Abstract:
Rate responsive pacing systems that employ a time-dependent AV delay in the pacing hearts in Congestive Heart Failure (CHF) with Dilated Cardiomyopathy (DCM) (a CHF/DCM heart) during a post-implant Time-Adaptive period are disclosed. A starting or initial AV delay is set to an intrinsic AV delay time interval exhibited by the patient&#39;s heart at the time of implant. A chronic AV delay is then set to a therapeutic AV delay time interval that is shorter than the intrinsic AV delay time interval and alleviates symptoms of the CHF/DCM heart. A Time-Adaptive AV delay (TA-AV delay) is employed during a post-implant Time-Adaptive period that gradually changes the initial AV delay to the chronic AV delay at the end of the post-implant Time-Adaptive period.

Description:
FIELD OF THE INVENTION 
     The present invention relates to dual chamber pacing systems, including rate responsive pacing systems, and more particularly to the employment of a time-dependent AV delay for pacing hearts in Congestive Heart Failure (CHF) with Dilated Cardiomyopathy (DCM). 
     BACKGROUND OF THE INVENTION 
     Dual chamber pacing systems operating in the multi-programmable, DDD and DDDR pacing modes have been widely adopted in implantable dual chamber pacemakers and certain implantable cardioverter/defibrillators (ICDs) for providing atrial and ventricular (AV) synchronized pacing on demand. A DDD pacemaker implantable pulse generator (IPG) includes an atrial sense amplifier to detect atrial depolarizations or P-waves and generate an atrial sense event (A-EVENT) signal, a ventricular sense amplifier to detect ventricular depolarizations or R-waves and generate a ventricular sense event (V-EVENT) signal, atrial and ventricular pacing pulse generators providing atrial and ventricular pacing (A-PACE and V-PACE) pulses, respectively, and an operating system governing pacing and sensing functions. If the atria fail to spontaneously beat within a pre-defined time interval (atrial escape interval), the pacemaker supplies an A-PACE pulse to the atria through an appropriate lead system. The IPG supplies a V-PACE pulse to the ventricles through an appropriate lead system at the time-out of an AV delay timed from a preceding A-EVENT or generation of an A-PACE pulse unless a non-refractory V-EVENT is generated in response to an R-wave during the AV delay. Such AV synchronous pacemakers which perform this function have the capability of tracking the patient&#39;s natural sinus rhythm and preserving the hemodynamic contribution of the atrial contraction over a wide range of heart rates. 
     The rate-adaptive DDDR pacing mode functions in the above-described manner but additionally provides rate modulation of a pacing escape interval between a programmable lower rate and an upper rate limit (URL) as a function of a physiologic signal or rate control parameter (RCP) developed by one or more physiologic sensors and related to the need for cardiac output. In the DDDR pacing mode, reliance on the intrinsic atrial heart rate is preferred if it is appropriately between the URL and the programmed lower rate. At times when the intrinsic atrial rate is inappropriately high, a variety of “mode switching” schemes for effecting switching between tracking modes and non-tracking modes (and a variety of transitional modes) based on the relationship between the atrial rate and the sensor derived pacing rate have been proposed as exemplified by commonly assigned U.S. Pat. No. 5,144,949, incorporated herein by reference in its entirety. 
     The DDD and DDDR pacing modes were initially perceived to be of greatest benefit to cardiac patients whose hearts have an intact sinoatrial (SA) node that generates the atrial depolarizations detectable as P-waves, but also suffer defective A-V conduction, or AV block, wherein the ventricles fail to depolarize in synchrony with the atria. The DDD pacing mode paces the ventricles in synchrony with the atria after a timed out AV delay and is generally adequate to restore cardiac output for sedentary patients. Active patients with Sick Sinus Syndrome (SSS) have an atrial rate which can be sometimes appropriate, sometimes too fast, and sometimes too slow. For SSS patients, the DDDR pacing mode provides some relief by pacing the atria and ventricles at a physiologic rate determined by an algorithm responsive to the RCP indicative of the patient&#39;s metabolic needs. 
     A loss of A-V electrical and mechanical synchrony can result in series of asynchronous atrial and ventricular depolarizations at independent rates that periodically result in an atrial depolarization that closely follows a ventricular depolarization. When this occurs, the left atrium contracts against a closed mitral valve, resulting in impeded venous return from the pulmonary vasculature due to increased atrial pressure and possibly even retrograde blood flow into the pulmonary venous circulation. As a result, the volume and pressure in the pulmonary venous circulation rise. Increased pulmonary pressures may lead to pulmonary congestion and dyspnea. Distention of the pulmonary vasculature may be associated with peripheral vasodilation and hypotension. In addition, the concomitant atrial distention is associated with increased production of atrial natriuretic factor and increases the susceptibility to atrial arrhythmias and possibly rupture of the atrial wall. Finally, turbulence and stagnation of blood within the atrium increase the risk of thrombus formation and subsequent arterial embolization. Maintenance of AV mechanical synchrony is therefore of great importance as set forth in greater detail in commonly assigned U. S. Pat. No. 5,626,623, incorporated herein by reference in its entirety. 
     Theoretically, AV synchrony is best maintained during dual chamber cardiac pacing by setting the AV delay interval in a physiological range related to the spontaneous atrial rate or the sensor derived rate, depending on which is the controlling pacing mode. However, while “physiological” AV delays may ensure right heart AV electrical synchrony, in patients with significant interatrial and/or interventricular conduction delays, left heart electrical and mechanical synchrony, and thus hemodynamic performance, may be significantly compromised. 
     The maintenance of AV mechanical synchrony is of vital importance in patients with compromised cardiac function, including CHF, DCM, hypertrophic cardiomyopathy, hypertensive heart disease, restrictive cardiomyopathy, and other disorders that are characterized by significant diastolic dysfunction. In such patients, passive ventricular filling is reduced due to poor ventricular compliance and incomplete or delayed relaxation. Consequently, there is increased reliance on atrial contraction for ventricular filling sufficient to achieve adequate stroke volume and maintain low atrial and pulmonary pressure. 
     Carefully controlled AV delays have been found to be beneficial to increase cardiac output of hearts of certain patients that exhibit cardiomyopathy and forms of CHF, and in particular Hypertrophic Obstructive Cardiomyopathy (HOCM). HOCM is characterized by a narrowed left ventricular outflow tract (LVOT), which causes a significant increase in the left ventricular end systolic pressure. The narrowed LVOT is caused by an increased thickness of the interventricular septum which obstructs blood flow during systole, the time of cardiac ejection. 
     Symptomatic improvement of patients with HOCM can be obtained in some cases with the use of standard pharmacotherapy. However, drugs in use for this therapy have disadvantages which have been cited in the literature. Likewise, surgical intervention, e.g., septal myectomy or mitral valve replacement, is another optional treatment. However, such surgical treatments carry a significant operative mortality and have not been shown to alter the natural history of the disease. See, for example, “Permanent Pacing As Treatment For Hypertrophic Cardiomyopathy,” by Kenneth M. McDonald et al.,  American Journal of Cardiology , Vol. 68, pp. 108-110, July 1991. 
     The value of dual chamber cardiac pacing and treatment of patients suffering from HOCM has been recognized in the literature. Studies have indicated that patients suffering from HOCM may benefit from a specific mode of dual chamber pacing, wherein a ventricular pacing pulse is delivered in timed synchrony with the sensed or paced atrial depolarization. Pacing the right ventricular apex before spontaneous atrio-ventricular conduction activates the ventricles is understood to alter the ventricular septal activation pattern. Since the right ventricle is caused to contract first, it pulls the septum toward the right ventricle thereby reducing the LVOT obstruction. The literature uniformly acknowledges the potential advantages of synchronized AV pacing for HOCM patients, stressing the importance of achieving ventricular capture. Causing “complete ventricular capture” is important to obtain the above-described septal movement, while selecting the longest AV delay that results in complete ventricular capture is important in order to maximize the atrial contribution to ventricular filling. See, for example, commonly assigned U.S. Pat. No. 5,507,782, and the literature articles referenced therein. The delivered pacing pulse should provide “pre-excitation,” i.e., depolarization of the ventricular apex before the septum. This altered pattern of septal contraction, as well as optimal left ventricular filling, is generally recognized as being important to this mode of pacemaker treatment. 
     The literature suggests that the AV delay should be set at the longest duration that maintains ventricular capture at different exercise levels. See the above-cited McDonald article. It has been suggested that the AV delay that allows for maximal pre-excitation of the ventricle by the pacing pulse can be selected by determining the AV delay that produces the widest paced QRS complex duration, as seen on a surface electrocardiogram. See, for example, “Impact of Dual Chamber Permanent Pacing in Patients With Obstructive Hypertrophic Cardiomyopathy With Symptoms Refractory to Verapamil and beta.-Adrenergic Blocker Therapy,” by Fananapazir et al.,  Circulation , Vol. 8, No. 6, June 1992, pp. 2149-2161. 
     The prior art techniques for AV synchronous pacing of HOCM patients recognize the necessity to periodically evaluate the pacing AV delay. The patient&#39;s spontaneous atrio-ventricular conduction time generally will change with heart rate, i.e., from rest to exercise. Moreover, simultaneous drug treatment such as beta blockers may also modify A-V conduction time and require renewed evaluation of the AV delay. The importance of periodically making an accurate determination of the optimized pacing AV delay thus takes on significance. If the AV delay is adjusted to a value which is too short, in order to ensure complete ventricular capture, the atrial contribution to ventricular filling may be compromised. However, if the AV delay is adjusted to too great a value, ventricular capture is compromised, and there may be episodes of no ventricular pacing or the ventricular pace may not contribute the best possible reduction of the LVOT obstruction. Accordingly, it is important in this therapy to be able to continuously or periodically adjust the AV delay to optimize it for HOCM therapy. Commonly assigned U.S. Pat. Nos. 5,534,506, 5,626,620, 5,626,623, 5,716,383, and 5,749,906 disclose ways of optimizing the pacing AV delay. 
     However, AV synchronized pacing of CHF hearts exhibiting DCM (a CHF/DCM heart) do not necessarily benefit from the variable, and typically long AV delay that is determined to be optimal for HOCM patients. Frequently, CHF/DCM hearts exhibit intrinsic A-V (alternatively referred to as P-Q) conduction intervals between 180 ms-260 ms with LBBB patterns or Inter-Ventricular Conduction Delay (IVCD), and widened QRS complexes &gt;120 ms, and also exhibit A-V conduction defects, including 1° AV Block (AVB). In time the 1° AV Block can degenerate to 2° AV Block or 3° AV Block. Widened QRS Complexes (&gt;120 ms), caused either by LBBB, IVCD, or RV paced evoked response, represent a significant delay in LV electrical activation and thus a significant delay in LV mechanical activation. FIG.  7 . illustrates the intrinsic cardiac sinus rhythm of a patient&#39;s heart (at a 65 bpm heart rate, for example) with intrinsic LBBB, 1° AV block, LA to LV asynchrony, and reduced LV filling time with subsequent fusion of transmitral inflow rapid filling phase (E wave) and active filling phase (A wave). 
     Optimal AV delay timing is obtained when the onset of LV contraction occurs immediately upon completion of the LA contribution (Left Atrial Kick) in late diastole. At this moment, the LV filling (preload) is maximum, and the Frank Starling Relationship between LV stretch and LV contraction is the greatest. This will result in maximum LV stroke volume ejection, and thus maximum Cardiac Index/Cardiac Output to be realized. To realize this exact Atrial-Ventricular Sequential timing, the AV delay must be fully optimized. FIG.  8 . illustrates the cardiac rhythm of a heart having a minor LA to LV asynchrony and sub-optimal, too long, AV delay timing, partial fusion of E and A waves, and increased LV Filling (LVFT). Any delay between the completion of atrial contribution and the start of LV contraction (indicated as δ in FIG.  8 ), can lead to “Pre-Systolic” mitral regurgitation, resulting in loss of effective LV filling and thus loss of LV stroke volume and reduced cardiac output. In addition, a too long AV delay reduces the diastolic time available for proper LVFT) as observed on the diastolic Transmitral Inflow Pattern, resulting in a fusion (competitive action) of the E wave vs. the A wave of the Mitral Flow Relationship (also shown in FIG.  8 .). 
     FIG.  9 . illustrates a desirable exact LA to LV synchrony restored in a cardiac rhythm due to a short, optimized AV delay, LV contraction occurring upon completion of A wave, and maximum diastolic LVFT. A short, optimized AV delay, however, will allow maximum defusion of E and A waves, and a maximum LVFT to be realized at any given heart rate, contributing to increased cardiac output (see FIG.  9 ). Recent findings of studies of such hearts has determined that each CHF/DCM heart has an optimal short AV delay that generates the highest cardiac output and provides the most physiologic hemodynamics as measured using echocardiography. See, “Effect of pacing chamber and atrioventricular delay on acute systolic function of paced patients with congestive heart failure” by Auricchio A, Stellbrink C, et al.,  Circulation  1999, June 15;99(23):2993-3001. 
     Short AV delays in the range of 60 ms-140 ms appear to be superior to the 180 ms-240 ms AV delays that have been typically either preset or calculated using the algorithms described above for determining the AV delay for HOCM hearts. Consequently, it is recommended that the AV delays of the implanted DDD and DDDR pacemakers be set to the relatively short AV delays determined in testing the cardiac output at differing AV delays. 
     But, abruptly commencing AV pacing with such a short AV delay represents a significant change in the function of and load on the CHF/DCM heart wherein, prior to pacing, the ventricles depolarized after a longer intrinsic AV delay. FIG. 1 illustrates the abrupt change from a chronic, prolonged, intrinsic AV delay of 225 ms, for example, exhibited in a CHF/DCM heart that is well above the normal, healthy heart, intrinsic AV delay of 125 ms. After years of gradually increasing intrinsic AV delay, the patient&#39;s heart is subjected to a programmed chronic AV delay of 100 ms, for example, for a sense AV delay (SAV delay) or a pace AV delay (PAV delay) or both. 
     This means that the heart is suddenly forced to change from a situation with long AV to short AV delay. In the long AV delay situation, the filled left ventricle has more time to let blood flow back into the left atrium before contraction starts (mitral regurgitation), which on the one hand reduces the cardiac output but on the other hand may serve as a kind of ‘pressure relief valve’ to limit the LV end diastolic pressure, which is extremely elevated in these patients. In the short AV delay situation, the maximum cardiac output requirements (exact synchronized filling of LV, optimal LV filling period, and optimal preloading of LV) are met, but the pressure may become high in the LV that is not ‘used’ to that. 
     SUMMARY OF THE INVENTION 
     The present invention is therefore particularly directed to a method and apparatus for avoiding or alleviating stress of a patient&#39;s heart induced by programming a relatively short AV delay in comparison to an intrinsic, prolonged AV delay. In accordance with the present invention, a Time-Adaptive AV delay (TA-AV delay) determining algorithm is started upon implantation of a DDD or DDDR pacing system in a patient having a CHF/DCM heart. At or about the time of implantation, the AV delay is initially set at a relatively long starting or initial AV delay that may be correlated with any intrinsic AV delay that the patient&#39;s heart exhibits. Thereafter, the TA-AV delay is incrementally decreased or decrement over a post-implant Time-Adaptive (TA) period of time of hours, days or weeks until a programmed, relatively shorter, AV delay is reached. Then, the programmed, chronic, AV delay is maintained. 
     The TA-AV delay is either linearly or non-linearly decremented in time interval from the initial AV delay to the chronic AV delay in decrement steps over the post-implant TA period. Preferably, a separate SAV delay is commenced by the A-EVENT signal and a PAV delay is commenced at time-out of an atrial pace escape interval and delivery of the A-PACE pulse. The present invention may be implemented in such a way that the Time-Adaptive feature is programmed on to operate for establishing a TA-SAV delay or a TA-PAV delay or both during the post-implant period. 
     In this way, the heart gradually adapts to the optimal chronic AV delay and stress is reduced. This gradual process may aid in the remodeling process of the CHF/DCM heart. The process of remodeling is a gradual adaptation of the muscle cells of the heart to a new situation of different wall stresses, volume loading, and/or contraction patterns. Some relevant references include the following: “Asynchronous Electrical Activation Induces Asymmetrical Hypertrophy of the Left Ventricular Wall”, by Oosterhout, Prinzen, et al.,  Circulation , 1998;98:588-595; and “Redistribution of myocardial fiber strain and blood flow by asynchronous activation” by Prinzen et al.,  American Journal of Physiology , 1990;259:H300-H308. 
     Preferably, the TA-AV delay is further modified or altered during the TA period to provide a more physiologic AV delay under certain conditions where a need for increased cardiac output causes the pacing system to increase its pacing rate. In this case, the TA-AV delay is calculated but can be temporarily altered by a rate response pacing algorithm. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other advantages and features of the present invention will be more readily understood from the following detailed description of the preferred embodiments thereof, when considered in conjunction with the drawings, in which like reference numerals indicate identical structures throughout the several views, and wherein: 
     FIG. 1 is a time chart illustrating the current manner of setting an AV delay used in a DDD or DDDR pacing system for pacing a CHF/DCM heart; 
     FIG. 2A is a time chart illustrating the manner of time-adapting the AV delay used in a DDD or DDDR pacing system for pacing a CHF/DCM heart in accordance with the present invention; 
     FIG. 2B is a magnification of a portion of the TA-AV delay during the TA period to show incremental reductions in response to a rate adaptation in a rate responsive pacing mode; 
     FIG. 3 is a view of one embodiment of an implantable DDD/DDDR pacemaker implanted subcutaneously in a patient&#39;s body in which the present invention is advantageously implemented; 
     FIG. 4 is a schematic block diagram of major functional blocks of one embodiment of a DDD/DDDR pacemaker operating system that can be employed to carry out the present invention; 
     FIG. 5 is a flow chart illustrating the DDD/DDDR pacing mode operating steps in accordance with the present invention; 
     FIGS. 6A and 6B are a flow chart illustrating the calculation of TA-AV delays employed in the DDD/DDDR pacing mode operating steps of FIG.  5 . 
     FIG. 7 illustrates an intrinsic cardiac sinus rhythm of a patient&#39;s heart with intrinsic LBBB, 1° AV block, LA to LV asynchrony, and reduced LV filling time with subsequent fusion of transmitral inflow rapid filling phase (E wave) and active filling phase (A wave); 
     FIG. 8 illustrates a cardiac rhythm evidencing minor LA to LV asynchrony and sub-optimal, too long AV delay timing, partial defusion of E and A waves, and with increased LV Filling (LVFT); and 
     FIG. 9 illustrates a desirable cardiac rhythm having exact LA to LV synchrony due to a short, optimized AV delay, LV contraction occurring upon completion of A wave, and maximum diastolic LVFT. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 is a time chart illustrating the current manner of setting an AV delay used in a DDD or DDDR pacing system for pacing a CHF/DCM heart. 
     FIGS. 2A and 2B illustrate the manner of time-adapting the AV delay used in a DDD or DDDR pacing system for pacing a CHF/DCM heart in accordance with the present invention. The initial or starting AV delay, which may be the SAV delay and/or the PAV delay, is set to be the same or nearly the same length as the intrinsic AV delay, 225 ms in the illustrated case. The AV delay is gradually decreased as a TA-AV delay over a post-implant TA period of hours, days, weeks or months until the programmed, base or chronic, 100 ms, AV delay is reached. This Time-Adaptive shortening of the AV delay allows the heart to grow accustomed to the short AV delay over the TA period and may contribute to remodeling of the heart function. Either a single TA-AV delay is calculated and used during the post-implant period or separate TA-SAV and/or TA-PAV delays are calculated and used over the post-implant period until the expiration of the post-implant period. In practice if the PAV and SAV are set to the same value, then a single TA-AV delay is calculated employed and timed out after either an atrial pace or an atrial sense event. 
     In a rate responsive (RR) pacing mode, the TA-AV delays can be automatically shortened when an increased activity or heart rate is detected. When such RR modulation of PAV and SAV intervals occurs, these AV delay incremental modulations are superimposed as shown in FIG. 2B on the slow change of the basic TA-AV delay as illustrated in FIG. 2A in accordance with the TA algorithms described further below in reference to FIGS. 6A and 6B. 
     FIGS. 2A and 2B are simply exemplary of a manner in which the TA-AV (TA-PAV or TA-SAV) delay can be decreased with or without the RR modulation of the TA-AV delay. In typical DDD pacing systems, the PAV and SAV delays are separately programmable and there is typically an offset difference between the PAV and SAV delays when they are either fixed or vary in a RR mode dependent upon the RCP or intrinsic atrial heart rate. For simplicity, only a single AV delay, including the TA-AV delay are shown in FIG.  2 A. 
     The TA-AV delay decrease is effected at specific times (determined by a real time clock) during the post-implant period and typically would be accomplished by decreasing the AV interval by one-half to one or more clock cycle intervals as described further below. The Time-Adaptive curve can therefore take many forms, including a relatively straight ramp a curve, or periodic discrete step or increment decreases. 
     The present invention can be incorporated into external and implantable pacing systems, and into both pacemakers and ICDs having dual chamber pacing capability. For example, FIG. 3 depicts the configuration of a typical DDD/DDDR pacing system comprising IPG  100  and unipolar or bipolar right atrial (RA) and right ventricular (RV) leads  114  and  116  (bipolar leads are depicted), in which the present invention may be implemented to synchronously pace the atria and ventricles in a RA-RV sequence. The present invention may also be incorporated into other dual chamber pacing systems employing left atrial (LA) and/or left ventricular (LV) leads for dual chamber pacing using the TA-AV delay in the sequences LA-RV, LA-LV or RA-LV. 
     The TA-AV delay may also be employed in bi-atrial and bi-ventricular pacing systems having LA and/or r LV leads of the types described, for example, in commonly assigned U.S. Pat. No. 5,902,324 and commonly assigned U.S. patent application Ser. No. 09/067,729 filed Apr. 28, 1998 for MULTIPLE CHANNEL, SEQUENTIAL, CARDIAC PACING SYSTEMS filed in the names of C. Struble et al. The TA-AV delay algorithms of the present invention may be incorporated into such three and four chamber DDD/DDDR pacing systems wherein the pacing sequences can include RA-(RV+LV), LA-(RV+LV), (RA+LA)-RV, (RA+LA)-LV, and (RA+LA)-(RV+LV). 
     Typically, the DDDR pacing system described below in reference to FIGS. 3-6 comprises a microcomputer controlled, digital controller/timer circuit that defines and times out a V-A interval upon a V-EVENT or V-PACE pulse followed by an AV delay upon an A-EVENT or A-PACE pulse as well as a number of other intervals. Preferably, the SAV delay is commenced by the A-EVENT signal and the PAV delay is commenced upon an A-PACE pulse, and both are set in a Time-Adaptive manner. But the present invention may be implemented in such a way that the Time-Adaptive feature is only programmed on to operate for the setting of the SAV or the PAV or for a single AV delay for both. While the present invention is believed optimally practiced in a DDD or DDDR pacing mode, in some patients there may also be a benefit to practicing the invention in a VDD/DDR or DVI/DVIR pacing mode, depending upon the specific underlying heart condition of the patient. It is contemplated that these features of the invention are most likely to be implemented in either a DDD or DDDR pacemaker IPG or pacing system of an ICD IPG which may be programmed to these alternate pacing modes. 
     Additional timed intervals include atrial and ventricular sense amplifier blanking periods following delivery of an atrial and/or ventricular pacing pulse to disable atrial and ventricular amplifier sensing of the evoked response to the delivered pacing pulse. In addition, a number of sense amplifier refractory periods are timed out on atrial and ventricular sense event signals and generation of A-PACE and V-PACE pulses, such that “refractory” A-EVENT and V-EVENT signals during such refractory periods are selectively ignored or employed in a variety of ways to reset or extend time periods being timed out. Atrial and ventricular refractory periods (ARP and VRP) are commenced upon an A-EVENT or V-EVENT signal or generation of an A-PACE or V-PACE pulse, respectively. The ARP extends through the SAV delay or the PAV delay until a certain time following a V-EVENT signal terminating the SAV or PAV delay or generation of a V-PACE pulse at the expiration of the SAV or PAV delay. 
     In addition, a post-ventricular atrial refractory period (PVARP) is commenced by a V-PACE pulse or V-EVENT signal so that A-EVENT signals sensed during its time-out are presumed to reflect a retrograde conduction of the evoked or spontaneous ventricular depolarization wave and are ignored and not employed to reset an escape interval and commence an SAV delay. Retrograde conduction is a condition where the depolarization of the ventricles propagates backwards into the atria, causing the atria to depolarize, which atrial depolarization in turn propagates through the AV node into the ventricles, causing the ventricles to depolarize. If retrograde conduction originating from a PVC continues over several cardiac cycles, a tachycardia may result. 
     Thus, DDD and DDDR pacemaker systems sense and pace in the atrial and ventricular chambers, and pacing is either triggered and inhibited depending upon sensing of intrinsic, non-refractory atrial and ventricular depolarizations during the sequentially timed V-A interval and AV delay, respectively, as is well known in the art. Such DDD and DDDR pacemaker IPGs effectively function in a VDD pacing mode when the sinus atrial heart rate varies within the lower rate and the upper rate limit and such intrinsic atrial depolarizations are consistently sensed. The following description is thus intended to encompass the various types of dual chamber pacemaker systems in which the present invention can be implemented in both implantable pacemakers and in dual chamber ICDs. 
     The IPG  100  depicted in FIG. 3 is provided with a hermetically sealed enclosure  118 , typically fabricated of bio-compatible metal such as titanium, enclosing the dual chamber IPG circuit  300  depicted in FIG. 4. A connector block assembly  112  is mounted to the top of the enclosure  118  to receive electrical connectors located on the proximal connector ends of the depicted bipolar atrial and ventricular pacing leads  114  and  116 . 
     The bipolar atrial pacing lead  116  extends between its proximal connector coupled to IPG  100  and distal atrial pace/sense electrodes  120  and  122  located in the right atrium  12  of heart  10  to sense P-waves and to deliver atrial pacing pulses to the right atria. Atrial pacing pulses may be delivered between electrodes  120  and  122  in a bipolar pacing mode or between electrode  122  and the housing  118  of the IPG  100  in a unipolar pacing mode. Sensing of P-waves may occur between electrode  120  and electrode  122  in a bipolar sensing mode or between either of electrode  120  and  122  and the housing  118  of the IPG  100  in a unipolar atrial sensing mode. 
     Similarly, the bipolar ventricular pacing lead  114  extends between its proximal connector coupled to IPG  100  and distal ventricular pace/sense electrodes  128  and  130  located in the right ventricle  16  of heart  10  to both sense R-waves and to deliver ventricular pacing pulses to the ventricles. Ventricular pacing pulses may be delivered between electrodes  128  and  130  in a bipolar pacing mode or between electrode  130  and the housing  118  of the IPG  100  in a unipolar pacing mode. Sensing of R-waves may occur between electrodes  128  and  130  in a bipolar sensing mode or between either of electrode  128  and  130  and the housing  118  of the IPG  100  in a unipolar ventricular sensing mode. 
     In accordance with a preferred embodiment of the invention, the IPG  100  or one of the leads  114  or  116  includes one or more physiologic sensor that is employed to derive a physiologic RCP signal that relates to the need for cardiac output. The use of physiologic sensors to provide variation of pacing rate in response to sensed physiologic parameters, such as physical activity, oxygen saturation, blood pressure and respiration, has become commonplace. Commonly assigned U.S. Pat. Nos. 4,428,378 and 4,890,617, incorporated herein by reference in their entireties, disclose activity sensors which are employed to vary the pacing escape interval in single and dual chamber pacemaker IPGs in response to sensed physical activity. Similarly, the SAV and PAV delays are varied in response to the heart rate and/or the sensor input. As described further below, the TA-AV delay may also be altered during the TA period of FIG. 2A as a function of the RCP derived physiologic pacing rate. Such an activity sensor  316  is coupled to the inside surface of the I PG housing  118  and may take the form of a piezoelectric crystal transducer as is well known in the art. 
     The preferred embodiment of the IPG  100  preferably operates in a DDD or DDDR pacing mode, described above wherein pacing pulses are delivered to both right atrium  12  and right ventricle  16  in AV synchrony. Sensed atrial and ventricular depolarizations are both effective to inhibit delivery of the next scheduled pacing pulse in the chamber in which they are detected or in any related mode where the AV delay is employed, including the related VDD, DDI, DVI, DVIR and DDIR modes. 
     Typically, the AV delay in such DDD and DDDR pacemakers is either fixed or varies with the prevailing intrinsic atrial rate, measured as an A-A interval, or a physiologic sensor derived atrial escape interval corresponding to the sensor derived atrial pacing rate. As shown in FIG. 2A, the common AV delay or one or both of the SAV delay and the PAV delay are decremented from an initial implant time interval to a final time interval which is relatively short. 
     Preferably, during the TA period, the actual TA-AV delay is influenced in a physiologic manner by the same inputs (A-A interval and/or sensor input) as in the ‘normal’ DDDR mode during the TA time period. In other words, while the TA-AV delay is progressively shortened as shown in FIG. 2A during the TA period toward the final 100 ms AV delay, the prevailing TA-AV delay at any point during the TA period may be shortened further in response to a rate response algorithm responding to the RCP or a high intrinsic atrial heart rate. The TA-AV delay shortening algorithm derives an RA-AV (or RA-PAV and RA-SAV) delay that provides a more physiologic AV delay during exercise affecting the RCP or stress causing the intrinsic heart rate to increase. The degree of alteration of the TA-AV delays are programmable as in any DDDR device. 
     The IPG circuit  300  within IPG  100  and the bipolar atrial and ventricular leads  114  and  116  are depicted in FIG. 4 in relation to heart  10 . The IPG circuit  300  is divided generally into a microcomputer circuit  302  and a pacing input/output circuit  320 . The input/output circuit  320  includes the digital controller/timer circuit  330 , the atrial and ventricular pacing pulse output circuit  340  and the atrial and ventricular sense amplifier circuit  360 , as well as a number of other components and circuits described below. Control of timing and other functions within the input/output circuit  320  is provided by the digital controller/timer circuit  330 . Digital controller/timer circuit  330 , operating under the general control of the microcomputer circuit  302 , includes a set of timing and associated logic circuits, of which certain ones pertinent to the present invention are depicted and described further below. 
     The atrial and ventricular pacing pulse output circuit  340  and sense amplifier circuit  360  contain pulse generators and sense amplifiers corresponding to any of those presently employed in commercially marketed cardiac pacemakers for atrial and ventricular pacing and sensing. The bipolar leads  114  and  116  are illustrated schematically with their associated electrode sets  120 , 122  and  128 ,  130 , respectively, as coupled directly to the atrial and ventricular pacing pulse output circuit  340  and sense amplifier circuit  360  of pacing circuit  320 . 
     Digital controller/timer circuit  330  also controls sensitivity settings of the atrial and ventricular sense amplifiers  360  by means of sensitivity control  350  and times out an atrial blanking (A-BLANK) signal and a ventricular blanking (V-BLANK) signal. In the absence of an A-BLANK signal, atrial depolarizations or P-waves in the A-SENSE signal that are detected by the atrial sense amplifier result in an A-EVENT signal that is communicated to the digital controller/timer circuit  330 . Similarly, in the absence of a V-BLANK signal, ventricular depolarizations or R-waves in the V-SENSE signal that are detected by the ventricular sense amplifier result in a V-EVENT signal that is communicated to the digital controller/timer circuit  330 . The A-EVENT signal is characterized as a refractory A-EVENT signal if it occurs during time-out of an ARP or a PVARP or a non-refractory A-EVENT signal if it occurs after time-out of these atrial refractory periods. Similarly, a V-EVENT signal is characterized as a refractory V-EVENT signal if it occurs during time-out of a VRP or a non-refractory V-EVENT signal if it occurs after time-out of this ventricular refractory period. Refractory A-EVENT signals and V-EVENT signals are typically ignored for purposes of resetting timed out AV delays and V-A intervals, although diagnostic data may be accumulated related to their occurrences. 
     Digital controller/timer circuit  330  also interfaces with other circuits of the input output circuit  320  or other components of IPG circuit  300 . Crystal oscillator circuit  338  provides the basic timing clock and battery  318  provides power for the pacing circuit  320  and the microcomputer circuit  302 . Power-on-reset circuit  336  responds to initial connection of the circuit to the battery  318  for defining an initial operating condition and similarly, resets the operative state of the IPG circuit  300  in response to detection of a low battery condition. Reference mode circuit  326  generates stable voltage reference and currents for the analog circuits within the pacing circuit  320 . ADC (analog to digital converter) and multiplexer circuit  328  digitizes analog signals and voltage to provide real time telemetry of cardiac signals from sense amplifiers  360 , for uplink transmission via RF transmitter and receiver circuit  332 . Voltage reference and bias circuit  326 , ADC and multiplexer  328 , power-on-reset circuit  336  and crystal oscillator circuit  338  may correspond to any of those presently used in current marketed implantable cardiac pacemakers. 
     Data transmission to and from an external programmer (not shown) is accomplished by means of the telemetry antenna  334  and an associated RF transmitter and receiver  332 , which serves both to demodulate received downlink telemetry and to transmit uplink telemetry. Uplink telemetry capabilities will typically include the ability to transmit stored digital information, e.g. operating modes and parameters, EGM histograms, and other events, as well as real time EGMs of atrial and/or ventricular electrical activity and Marker Channel pulses indicating the occurrence of sensed and paced depolarizations in the atrium and ventricle, as are well known in the pacing art. The telemetry transceiver system disclosed in commonly assigned U.S. Pat. No. 5,354,319, incorporated herein by reference, may be employed to provide the uplink and downlink telemetry from and to the implanted medical device in the practice of the present invention. 
     The activity sensor  316  coupled to the implantable pulse generator housing  118  generates an output signal in response to certain patient activities, e.g. ambulating, that is processed and used as the RCP. If the IPG operating mode is programmed to a rate responsive mode, the patient&#39;s activity level developed in the patient activity circuit (PAS)  322  is monitored, and a sensor derived V-A interval is derived proportionally thereto. A timed interrupt, e.g., every two seconds, may be provided in order to allow the microprocessor  304  to analyze the output of the activity circuit PAS  322  and update the basic V-A (or A-A or V-V) escape interval employed to govern the pacing cycle and to adjust other time intervals as described below. 
     Microcomputer  302  contains a microprocessor  304  and associated system clock  308  and on-processor RAM and ROM chips  310  and  312 , respectively. In addition, microcomputer circuit  302  includes a separate RAM/ROM chip  314  to provide firmware and additional RAM memory capacity. Microprocessor  304  is interrupt driven, operating in a reduced power consumption mode normally, and awakened in response to defined interrupt events, which may include the A-TRIG, V-TRIG, A-EVENT and V-EVENT signals. 
     Microcomputer  302  controls the operational functions of digital controller/timer  324 , specifying which timing intervals are employed in a programmed pacing mode via data and control bus  306 . The specific values of the intervals timed by the digital controller/timer circuit  330  are controlled by the microcomputer circuit  302  by means of data and control bus  306  from programmed-in parameter values. The microcomputer  302  also calculates a number of intervals, including the V-A interval, the AV delay, and the ARP, PVARP and VRP, as a function of the RCP or the intrinsic atrial heart rate. During the TA-period, the TA-AV delay may also be shortened as a function of the RCP or intrinsic atrial heart rate. 
     The depicted counters and timers within digital controller/timer circuit  330  include a real time counter/timer  366  that is used to time out the TA period and maintain a the count of TA-AV delay adjustments so that the TA-AV or TA-PAV and/or the TA-SAV delay can be periodically adjusted at particular time increments and counts. Digital controller/timer circuit  330  also includes intrinsic interval timers  368  for timing average intrinsic A-A and V-V intervals from A-EVENTs and V-EVENTs, escape interval timers  370  for timing A-A, V-A, and/or V-V pacing escape intervals, and an AV delay timer  372  for timing the SAV delay from a preceding A-EVENT or the PAV delay from a preceding A-TRIG or a common AV delay. Digital controller/timer circuit  330  also includes post-event interval timers  374  for timing the post-event time intervals, including the A-BLANK, V-BLANK, ARP, PVARP and VRP intervals. Finally, digital controller/timer circuit  330  also includes and a TA-AV calculator  376  for calculating the TA-AV delay (or TA-SAV delay and/or TA-PAV delay) that is employed by the AV interval timer  372  as a function of the controlling algorithm and the real time or count provided by the real time timer/counter  366 . Digital controller/timer circuit  330  starts and times out these intervals that are calculated by microcomputer circuit  302  for controlling the above-described operations of the atrial and ventricular sense amplifiers in sense amplifier circuit  360  and the atrial and ventricular pace pulse generators in output amplifier circuit  340 . 
     In order to trigger generation of a V-PACE pulse, digital controller/timer circuit  330  generates a V-TRIG signal at the end of a PAV or SAV delay provided by AV delay timer  372 . Similarly, in order to trigger an atrial pacing or A-PACE pulse, digital controller/timer circuit  330  generates an A-TRIG signal at the termination of the V-A interval timed out by escape interval timers  370 . Typically, digital controller/timer circuit  330  also times out associated intervals including the A-BLANK interval following delivery of an A-TRIG pulse or V-TRIG pulse, during which atrial sensing is disabled, as well as the V-BLANK interval following a V-TRIG atrial pulse, during which ventricular sensing is disabled. 
     The post-event interval timers  374  time the ARP from an A-TRIG pulse or A-EVENT during which a sensed A-EVENT is ignored for the purpose of resetting the V-A interval. The ARP extends from the beginning of the SAV or PAV delay following either an A-EVENT or an A-TRIG and until a predetermined time following a V-EVENT or a V-TRIG. The post-event interval timers  374  also time the PVARP from a V-TRIG pulse or V-EVENT during which a sensed A-EVENT is also ignored for the purpose of resetting the V-A interval. The VRP is also be timed out by the post-event interval timers  374  after a V-EVENT or V-TRIG signal so that a subsequent, closely following V-EVENT is ignored for the purpose of restarting the V-A interval. 
     The base ARP, PVARP and VRP that prevails at the lower rate of 60-70 bpm, for example, are either default or programmed parameter values stored in the microcomputer  302 . These refractory period parameter values can be fixed for the full operating range of pacing rates between the lower rate and the URL, which may be 120 bpm, for example, or they can be programmed to follow the algorithm for automatically shortening in duration as the paced or intrinsic heart rate increases to ensure that the long refractory periods during the diminishing escape intervals do not cause loss of sensing of valid intrinsic P-waves and R-waves. 
     The illustrated IPG circuit  300  of FIG. 4 is merely exemplary, and corresponds to the general functional organization of most multi-programmable microprocessor controlled DDD and DDDR cardiac pacemaker IPGs presently commercially available. It is believed that the present invention is most readily practiced in the context of such an IPG, and that the present invention can therefore readily be practiced using the basic hardware of existing microprocessor controlled dual chamber pacemakers, as presently available, with the invention implemented primarily by means of modifications to the software stored in the ROM  310  of the microcomputer circuit  302 . However, the present invention may also be usefully practiced any combination of hardware and software or by means of a full custom integrated circuit, for example, a circuit taking the form of a state machine, in which a state counter serves to control an arithmetic logic unit to perform calculations according to a prescribed sequence of counter controlled steps. As such, the present invention should not be understood to be limited to a pacemaker IPG or pacing system of an ICD having an architecture as illustrated in FIG. 4, and a circuit architecture as illustrated in FIG. 4 is not believed to be a prerequisite to enjoying the benefits of the present invention. 
     FIG. 5 is a functional flow chart of the overall pacing cycle timing operation of the DDDR pacemaker IPG illustrated in FIG. 4 in the DDD or DDDR pacing modes. In the flow chart of FIG. 5, it is assumed that the A-A or V-V escape interval, cardiac cycle timing of the IPG ranges between a programmed lower rate and a programmed upper rate limit (URL) and is based on the definition of a V-A interval and an AV delay, specifically either the SAV or the PAV delay. The operations of the flow chart may also incorporate any of the mode switching and sinus preference algorithms of the prior art described above to switch between the use of the sensor or the atrial rate derived escape intervals. The steps depicted in FIG. 5 set forth the primary timing functions and actions of the IPG circuit  300  which recycle continuously. However the algorithm is specifically implemented, it is understood to incorporate the TA-AV delay setting algorithm of the present invention as described hereafter. During the post-implant TA period shown in FIG. 2A, the appropriate TA-AV delay is calculated in steps S 116  and S 122  as further described in reference to FIGS. 6A and 6B and employed in the operating algorithm depicted in FIG.  5 . 
     At step S 100 , the V-A interval being timed out in step S 112  is reset in response to a non-refractory A-EVENT in step S 118  or an A-TRIG in step S 116 , and timing of the current PAV or SAV delay is commenced. In the flow chart of FIG. 5, it is assumed that the basic timing of the pacing system is based around of the definition of an atrial escape interval (A-A escape interval or V-A escape interval) which may be fixed in the DDD mode or may vary as a function of the RCP. This A-A escape interval, the post-atrial time periods, and the current SAV delay or PAV delay are restarted at step S 100  due to an A-EVENT sensed in step S 118  or an A-PACE delivered in step S 114 , respectively, which terminate the V-A escape interval being timed out in step S 106 . During step S 102 , the system awaits either time out of the current AV delay (which may be a TA-AV delay during the TA period) or a non-refractory V-EVENT in step S 104 . The time-out of the AV delay in step S 102  is terminated if a non-refractory V-EVENT is sensed at step S 104 . A V-TRIG is generated and the V-PACE is delivered at step S 106  at the end of the AV delay if a non-refractory V-EVENT does not occur at step S 104  prior to AV time-out in step S 102 . 
     A V-A escape interval is started in step S 106  in substitution for the A-A escape interval started in step S 100  when either a V-TRIG or a V-EVENT occurs. The post-ventricular time periods, e.g., the PVARP, PVABP, VRP, VBP, URI, are started in step S 110  in response to either the time-out of the AV delay or the V-EVENT sensed in step S 104 . The V-A escape interval started in step S 106  or the A-A escape interval set in step S 100  continues to time out in step S 112 , and the atrial and ventricular sense amplifiers are enabled to detect A-SENSE and V-SENSE depolarization waves after the PVABP and VBP, respectively, time out. 
     The A-TRIG signal is generated by digital controller/timer circuit  330  to trigger delivery of an A-PACE if the A-A or V-A escape interval does time out, as determined in step S 112 , without a non-refractory A-EVENT or V-EVENT outputted by the atrial or ventricular sense amplifiers. When an A-PACE pulse is delivered in step S 114 , the next succeeding AV delay is defined to be equal to PAV at step S 116 , and the A-A escape interval and the PAV delay are restarted at step S 100  to commence the next pacing cycle. 
     The V-A escape interval commenced in step S 106  or the A-A escape interval still being timed out in step S 100  can time-out in step S 112  as described above or be terminated by a non-refractory A-EVENT or V-EVENT output by the atrial or ventricular sense amplifier, respectively, prior to the time-out as determined in step S 118  or S 120 , respectively. If an A-EVENT is provided by the atrial sense amplifier at step S 118  prior to expiration of the A-A escape interval or V-A escape interval, then the subsequent AV delay is defined to be equal to SAV at step S 122 , and the A-A escape interval and the SAV delay are restarted at step S 100 . The V-A escape interval is restarted in step S 106 , and steps S 110 -S 122  are repeated if a non-refractory V-EVENT (presumably a PVC) is sensed at step S 120  prior to expiration of the escape interval. 
     FIGS. 6A and 6B depict steps S 116  and S 122  in greater detail, particularly showing the periodic recalculation of the TA-PAV delay and the TA-SAV delay in steps S 206 -S 210  when the function is determined to be programmed ON in step S 200  and the post-implant TA period has not expired as determined in step S 202  and recalculation is otherwise enabled in step S 204 . The programmed fixed PAV delay and SAV delay (e.g., the 100 ms delay depicted in FIG. 2) of step S 202  are employed in step S 100  when these conditions are not met and a rate response function (RR) is not programmed ON. 
     When these conditions of steps S 200 -S 204  are met, the recalculation of the TA-PAV delay and the TA-SAV delay takes place either each time that the program cycles to steps S 116  and S 122  or once every N times the program cycles to steps S 116  and S 122 . Alternatively, the recalculation of the TA-PAV delay and the TA-SAV delay takes place only once an hour or number of hours or day or number of days or on any periodic schedule that may be programmed as a recalculation time. The recalculation takes place when the programmed recalculation time and/or count occurs as determined in step S 206 . The last calculated TA-PAV delay and TA-SAV delay is retained in a register in step S 210  and employed in steps S 114  and S 122 , respectively, if other conditions of steps S 216 -S 224  of FIG. 6B are satisfied, until they are recalculated. 
     A TA factor is determined and a the count of TA-AV decrement cycles is incremented in step S 208 . For example, a decrement step is determined to be appropriate in step S 206  when the recalculation time occurs and a TA factor that is other than zero is calculated in step S 208 . The TA factor can be directly related to the number of TA adjustments that have been made or the actual elapsed post-implant time. The TA factor may be a linear or nonlinear function of the difference between the initially programmed implant AV, PAV and SAV delay (225 ms in the example of FIG. 2A) and the chronic AV, PAV and SAV delay (100 ms in the example of FIG. 2A) and the number of recalculations to be made over the post-implant period or the elapsed time of the post-implant period. 
     The calculated TA factor is subtracted from the initially programmed AV, PAV and SAV delay (or the last calculated TA-AV delay) in step S 210  to derive the recalculated TA-AV delay that is retained until recalculated again. The recalculated TA-AV delay is compared to the fixed or chronic AV delay in step S 212 , and the recalculated TA-AV delay is supplied to step S 216  as long as it is not shorter than the fixed or chronic AV delay. In this way, the TA-AV delay is linearly or non-linearly decremented from the initial AV delay to the chronic AV delay in decrement steps over the post-implant TA period. 
     As set forth in step S 218 , the PAV delay is set to the recalculated TA-PAV delay in step S 116  and the SAV delay is set to the recalculated TA-SAV delay in step S 122  as long as a RR mode is not programmed on as determined in step S 216 . If a RR mode is programmed on, then a RR-AV delay decrement is calculated as a function of the RCP or intrinsic atrial heart rate in step S 220 . At most times, there is no need to elevate pacing rate to increase cardiac output, and the calculated RR-AV delay decrement is zero. The RR-AV delay decrement becomes greater than zero only when the RCP or an elevated intrinsic atrial heart rate signify the need for increased pacing rate and cardiac output. The TA-PAV and TA-SAV delays are decremented by the RR-AV delay decrement in step S 222  under such conditions requiring increased cardiac output to derive respective RA-PAV and RA-SAV delays that are used in steps S 116  and S 122 , respectively pursuant to step S 224 . When the RR-AV delay decrements exceed zero, they cause temporary decreases in the TA-PAV and TA-SAV delays as shown, for example, in FIG.  2 B. 
     A similar process takes place to determine the PAV and SAV delays employed in steps S 114  and S 122  if the conditions of steps S 200  or S 202  or S 212  are not met. Either a RR varying PAV delay and SAV delay or the programmed chronic PAV delay and SAV delay are employed as determined in steps S 226 -S 232 . 
     In the claims, means-plus-function clauses are intended to cover the structures described herein as performing the recited function and not only structural equivalents but also equivalent structures. Thus, although a nail and a screw may not be structural equivalents in that a nail employs a cylindrical surface to secure wooden parts together, whereas a screw employs a helical surface, in the environment of fastening wooden parts a nail and a screw are equivalent structures. 
     All patents listed hereinabove are hereby incorporated by reference into the specification hereof in their respective entireties. As those of ordinary skill in the art will appreciate readily upon reading the Summary of the Invention, Detailed Description of the Preferred Embodiments and Claims set forth herein, at least some of the devices and methods disclosed in those patents may be modified advantageously in accordance with the teachings of the present invention.