient and responsive to patient activity to produce an activity signal;
a pacing rate computing circuit, coupled to said activity circuit and responsive to said activity signal to compute a pacing rate;
a work computing circuit, coupled to said activity circuit and to said pacing rate computing circuit, said work computing circuit responsive to said activity signal to modulate decreases in said pacing rate;
a pulse generator, coupled to said rate computing circuit and responsive thereto to generate stimulating pulses at said computed pacing rate.

4. A rate-responsive cardiac pacemaker comprising:
an activity sensor producing an activity signal indicative of detected levels of a patient's activity;
a processor, coupled to the activity sensor, responsive to said activity signal to compute a pacing rate and a work value;
a pulse generator, coupled to said processor, for a producing stimulating pulses at said computed pacing rate;
a pacing lead, coupled to said pulse generator, delivering said stimulating pulses to the patient's heart;
wherein said work value comprises a quantification of the patient's activity over time;

and wherein deceleration of said pacing rate rate is modulated by said work value.

5. An implantable, rate-responsive cardiac pacemaker, comprising:
a pulse generator, adapted to generate cardiac stimulating pulses in response to a trigger signal;
a pacing lead, coupled to said pulse generator and adapted to convey said stimulating pluses to a patient's heart;
a piezoelectric activity sensor, mechanically coupled to said patient and responsive to patient motion to produce an activity signal;
an activity circuit, coupled to said activity sensor and responsive to said activity signal to produce a stream, of activity pulses;
an activity pulse counting circuit, coupled to said activity circuit and adapted to periodically compute an activity count value corresponding to the number of activity pulses produced by said activity circuit during a predetermined time interval;
a target rate computing circuit, coupled to said counting circuit and adapted to periodically compute a target pacing rate value proportional to said activity count value;
a work computing circuit, coupled to said rate computing circuit and adapted to peridically compute a work value that is proportional to excesses of previously computed target pacing rate values over a predetermined threshold value;

a pacing rate computing circuit, coupled to said target rate computing circuit and to said work computing circuit, and adapted to periodically derive a pacing rate value from said target rate value and said work value;
wherein said pacing rate computing circuit is adapted to vary the rate of decrease in said pacing rate value in an inversely proportional relationship to said work value.

6. A method of controlling the pacing rate of a cardiac pacemaker comprising the steps of:
(a) increasing said pacing rate from an initial rate to a first target rate along a predetermined acceleration curve;
(b) associating a work value with said increase in pacing rate;
(c) decreasing said pacing rate from said first target rate along a deceleration curve having a plurality of phases with different deceleration rates, at least one of said plurality of phases having a duration modulated by said work value.

7. The method of claim 6, wherein said work value is proportional to a time interval during which said first target rate exceeds a predetermined rest rate, and proportional to the margin between said pacing rate and said rest rate during said time interval.