PMT DETECTING PACEMAKER

Abstract: A PMT is detected by calculating the correlations for typically 2 to 6 heartcycles between e.g. the PP-intervals (T3) and the pacemaker generated, predictably varied, AV-delays (D2). The correlation is low when no PMT is on, as a possible variation in PP (spontaneous atrial beats) is independent of the variation in AV-delay. Should a PMT be present the correlation is high, as the PP-interval then equals the PMT cycle time, viz. AV-delay plus retrograde conduction time in the heart tissue.

Description: PMT detecting pacemaker The present invention relates to a heart pacemaker, and in particular to a heart pacemaker comprising means for detecting a pacemaker-mediated tachycardia. Atrial synchronized pacing systems include an atrial (P- wave) sensing circuit, which, in connection with retrograde (ventriculoatrial, VA) heart tissue electrical conduction might cause a so called pacemaker-mediated tachycardia (PMT). A PMT results when the atrial sensing circuit detects a P-wave induced by e.g. a retrogradely conducted ventricular activity outside the sensing circuit's refrac¬ tory periods and the pacemaker subsequently initiates a paced ventricular beat. Repeated stimulation is sustained by heart tissue retrograde conduction and by pacemaker antegrade conduction. Although this complication is related to atrial synchro¬ nized systems in general, it can be exemplified with reference to dual chamber (DDD) pacemakers. With the advent of the first generation of such pacemakers with relatively short atrial refractory periods, PMT was identified as a significant problem, and later generations of such pacemakers therefore include methods for preventing PMT:s. One such known method involves the use of programmable atrial refractory periods, where the atrial refractory period is programmed to be longer than the retrograde con¬ duction interval. However, this method lowers the upper synchronized rate limit of the pacemaker and, in patients with a long retrograde conduction interval, the refractory period required is so long that the advantages with dual chamber pacing is seriously affected. Another known method is based on the fact that the majority of PMT:s are ihitiated by ventricular premature beats (VPB), viz. a ventricular event not preceded by an atrial beat. Therefore, this known method foresees that only a VPB triggers a prolonged atrial refractory period. Alternative¬ ly, a VPB could trigger a simultaneous atrial stimulation causing the atrium to be refractory when the retrograde conduction occurs. However, although this method generally allows shorter refractory periods, PMT:s remain a problem in patients where the initiating mechanism for PMT:s is un¬ known or is different from a VPB (Spontaneous Endless Loop Tachycardia by Oseran et al, PACE vol. 9, May-June 1986, pages 379 to 386). Further, it is known that a PMT, once established, can for instance be terminated by omitting ventricular beats or delivering critically timed ventricular premature beats. Here, however, PMT detection is critical, and a possible detection criterion such as a stable high frequency at, or in the vicinity of, the upper rate pacing limit cannot be reliably related to a PMT (Merits of Various Antipacemaker Circus Movement Tachycardia Features by den Dulk et al, PACE, vol. 9, Nov-Dec, Part II 1986). The object of the present invention is therefore to provide for a reliable PMT detecting device. The invention is defined in claim 1 and preferable embodi¬ ments thereof are defined in the dependent claims. In order that the invention may be more readily understood, an example of the invention will be described below with reference to the accompanying drawings. Fig. 1 is a functional block diagram of the pacemaker as connected to the heart. Fig. 2 is a functional block diagram of the pacemaker basic timing and logic unit 15. Fig. 3 is a functional block diagram of the calculating unit 16. Fig. 4 is a timing diagram illustrating various heart and pacer events as well as various time intervals relevant to the invention. According to the invention, the pacemaker investigates the presence of a PMT by measuring the correlation between an interval T 3 , for signals generated by the heart and an interval D 3 , for signals generated by the pacemaker. Refe¬ rence is now made to Fig. 4, where a non-PMT situation is illustrated to the left and a PMT situation is illustrated to the right. The lower part of the figure illustrates the atrial (A) and the ventricular (V) conduction levels and the dashed and the continuous lines indicate antegrade and retrograde conduction, respectively. The cycle time of a PMT comprises two delay intervals, one (O_ ) related to the retrograde heart tissue conduction, and the other one (D_) related to the pacemaker introduced interval or time delay between atrial and ventricular acti¬ vity (AV-interval) . D_ can be stable, vary regularly or sto- castically. However, for short time periods, D_ is limited and substantially constant. The AV-interval, D 2 , is in this invention made to vary stocastically or in accordance with a predetermined pattern. By varying the interval D 2 in a known way, the correlation between D 2 and T 3 is calculated for a short time period. Typically 2 to 6 heart cycles are needed to reach a rea¬ sonably safe decision that the value of the correlation ex- ceeds a predetermined value and, consequently, that a PMT is present. The high correlation for D 2 and T 3 when a PMT is present is explained by way of example below. Let the interval T 3 be the interval (PP) between two consecutive P-waves. If no PMT is present, the PP-interval is the interval between two spontaneous atrial beats, and the correlation between D 2 and T 3 is low as the PP-interval variation is independent of D 2 . If, in contrast, a PMT is present, the PP-interval is the sum of the retrograde conduction interval O_ and the pacemaker generated interval D 2 . Consequently, during a PMT, there is a very high correlation between D 2 and T 3 (the PP- interval) . Alternatively, instead of using the PP-interval as T 3 , quan¬ tities related thereto, for instance the atrial frequency or the interval between a ventricular stimulus and a P- wave, could be employed. In the last case, it is obvious that the expression "signals generated by the heart" , pre¬ viously used in connection with T 3 , is intended to comprise signals related to ventricular stimuli, and further, that the correlation between D 2 and T 3 is contrary to where T 3 denotes the PP-interval. In Fig. 1, the heart is designated 11 and the pacemaker is generally designated 10. The pacemaker 10 is connected to the heart through an electrode lead 112 for the atrium and an electrode lead 113 for the ventricle. The electrode leads 112, 113 are connected to signal sensors and ampli¬ fiers 12, 13 for the atrium and the ventricle, respective¬ ly. The electrode lead (or possibly a further, not indica¬ ted separate electrode lead) for the ventricle 113, is con¬ nected to a stimulating unit 14. The stimulating unit 14 delivers stimuli to the heart. A not indicated, further stimulating unit, could possibly be provided for stimulating the atrium through electrode lead 112, or, another also not indicated, separate electrode lead. The basic pacemaker timing and logic unit 15 is controlled by the sensed signals applied thereto through leads 121 and 131. The sensed signals are related to spontaneous heart activities, viz. atrial P-wave or ventricular R-wave. If the heart fails to beat normally, stimulation pulses are emitted by the pacemaker in order to maintain the normal heart function. It is also possible to deliver correctly timed stimulation pulses even if spontaneous heart beats exist. The stimulation unit 14 is connected to the basic unit 15 through lead 156. The basic logic and timing unit 15 (to be described later) is connected to the calculating unit 16 (to be described later) through data bus 161. In response to e.g. a sensed atrial signal on lead 121, the basic timing and logic unit 15 (Fig. 2) generates basic pacer escape intervals (PP-intervals) in the time base ge¬ nerator and time base register 151. A control signal on lead 152 triggers the AV-interval counter 153 simultaneous¬ ly with the triggering of the time base generator 151. After the AV-interval has been timed out, a control signal on lead 154 triggers the stimulation pulse width generator and time register 155, generating a control signal on lead 156, which controls the stimulating unit 14. A further control circuit, similar to the one just described and also starting from the time base generator 151 but delivering stimuli for the atrium through electrode lead 112 or another separate (not indicated) electrode lead for the atrium can also be provided. In order to prevent incorrect control due to false signal sensing after for instance a stimulation pulse has been delivered or a heart signal just sensed, the refractory period registers 122, 132 and their leads 123, 133 are provided in connection with atrial and ventricular signal sensing, respectively. A communication and data register unit 171 is provided for the pacemaker programming and functional control. The communication is preferably carried out by telemetry means 172. The data bus 161 provides for the internal pacemaker transmission of programmed parameter values, control signals and time register values. The calculating unit 16 (Fig. 3) comprises time registers, logic circuits and arithmetic processing circuits. Preferably, a microprocessor is employed, and the micropro¬ cessor is operated in accordance with a correlation calcu¬ lating program. Arithmetic processing, time measurement and control are then carried out from a RAM and ROM program me¬ mory 163 connected to a central processing unit (CPU) 162. Measured time intervals (D 2 , T 3 ) and calculation results are stored in read-write memory 164. Detected atrial and ven¬ tricular signals are received via interrupt registers 166, 167, respectively. Data from the microprocessor can be placed in the input-output register 168 via the internal bus 165 for subsequent communication with the basic timing and logic unit 15. It should be noted that, for the sake of explanation, the timing and logic unit 15 and the calculating unit 16 have been disclosed as separate units. It is, however, perfectly possible to arrange unit 16 to carry out the functions of timing and logic unit 15. The calculation of the correlation is carried out in ac¬ cordance with well-known mathematical theory and can be made in different ways. By way of example, the mean value and the deviation from the mean value is calculated for each interval T 3 and each interval D 2 , respectively. The deviation for each interval T 3 and for each corresponding interval D 2 are multiplied and the products added for all T 3 , D 2 intervals. The resulting sum is then divided by the sum of the absolute values of the deviation for the D 2 intervals. If the result of this division exceeds a predetermined value, a PMT is present. The PMT decision can also be referred to two correlation levels. Below the lowest level, there is no PMT and above the highest, a PMT is present. A value between the two levels indicates that a decision cannot be made with the intervals available, and therefore further intervals D 2 , T 3 should be included until a decision can be reached. Another way of calculating the correlation would be to sub¬ tract D 2 from T 3 for each corresponding D 2 , T 3 interval, cal¬ culate the mean value for the resulting differences and the deviation therefrom for each resulting difference. Corre¬ spondingly, the mean value and the deviations therefrom for the intervals D 2 are calculated. These last deviations are compared to the corresponding difference deviations, and, if substantially equal, the correlation is high. Finally, for a reliable detection of a PMT, the variation in D; should be of the same magnitude or greater than the variation in D,. However, the number of intervals are also of importance for the reliability, and should the variation in D_ be greater than that in D 2 , the number of intervals should be correspondingly increased.

Claims: Claims 1. A heart pacemaker (10) comprising at least sensing means (12) for detecting atrial events; stimulating means (14) for stimulating the ventricle; variable delay means (153) for generating a variable time delay between the de¬ tection of atrial events and the stimulation of the ven¬ tricle, c h a r a c t e r i z e d b y time measuring means (16) for measuring the time interval between consecutive signals of which at least one is generated by the heart, calculating means (16) for calculating the correlation between said time delay and said measured time interval, decision means (16) for at least deciding that a pacemaker- mediated tachycardia is present whenever the value of said correlation deviates from a first predetermined value. 2. A pacemaker according to claim 1, c h a r a c t e¬ r i z e d b y that said correlation value exceeds said first predetermined value. 3. A pacemaker according to claim 1 and 2, c h a r a c t e r i z e d b y that said decision means (16) further decides that a pacemaker-mediated tachycardia is not present when the value of said correlation falls below a second predetermined value which is lower than said first predetermined value. 4. A pacemaker according to any preceding claim c h a- r a c t e r i z e d b y that said variable delay means (153) are arranged to vary said delay in accordance with a predetermined/programmable pattern. 5. A pacemaker according to claim 4, c h a r a c t e- r i z e d b y that said correlation calculation is carried out for at least two heart cycles. 6. A pacemaker according to any preceding claim, c h a r a c t e r i z e d b y that the measured time interval is the interval between consecutive P-waves. 7. A pacemaker according to claims 5-6, c h a r a c t e¬ r i z e d b y that said correlation calculation is extended to include time delays and time measurements for further heart cycles when the value of the correlation falls between said first and second predetermined values. 8. A pacemaker according to any preceding claim c h a r a c t e r i z e d b y that the calculating means (16): calculate the mean value for the time delays and measured time intervals and the deviation from the mean value for each time delay and each mesured time interval, respectively; multiply the deviations for each time delay and each cor- responding measured time interval and add the products for all time delays and measured time intervals to a resulting sum; calculate the sum of the absolute values of said deviations of said time delays; divide said resulting sum by said sum of absolute values. 9. A pacemaker according to claim 1, c h a r a c t e¬ r i z e d b y that the measured time interval is the interval between consecutive ventricular stimulation pulses and P-waves, and that the correlation value falls below said first predetermined value. 10. A pacemaker according to claim 9, c h a r a c t e¬ r i z e d b y that said decision means (16) further decides that a pacemaker-mediated tachycardia is not present when the value of said correlation exceeds a second value which is higher than said first predetermined value.

Inventors: ELMQVIST HAAKAN, ELMQVIST, HAAKAN
Applicants: SIEMENS AG, SIEMENS ELEMA AB, SIEMENS-ELEMA AB
Application year: 1989
Main classification: A61N1
IPC codes: A61N1