AN ATRIAL SYNCHRONIZED PACEMAKER HAVING AN ADAPTIVE ATRIAL REFRACTORY PERIOD

Abstract: An atrial synchronized pacer having an atrial refractory period which is extended in response to the detection of retrograde activated P-waves for interrupting pacemaker meditated tachycardia. Atrial sensing amplifier 14 supplies atrial sense signals to A-V timer 20 and atrial refractory timer 30. After the A-V delay, timer 20 produces a ventricular pacing pulse, which may be delayed by upper rate limit timer 60. A V-A time measuring circuit 40 measures the times between ventricular and atrial pulses and if the times are both short and invariant, indicative of the detection of retrograde P-waves, atrial refractory control 50 operates to extend the atrial refractory period so that the pacer does not respond to the retrograde P-waves.

Description: AN ATRIAL SYNCHRONIZED PACEMAKER HAVING AN ADAPTIVE ATRIAL REFRACTORY PERIOD BACKGROUND OF THE INVENTION Field of the Invention - This invention relates generally to the field of heart pacemakers, and more specifically to improvements in atrial synchronized pacemakers. Background of the Prior Art - The body's demand for oxygenated blood is reflected by the rate at which the sinus node of the heart beats. The electrical signal generated by the sinus node causes the atria or upper chambers of the heart to contract, forcing blood into the lower chambers or ventricles of the heart. After a brief delay, the lower chambers of the heart contract forcing the blood throughout the body. Normally, the ventricles contract approximately 100-200 milliseconds after the atria contract. This delay period is generated by a structure known as the A-V node, which receives a signal from the atria and relays it to the ventricles. Pacemakers mimic the natural electrical system of the heart and provide therapeutic electrical stimulation pulses to the heart in response to cardiac conduction disturbances detected by the pacemaker. The pacers described throughout this Specification are identified by the three-letter code adopted by the Intersociety Commission for Heart Disease Resources (ICHD). In this three-letter code, the first letter indicates the chamber paced, the second letter indicates the chamber of the heart which is sensed by the pacemaker and the third letter indicates the mode of response of the pacemaker. Details of this code may be found in the American Journal of Cardiology, 34 (487, 1974). The earliest form of pacer operated in the VOO mode. Pacers of this type pace the heart asynchronously with respect to the natural activity of the heart. An example of an early asynchronous pacemaker is known from U.S. Patent No. 3,057,356 issued to W. Greatbatch. The later developed, demand or VVI mode pacemaker senses the ventricular contraction of the heart and provides stimulation to the ventricles only in the absence of a naturally occurring contraction of the ventricles. Such demand pacemakers are known from U.S. Patent No. 3,478,746 to W. Greatbatch. The bifocal or DVI mode pacemaker provides sequential stimulation of the atrium and the ventricle if naturally occurring cardiac activity is not detected in the ventricle. This form of pacemaker is known from U.S. Patent No. 3,595,242 to B. Berkovits. In each of the preceding pacemakers, electrical activity in the atrium is not sensed by the pacemaker. Consequently, these pacemakers do not utilize the information present in the atrial rate to govern the operation of the pacemaker. In contrast, the P-synchronous or VAT mode pacemaker does sense electrical activity in the atrium. This form of pacer provides a stimulus to the ventricle to ensure that the ventricles depolarize in sequence with, and in synchrony with the activity of the atrium. Such VAT mode pacemakers are known from U.S. Patent No. 3,253,596 to J.W. Keller. Another form of atrial synchronized pacemakers is the atrial synchronized, ventrical inhibited pacer or VDD mode pacer described in U.S. Patent No. 4,059,116 to J. M. Adams. More recently the ODD mode pacemaker known from U.S. Patent No. 4,312,355 to H. Funke, has been introduced which also operates in an atrial tracking mode. In the atrial tracking mode, the signal from the atrium is detected and utilized to time the qeneration of a a ventricular pacing stimulus. In this fashion the atrial tracking pacer mimics the operation of the heart's A-V node, described previously. An early form of the atrial tracking pacemaker is the VAT mode pacer known from U.S. Patent No. 3,253,596. This pacer tracks the naturally occurring atrial rate between a lower rate of typically 60 bpm to an upper rate of, for example, 120 bpm. Above the 120 bpm rate, the atrial refractory period built into the pacemaker would cause the pacemaker to pace the heart at one-half of the detected atrial rate. The blocking effect resulting from the atrial refractory period would cause the ventricular rate to suddenly drop from a relatively fast rate near 120 bpm to a relatively slow rate in the presence of an atrial rate above 120 bpm. Precipitous rate drops are uncomfortable for many patients and may ccanprchnise hemodynamic performance. This problem was addressed by the Adams pacer which provided an upper rate stabilization circuit which would cause the ventricular stimulation rate to approach an upper limit and remain there for detected atrial rates above the upper rate limit defined by the pacemaker. For example, in the early Keller pacemaker, if the atrial rate rose from a rate of 115 bpm to 125 bpm, the ventricular rate would increase from a rate of 115 bpm to 120 bpm and then drop precipitously to approximately 62 bpm as the atrial rate passed through 120 bpm. In contrast, the Adams pacemaker would track the atrial rate from 115 bpm up to 120 bpm and then stabilize at approximately 120 bpm as the atrial rate rose to 125 bpm. This discussion has addressed the upper rate characteristics of atrial tracking pacemakers when the atrial rate is determined by the sinus node of the heart. In the preceding description of the electrophysiology of the heart, the direction of conduction was assumed to be from the atrium to the ventricle. It appears, however, that in many patients there exists a retrograde conduction path from the ventricle into the atrium. This conductive path does not normally affect the operation of a pacemaker. However, it is possible for an atrial tracking pacemaker to detect this retrograde P-wave activity and synchronize upon it. This may result in the stimulation of the ventricles at a high rate and constitutes a pacemaker-mediated tachycardia which is undesirable. According to the invention there is provided a pacer for delivering stimulation pulses to the ventricle of a patient's heart in response to detected depolarizations of the atria of the patient's heart, comprising: an atrial terminal means for coupling said pacer to the atrium of the patient's heart; a ventricular terminal means for coupling said pacer to a ventricle of the patient's heart; atrial sense amplifier means coupled to said atrial terminal means for generating an atrial sense signal in response to the detection of electrical activity in the atria; A-V delay timer means coupled to said atrial sense amplifier means for generating an A-V delay period for generating a ventricular pace signal upon the expiration of said A-V delay period, said A-V delay period being initiated by an atrial sense signal; ; ventricular pulse generator means coupled to said ventricular terminal means and said A-V delay timer means for producing a stimulation pulse in response to said ventricular pace siqnal; an atrial refractory timer means arranged to prevent the initiation of said A-V delay timer until the expiration of an atrial refractory period; and detection means for detecting retrograde activated P-waves coupled to said atrial refractory timer means for selecting said atrial refractory period to prevent retro-P-waves from Initiating a ventricular stimulation pulse. The present invention is directed to a pacer which detects the existence of retrograde P-wave activity and which extends the atrial refractory period or sets the atrial refractory period of the pacemaker to prevent the synchronization upon retrograde P-activity, thus preventing the maintenance of the pacemaker-mediated tachycardia. The invention also provides a pacer for delivering stimulation pulses to the ventricle of a patient's heart in response to detected depolarizations of the atria of the patient's heart comprising: an atrial terminal means for coupling said pacer to the atrium of the patient's heart; a ventricular terminal means for cnuplinq said pacer to a ventricle of the patient's heart; atrial sense amplifier means coupled to said atrial terminal means for generating an atrial sense signal in response to the detection of electrical activity in the atria; A-V delay timer means coupled to said atrial sense amplifier means for timing an A-V delay period and for generating a ventricular pace signal upon the expiration of said A-V delay period, said A-V delay period being initiated by an atrial sense signal; ; ventricular pulse generator means coupled to said ventricular terminal means and said A-V delay timer means for producing a stimulation pulse in response to said ventricular pace signal; an upper rate limit timer means coupled to said ventricular pulse generator means for defining an upper rate limit interval for preventing the generation of a ventricular stimulating pulse durinq said upper rate limit interval, and for generating a ventricular pace signal at the expiration of said upper rate limit interval if a ventricular pace signal is generated during such upper rate limit period, said upper rate limit timer being initiated by the delivery of a ventricular stimulation pulse; ; an atrial refractory timer means responsively coupled to said A-V delay timer means for preventing the initiation of said A-V delay timer until the expiration Xf drl al r i nl refractory piario, said dtridl re fr ds tory period extending for a first predetermined tiie period after the occurrence of a ventricular stimulation pulse; a V-A measurement timer means responsively coupled to said A-V delay timer means and said upper rate limit timer for measuring V-A time intervals, said V-A time interval extending from the occurrence of a ventricular stimulation pulse to the subsequent initiation of said A-V delay timer; and atrial refractory control means coupled to said V-A measurement timer means and coupled to said atrial refractory timer means for extending said first predetermined time period when said V-A time intervals are both short and invariant. The invention also provides a pacer for delivering stimulating pulses to the ventricle and atria of a patient's heart in response to the presence or absence of the detection of electrical activity in the atria and ventricles of the patient's hearttcomprising: atrial terminal means for coupling said pacer to the atrium of the patient's heart; ventricular terminal means for coupling said pacer to a ventricle of the patient's heart; atrial sense amplifier means coupled to said dtrial terminal means for generating an atrial sense signal in response to the detection of electrical activity in the atria; ventricular sense amplifier means coupled to said ventricular terminal means for generating a ventricular sense signal in response to the detection of electrical activity in the ventricle; ; A-V delay timer means responsively coupled to said atrial sense amplifier means and said ventricular sense amplifier means for generating an A-V delay period, said A-V delay period being initiated by an atrial sense signal and said A-V delay timer being reset by said ventricular sense signal, and for generating a ventricular pace signal upon the expiration of said A-V delay period; vent-ricular pulse generator means coupled to said ventricular terminal means for generating a ventricular stimulation signal in response to a ventricular pace signal; an atrial pulse generator means coupled to said atrial terminal means for generating an atrial stimulation signal in response to an atrial pace signal; ; lower rate timer means coupled to said ventricular pulse generator means and said atrial pulse generator means and said ventricular sense amplifier means for defining lower V-V, and V-A rate escape intervals -and for producing an atrial pace signal at the expiration of said V-A time interval and for producing a ventricular pace signal at the expiration of said V-V¯ time interval, said lower rate timer being initiated by said ventricular sense signal or said ventricular pace signal; ; an atrial refractory timer means responsively coupled to said A-V delay timer means, and to said atrial sense amplifier means, and to, said ventricular pulse generator for preventing the initiation of said A-V delay timer until the expiration of an atrial refractory period, said atrial refractory period extending for a first predetetmi ned L im,' #L iod after the occurrence of a ventricular stimulation pulse; ; a ventricular refractory timer means coupled to said lower rate timer means and said ventricular sense amplifier means for defining a ventricular refractory period for preventing the initiation of said lower rate timer means during said ventricular refractory period, said ventricular refractory period being initiated on the occurrence of a ventricular stimulation pulse, and said ventricular refractory period terminating a first predetermined time after initiation; a V-A measurement timer means responsively coupled to said A-V timer and to said ventricular pulse generator for measuring the time period from the occurrence of a ventricular stimulation pulse to the subsequent initiation of said A-V delay timer; ; atrial refractory control means coupled to said V-A timer means and coupled to said atrial refractory timer means for extending said first predetermined time period when said V-A time periods are both short and invariant; and an upper rate limit timer means coupled to said ventricular pulse qenerator means for defining an upper rate limit interval for preventing the generation of a ventricular stimulating pulse during said upper rate limit interval, and for generating a ventricular pace signal at the expiration of said upper rate limit interval if a ventricular pace signal is generated during such upper rate limit period, said upper rate limit timer being initiated by the occurrence of a ventricular event. Certain embodiments of the invention will now be described by way of example and with reference to the accompanying drawings. DESCRIPTION OF THE DRAWINGS FIG. 1 represents a functional block diagram of a simple VAT mode pacer incorporating the invention; FIG, 2 is a schematic timing diagram illustrating the upper rate behavior of the pacer of FIG. 1 at high sinus rates; FIG. 3 is a schematic timing diagram illustrating the behavior of the pacer of FIG. 1 in the presence of retrograde activated atrial activity; FIG. 4 is a diagram illustrating the detection procedure of the invention; and FIG. 5 is a functional block diagram of a DDD mode pacer incorporating the invention. DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS As previously described, the present invention may be utilized in any atrial tracking pacemaker. In the subsequent description, the invention will be described within the context of a VAT or atrial synchronized pacer. First, the general operation of such a VAT mode pacemaker will be discussed and subsequently the operation of such a pacemaker at high sinus (atrial) rates will be described. Next the operation of the pacemaker in response to the retrograde conduction of P-waves will be described illustrating the operation of the invention. FIG. 1 shows a VAT mode pacemaker which includes the present invention. The pacer is coupled to the atrium through an atrial terminal 10 and a ventricular terminal 12. The atrial sense amplifier 14 (ASA) is coupled to the atrial terminal and generates an atrial sense signal (AS) in response to detected electrical activity within the atrium. This atrial sense signal is supplied through terminal 22 to an A-V delay timer 20 (AVT). The AVT 20 times out a programmed A-V delay time period and generates a ventricular pace signal (VP) at the expiration of the programmed A-V delay interval. This VP signal is supplied through terminal 24 to the ventricular pulse generator 16 (VPG). If the VPG is enabled, a ventricular pace signal supplied to the ventricular pulse generator will result in the generation of a ventricular pacing stimulus at ventricular terminal 12. These circuit elements cooperate to track the sensed atrial rate on a 1:1 basis to produce ventricular pacing stimuli in response to the detected atrial activity. There is also shown an upper rate limit timer 60 (URT) which is involved in the upper rate stabilization characteristics of this pacemaker. The upper rate limit timer times out a programmed upper rate limit time period. This timer is initiated by the generation of a ventricular stimulation pulse. During the upper rate limit time interval, an inhibit signal is supplied through terminal 64 to the ventricular pulse generator 16, which prevents the generation of venticular pacing stimuli during the upper rate limit time period. The interaction between this circuitry and the sensed atrial activity of the heart is depicted and described in connection with FIG. 2 and FIG 3. There is also shown an atrial refractory timer 30 (ART). The ART times out an atrial refractory period which prevents electrical activity present in the atrium from affecting the pacemaker's operation during the atrial refractory period. This objective may be achieved by the application of a control signal through terminal 31 of the ART which is applied to the atrial sense amplifier. This signal may disable the atrial sense amplifier or may be utilized to inhibit the generation of an atrial sense signal during the atrial refractory period. The atrial refractory period is initiated by the application of an atrial sense signal through terminal 32 of the ART. The atrial refractory period of the pacer shown normally ends 155 milliseconds after a VP signal is communicated to the ART through terminal 33. In FIG. 1 there is also shown a ventricular-atrlal timer 40 (VAM) which measures the time interval from the generation of a ventricular stimulus to the subsequent reinitiation of the A-V delay timer 20. The VAM 40 provides heat-to-beat measurements of the V-A time interval, which is supplied to an atrial refractory control circuit 50 (ARC) through terminal 54. The ARC 50 additionally receives information through terminal 42 from the URT 60 whenever a VP signal is generated by the URT. If the measured V-A time intervals are invariant within a small tolerance and the ventricular pace signals are occurring at the upper rate limit, then the ARC 50 will generate a control signal supplied to the ART 30 through terminal 38 to extend the refractory interval. As will be shown in connection with FIGs. 3 and 4, these criteria indicate that the pacemaker is participating in or mediating a tachycardia and the extension of the atrial refractory period will prevent the pacemaker from triggering on the retrograde P-wave activity. Turning to FIG. 2 there is shown a schematic timing diaqram illustrating the operation of the pacemaker of FIG. 1 at high atrial rates. In this example, the atrial activity is generated by the normal conduction of electrical activity from the atrium to the ventricle. In the figure, atrial sense events detected by the ASA 14 are shown as vertical marks under the schematic P-wave shown on the EKG trace of the diagram. The duration of the atrial refractory period set by the ART 30 is shown as a block extending from the atrial sensed event and terminating 155 milliseconds after the occurrence of the ventricular paced event. The ventricular pace signal is shown under the pacing artifact adjacent the stimulated R-wave in the EKG. The time period associated with the A-V delay timer 20 is shown by the triangular figures on the diagram. This depicts the incrementing of a counter from zero to a programmed value shown on the figure as 150 milliseconds. The time period associated with the upper rate timer URT 60 is shown by the blocks on the diagram adjacent the URT legend. The ventricular-atrial time periods measured by the pacemaker through the operation of VAM 40 are shown after the VAM legend on the diagram. In operation, the first P-wave shown in the schematic EKG trace is detected by the atrial sense amplifier 14 giving rise to an atrial sense signal. This atrial sense signal starts the atrial refractory period and the A-V delay timer. The A-V delay timer times out generating a ventricular pace signal shown in the figure 150 milliseconds after the detection of the P-wave. This VP signal is communicated to the ventricular pulse qenerator 16 and results in the first stimulated R-wave shown in the figure. The generation of this ventricular stimulus starts the upper rate limit timer which times out a programmed value of 500 milliseconds. The V-A time interval measurement timer 40 also starts at this time. Approximately 400 milliseconds after the first P-wave, another P-wave is generated by the atrium which is detected by the atrial sense amplifier generating the atrial sense signal shown in the figure. This P-wave initiates the operation of the A-V delay timer which aqain times out a 150 millisecond time interval. At the expiration of this time period the AVT generates a ventricular pace signal. However, since the generation of this signal falls within the upper rate limit time period of 500 milliseconds, the event is latched or stored within the URT 60. Therefore no ventricular pacing stimulus is generated by the ventricular pulse generator during the upper rate time period. This may be achieved by the application of an inhibit signal produced by the URT 60, and communicated to the ventricular pulse generator 16 through terminal 64. At the expiration of the upper rate limit time period of 500 milliseconds, the URT 60 generates a ventricular pace event which results in the second stimulated R-wave in the schematic EKG trace. Note that the operation of the upper rate limit timer has resulted in the extension of the A-V delay time shown on the EKG trace. In a similar fashion, the third P-wave shown in the EKG trace is detected by the atrial sense amplifier which ultimately results in the third stimulated R-wave shown in the EKG trace. The gradual prolongation of the A-V delay time observed with this pacemaker ensures that eventually an atrial event occurs during the atrial refractory period of the pacemaker and therefore will not result in a ventricular stimulation. In this fashion, an occasional atrial beat is ignored at high rates thus stabilizing the ventricular stimulation rate at the upper rate limit. In FIG. 2, the seven detected atrial P-waves have resulted in five stimulated ventricular events. This stimulation regime is called pseudo-Wenkebach pacing. It is important to note in this sequence that the measured ventricularatrial time period has progressively decreased due to the interaction of the upper rate timer with the detected P-waves. In this example, the V-A time period measurements are not invariant. In summary, the functional elements previously described cooperate to form an atrial synchronized pacemaker which tracks naturally occurrinq atrial activity up to an upper rate limit defined by the upper rate limit time period set by the URT 60. The atrial refractory timer, ventricular-atrial measurement timer, and atrial refractory control circuit cooperate to determine whether the pacemaker is participating in or mediating a tachycardia. As described in connection with FIG. 2, the pacer is not mediating the tachycardia, and the atrial refractory period is not altered. FIG. 3 shows the interaction of the pacemaker of FIG. 1 with the heart in the presence of retrograde activated P-waves. In the EKG trace, the inverted P-waves indicate retrograde activated atrial activity. These P-waves result from electrical activity conducted from the ventricle to the atrium which results in an atrial depolarization. Studies have shown that such retrograde P-waves occur on the order of 160 to 330 milliseconds after ventricular stimulation. In the diagram of FIG. 3, the retrograde P-waves are shown occurring approximately 200 milliseconds after the ventricular stimulating pulse. In the figure, the first sinus P-wave is detected by the atrial sense amplifier 14 as indicated by the atrial sense spike under the P-wave in the diagram. This atrial sensed event starts the atrial refractory timer 30 as well as the A-V delay timer 20. After the expiration of the programmed A-V time interval shown as 150 milliseconds on the diagram a ventricular pace signal is generated by the AVT 20 which results in the application of a ventricular stimulating pulse to the heart. This pacing pulse is shown as a pacer artifact on the EKG line of the diagram and as a mark on the VP line of the diagram. The generation of this ventricular pace signal by the AVT 20 initiates both the upper rate timer 60 and the V-A measurement timer 40 as shown on the diagram. The atrial refractory period ends 155 milliseconds after the ventricular stimulus. Shortly thereafter, the retrograde activated P-wave occurs shown by the inverted P-wave labeled RP on the EKG. This retro-P-wave is detected by the sense amplifier as indicated by the slash on the atrial sense line of the diagram and once again initiates the atrial refractory timer 30 as well as the A-V delay timer 20. Once again, the A-V delay timer times out a programmed time interval and generates a ventricular pace signal at the end of the time interval. However, since the upper rate limit timer has not yet timed out, this ventricular pace event is stored within the URT 60 and does not result in the generation of a ventricular pacing event. At the expiration of the programmable upper rate limit time period the URT 60 generates a ventricular pace signal which does generate a ventricular stimulation as shown by the second R-wave complex on the EKG line of FIG. 3. As shown in the diagram, approximately 200 milliseconds after this ventricular stimulation event occurs, there is a retrograde activated P-wave which once again is sensed by the pacemaker reinitiating the process. This sensed P-wave also results in a ventricular stimulating pulse at the expiration of the upper rate limit time interval. In this fashion, the first four retrograde conducted P-waves shown on the EKG trace of FIG. 3 result in ventricular stimulation at the upper rate limit of the pacemaker. Note, however, that unlike FIG. 2, the V-A time interval measurement generated by VAM 40 remains constant and varies only with the actual retrograde conduction time of the heart. It is the relative uniformity of the V-A time interval measurements which distinguish retrograde P-wave activity from the sinus tachycardia shown in FIG. 2. The preferred detection procedure for detecting the retrograde conduction includes the measurement of at least four sequential V-A time interval measurements and forming a running average based upon these measurements. Each of the four measurements is then compared with the running average to determine the variance in the data. If the successive time interval measurements are equal within approximately 15% of the average value, then it is likely that the pacemaker is participating in a tachycardia. In operation, logic within the atrial refractory control circuit 50 will perform these calculations and generate a control signal delivered to the atrial refractory timer 30 through terminal 38 to extend the atrial refractory period by an amount which is sufficient to prevent synchronization on the next occurring retrograde activated P-wave. This feature is shown in FIG. 3 by the 50 millisecond extension of the atrial refractory period shown in the figure. This retrograde P-wave may be detected by the sense amplifier as before, however, since the detection falls within the atrial refractory period, the A-V delay timer is not initiated and there is no ventricular stimulation provided to the heart in response to the retrograde activated P-wave. In one embodiment of the invention, the atrial refractory time interval may be extended incrementally until it reaches a maximum value or until the pacemaker mediated tachycardia is interrupted. Note that in the cycle in which the retro-P-wave is blocked, the V-A time interval measurement extends appreciably; therefore, the pacer will not make any further extensions to the atrial refractory period. Since it is desirable to have as short an atrial refractory period as possible to permit tracking of the atrial rate to high rates, it would be desirable to make the incremental extensions to the atrial refractory period relatively small, on the order of 10 milliseconds. However, since at least four cardiac cycles are required to determine whether a pacemaker mediated tachycardia is occurring, the time period reauired to extend the atrial refractory period may be quite long and therefore pose a risk to the patient. In a second embodiment, the V-A time interval average value calculated by the pacemaker, increased by a fixed percentage or a fixed amount, on the order of 15X or 30 milliseconds, may be used to provide an abrupt increase in the atrial refractory period thus interrupting the tachycardia promptly after it is first detected. In either embodiment, the atrial refractory period extending from the application of a ventricular pacing event to the heart is incremented to or set to a value which prevents the retrograde conducted P-wave from initiating an A-V delay time interval. An additional modification to the procedure may be required to distinguish between retrograde P-waves and a sinus tachycardia at a rate just above the rate limit of the pacemaker. Returninq to FIG. 2, note that the V-A time interval measurement decreased as the pacemaker interacted with the heart at the atrial rate. This decrease will be monotonic for a sinus tachycardia. However, the rate at which the V-A time interval measurements decrease may be quite slow if the atrial rate is just above the rate set by the upper rate limit timer. As a consequence, four or more beats may be invariant within 15% of their ensemble average and thus be indistinguishable from retrograde P-wave activity under the procedure presented. It is anticipated, however, that the sequence of V-A time interval measurements associated with the retrograde P-wave activity will not decrease monotonically, but on the contrary will be distributed at random or in a monotonically increasing order. This aspect may be incorporated in the detection procedure as shown in connection with FIG. 4, wherein two hypothetical strings of V-A time interval measurements are compared. In FIG. 4a the time interval measurements shown as M1-M4 result in a ensemble average of 233.75 milliseconds. Each of these values lies within a maximum and a minimum based upon a 15X variation around the average value. However, the measurements occur in a scattered order. As a result, this group of V-A measurements would result in the extension of the atrial refractory period. In contrast, the same time interval measurements occurring in the sequence shown in FIG. 4b will not result in the extension of the atrial refractory period. In this instance, the time interval measurements form a monotonically decreasing function which is more likely to be the result of a sinus tachycardia at the programmed upper rate limit than retrograde P-wave activity. It appears that the pathways associated with retrograde activation of atrial activity fatigue with the tachycardia and the conduction time becomes longer, therefore, the detection procedure will detect scattered data or monotonically increasing data and interpret this as originating from retrograde P-wave activity. Although the present invention has been described within the context of d VAT pacemaker for simplicity, It should be clear that the same technique can be applied to a dual pace, dual sense pacemaker as shown in FIG. 5. The pacer of FIG. 5 can operate in a variety of modes including the VAT mode described in connection with FIG. 1. This pacer may also operate in the VDD or ASVIP mode as well as DDD modes. In the VDD mode the pacer tracks the spontaneous atrial rate detected by ASA 14 from a lower rate to an upper rate. The lower rate is set by the low rate timer 70 and the upper rate is set by the upper rate limit timer 60. The lower rate timer 70 is started by a ventricular event i.e. VS signal or VP signal applied to terminal 76 or 74 respectively, and times out a programmable escape interval. At the expiration of this interval the LRT 70 generates a ventricular stimulus. In this fashion the LRT 70 defines the V-V interval of the pacer in the VDD mode. The upper rate timer 60 is also started by a ventricular event applied to URT 60 through terminals 66 or 62. The URT timesCuta programmable time period called the upper rate limit interval. During this time period the URTt60 applies an inhibit signal through terminal 64 to the ventricular pulse generator 16 to prevent the generation of a ventricular stimulus during the upper rate time interval. If a VP signal is generated during the upper rate limit interval a latch within the URT 60 stores this information and the URT 60 will generate a VP signal at the expiration of the upper rate limit interval. This feature results In the pseudo-Wenkebach behavior described more fully in connection with FIG. 2. In the VDD mode the ventricular sense amplifier 17 is utilized to detect electrical activity within the ventricle. If a spontaneous atrial beat is followed quickly by a normally conducted ventricular beat, then the VSA 17 will detect this event and generate a VS signal which will reset the LRT 70 as well as the AVT 20. This VS signal will also start the ventricular refractory timer 80. The ventricular refractory timer times out a refractory interval which renders the pacer insensitive to any electrical signals in the ventricle. During this interval the VRT 80 generates an inhibit signal applied to the VSA 17 through terminal 86, which prevents the generation of a VS signal during the refractory period. In the VDD mode the retrograde conduction detection circuitry shown in FIG. 5 operates as described with reference to FIG. 3. When the pacemaker of FIG. 5 is operating in the DDD mode, both the atrial and ventricular pulse generators 15, 16 and the atrial and ventricular sense amplifiers, 14,17 are activated. The sense amplifiers permit the pacemaker to monitor the naturally occurring activity of the heart and to conform the stimulation regime of the pacemaker to the requirements of the patient. If there is no spontaneous electrical activity in either the atrium or the ventricle, the pacemaker will sequentially pace the atrium and ventricle at a rate determined by the LRT 70. In operation, the LRT V-V time interval is initiated by a ventricular event. If no additional activity is sensed by the pacemaker the LRT will generate an AP signal delivered through terminal 72 to the APG 15 which results in an atrial stimulus. After a short A-V delay period, the LRT will then issue a VP signal through terminal 74 which will generate a ventricular stimulus throuqh the ventricular pulse generator 16. This sequential A-V pacing will occur at the programmed rate associated with the lower rate timer. If the heart is generating a spontaneous atrial rhythm which ranges between the rate set by the LRT 70 and the upper rate set by the URT 60 then the pacemaker will respond in the atrial synchronous ventricular inhibited or VDD mode as previously described. It is clear from the foregoing description that numerous modifications may be made, without departing from the scope of the invention.

Claims: CLAIMS 1. A pacer for delivering stimulation pulses to the ventricle of a patient's heart in response to detected depolarizations of the atria of the patient's heart, comprising: an atrial terminal means for coupling said pacer to the atrium of the patient's heart; a ventricular terminal means for coupling said pacer to a ventricle of the patient's heart; atrial sense amplifier means coupled to said atrial terminal means for generating an atrial sense signal in response to the detection of electrical activity in the atria; A-V delay timer means coupled to said atrial sense amplifier means for generating an A-V delay period for generating a ventricular pace signal upon the expiration of said A-V delay period, said A-V delay period being initiated by an atrial sense signal; ; ventricular pulse generator means coupled to said ventricular terminal means and said A-V delay timer means for producing a stimulation pulse in response to said ventricular pace siqnal; an atrial refractory timer means arranged to prevent the initiation of said A-V delay timer until the expiration of an atrial refractory period; and detection means for detecting retrograde activated P-waves coupled to said atrial refractory timer means for selecting said atrial refractory period to prevent retro-P-waves from initiating a ventricular stimulation pulse. 2. The pacer of claim 1 wherein said pacer further comprises: a lower rate limit timer means coupled to said ventricular pulse generator means for defining a lower rate escape interval and for producing a ventricular pace signal upon the expiration of said lower rate escape interval, said lower rate escape timer means being initiated by the occurrence of a ventricular stimulating pulse. 3. A pacer as claimed in claim 1 or 2 wherein the detection means is arranged to extend the atrial refractory period beyond the time after a ventricular stimulation pulse at which a retrograde P-wave occurs. 4. A pacer as claimed in claim 1, 2 or 3 wherein said detection means comprises a V-A time measuring means arranged to measure the time between the occurrence of a ventricular stimulating pulse and the detection of electrical activity in the atria. 5. A pacer as claimed in claim 4 wherein the detection means is arranged to determine the running average of a plurality of V-A time measurements and to detect retrograde P-waves on the basis of the relation of individual time measurements to the running average. 6. A pacer as claimed in claim 5 wherein the detection means is arranged to determine that retrograde P-waves are present only if successive V-A times are not monotonically decreasing. 7. A pacer for delivering stimulation pulses to the ventricle of a patient's heart in response to detected depolarizations of the atria of the patient's heart, comprising: an atrial terminal means for coupling said pacer to the atrium of the patient's heart; a ventricular terminal means for coupling said pacer to a ventricle of the patient's heart; atrial sense amplifier means coupled to said atrial terminal means for generating an atrial sense signal in response to the detection of electrical activity in the atria; A-V delay timer means coupled to said atrial sense amplifier means for timing an A-V delay period and for generating a ventricular pace signal upon the expiration of said A-V delay period, said A-V delay period being initiated by an atrial sense signal; ; ventricular pulse generator means coupled to said ventricular terminal means and said A-V delay timer means for producing a stimulation pulse in response to said ventricular pace signal; an upper rate limit timer means coupled to said ventricular pulse generator means for defining an upper rate limit interval for preventing the generation of a ventricular stimulating pulse durinq said upper rate limit interval, and for generating a ventricular pace signal at the expiration of said upper rate limit interval if a ventricular pace signal is generated during such upper rate limit period, said upper rate limit timer beinq initiated by the delivery of a ventricular stimulation pulse; ; an atrial refractory timer means responsively coupled to said A-V delay timer means for preventing the initiation of said A-V delay timer until the expiration of an atrial refractory period, said atrial refractory period extending for a first predetermined time period after the occurrence of a ventricular stimulation pulse; a V-A measurement timer means responsively coupled to said A-V delay timer means and said upper rate limit timer for measuring V-A time intervals, said V-A time interval extending from the occurrence of a ventricular stimulation pulse to the subsequent initiation of said A-V delay timer; and atrial refractory control means coupled to said V-A measurement timer means and coupled to said atrial refractory timer means for extending said first predetermined time period when said V-A time intervals are both short and invariant. 8. The pacer of claim 7 further comprising: a lower rate limit timer means coupled to said ventricular pulse generator means for defining a lower rate V-V escape interval and for producing a ventricular pace signal upon the expiration of said lower rate escape interval, said lower rate escape timer means being initiated by the occurrence of a ventricular stimulating pulse. 9. A pacer for delivering stimulating pulses to the ventricle and atria of a patient's heart in response to the presence or absence of the detection of electrical activity in the atria and ventricles of the patient's heart, comprising: atrial terminal means for coupling said pacer to the atrium of the patient's heart; ventricular terminal means for coupling said pacer to a ventricle of the patient's heart; atrial sense amplifier means coupled to said atrial terminal means for generating an atrial sense signal in response to the detection of electrical activity in the atria; ventricular sense amplifier means coupled to said ventricular terminal means for generating a ventricular sense signal in response to the detection of electrical activity in the ventricle; ; A-V delay timer means responsively coupled to said atrial sense amplifier means and said ventricular sense amplifier means for generating an A-V delay period, said A-V delay period being initiated by an atrial sense signal and said A-V delay timer being reset by said ventricular sense signal, and for generating a ventricular pace signal upon the expiration of said A-V delay period; ventricular pulse generator means coupled to said ventricular terminal means for generating a ventricular stimulation signal in response to a ventricular pace signal; an atrial pulse generator means coupled to said atrial terminal means for generating an atriat stimulation signal in response to an atrial pace signal; ; lower rate timer means coupled to said ventricular pulse generator means and said atrial pulse generator means and said ventricular sense amplifier means for defining lower V-V, and V-A rate escape intervals and for producing an atrial pace signal at the expiration of said V-A time interval and for producing a ventricular pace signal at the expiration of said V-V time interval, said lower rate timer being initiated by said ventricular sense signal or said ventricular pace signal; ; an atrial refractory timer means responsively coupled to said A-V delay timer means, and to said atrial sense amplifier means, and to said ventricular pulse generator for preventing the initiation of said A-V delay timer until the expiration of an atrial refractory period, said atrial refractory period extending for a first predetermined time period after the occurrence of a ventricular stimulation pulse; ; a ventricular refractory timer means coupled to said lower rate timer means and said ventricular sense amplifier means for defining a ventricular refractory period for preventing the initiation of said lower rate timer means during said ventricular refractory period, said ventricular refractory period being initiated on the occurrence of a ventricular stimulation pulse, and said ventricular refractory period terminatina a first predetermined time after initiation; ; a V-A measurement timer means responsively coupled to said A-V tiller and to said ventricular pulse generator for measuring the time period from the occurrence of a ventricular stimulation pulse to the subsequent initiation of said A-V delay timer; dtrial refractory control means coupled to said V-A timer means and coupled to said atrial refractory timer means for extending said first predetermined time period when said \'-A time periods are both short and invariant; and an upper rate it timer leans coupled to said ventricular pulse generator means for defining an upper rate limit interval for preventinq llle generation of a ventricular sti: :nulating pulse during said upper rate limit interval, and for generating d ventricular pace signal at the expiration of said upper rate limit interval if a ventricular pace signal is generated durinq such upoer rate limit period, Said tipper rate limit timer being initiated #y the occurrence of a ventricular event. 10. A pacer as claimed in any of claims 7 to 9 wherein: said atrial refractory control means utilizes the occurrence of two or more ventricular stimulating signals at said upper rate limit to ascertain that the V-A time period measurment s meet the shortness criteria; said atrial refractory control means averages two or more successive V-A time period measurements to generate an average V-A me period me#surment; 5 did atrial refractory control means compares each of said V-A ttime period measurements with s a id average to generate successive difference measurements; ; and said atrial refractory control means extends said atrial refractory period if said differences are less than a p@eestablished difference nd said success i ve ventricular pace signals are at said upper rate limit.

Inventors: CITRON, PAUL, DUFFIN, EDWING G.
Applicants: MEDTRONIC, INC.
Application year: 1984
Main classification: A61N1
IPC codes: A61N1