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Oversensing Pacemaker Beats

Figure 7.21. Crosstalk that occurs in the presence of a special crosstalk detection window triggers a ventricular output pulse rather than inhibiting it. This usually occurs at a shorter-than-programmed AV interval. Intermittent AV-interval shortening is seen in the two tracings. This reflects episodes of crosstalk triggering a ventricular output pulse. In the top tracing, for example, the second and ninth beats show AV-interval shortening.

Figure 7.21. Crosstalk that occurs in the presence of a special crosstalk detection window triggers a ventricular output pulse rather than inhibiting it. This usually occurs at a shorter-than-programmed AV interval. Intermittent AV-interval shortening is seen in the two tracings. This reflects episodes of crosstalk triggering a ventricular output pulse. In the top tracing, for example, the second and ninth beats show AV-interval shortening.

Atrial Electrogram Tracing

Figure 7.22. Surface ECG, atrial electrogram, ventricular electrogram, and event markers from a patient with a Guidant Discovery that was demonstrating inappropriate mode switching due to far-field R wave oversensing. This recording was obtained during performance of the ventricular capture threshold test as part of the pacing system evaluation. A discrete deflection is visible on the atrial lead coincident with the QRS and identified by the event markers (AS), which means that it is a sensed complex occurring within the refractory period. With loss of capture shown by the last complex on this, there is a disappearance of the second deflection on the AEGM and the marker (AS). Increasing the PVAB eliminated the far-field R wave sensing.

Figure 7.22. Surface ECG, atrial electrogram, ventricular electrogram, and event markers from a patient with a Guidant Discovery that was demonstrating inappropriate mode switching due to far-field R wave oversensing. This recording was obtained during performance of the ventricular capture threshold test as part of the pacing system evaluation. A discrete deflection is visible on the atrial lead coincident with the QRS and identified by the event markers (AS), which means that it is a sensed complex occurring within the refractory period. With loss of capture shown by the last complex on this, there is a disappearance of the second deflection on the AEGM and the marker (AS). Increasing the PVAB eliminated the far-field R wave sensing.

tricular channel. Circuits are being designed to manage this less common situation but it cannot be managed by extending the PVAB.

Pacemaker-Mediated Tachycardias: A pacemaker-mediated tachycardia (PMT) is a tachycardia that is sustained by the continued active participation of the pacemaker in the rhythm. A PMT is not the same as a pacemaker-induced tachycardia, in which the pacemaker induces a tachycardia by intentional or unintentional (undersensing) competition, but once the tachycardia has begun, the pacemaker is inhibited and no longer plays an active role in the rhythm. There are several forms of pacemaker-mediated tachycardias encountered in clinical practice. In the DDD mode, the normally functioning pacemaker is supposed to sense atrial activity and trigger a ventricular output in response to the detected P wave. The DDD pacemaker in a patient in whom atrial fibrillation or flutter develops may track the pathologic atrial signals (e.g., flutter or fibrillatory waves). This will drive the ventricular channel of the pacemaker at or near its maximum tracking rate (MTR).61

The first PMTs that became widely recognized in the literature were not due to tracking atrial fibrillation or atrial flutter or oversensing on the atrial channel. Instead, they were due to sensing retrograde atrial activity arising from a premature ventricular contraction (PVC). This set up a repetitive sequence of the sensed retrograde P wave triggering a ventricular output at the end of the maximum tracking rate interval. The delay created by waiting for the MTRI to complete before the ventricular stimulus was released allowed the atrium and AV node to physiologically recover. The depolarization resulting from a ventricular paced beat was again able to conduct in a retrograde direction.This next retrograde P wave would be sensed, triggering another ventricular output and resulting in a sustained PMT.62,63 Because this resembled the endless loop that can be seen in computers, Furman and associates labeled this rhythm an endless-loop tachycardia (ELT) to differentiate it from the other forms of PMT.64 The majority of PMTs in the literature are of the endless-loop variety, running either at or below the programmed maximum tracking rate (Fig. 7.23A).The onset of ELT requires at least transient AV dissociation to allow retrograde conduction to occur. Thus, the appearance of ELT should initiate a search for causes of AV dissociation such as atrial undersensing, atrial oversensing, loss of atrial capture, or magnet application to the pacemaker. Once initiated, an endless-loop pacemaker-mediated tachycardia will continue unless there is spontaneous VA block or loss of atrial sensing.

Algorithms to prevent ELT extend the PVARP after a sensed ventricular event that is not preceded by an atrial event (a PVC as defined by the pacemaker) to ensure that any retrograde P wave will fall in the refractory period and not be tracked. A number of adverse rhythms have been associated with automatic extension of the atrial refractory period, leading to sustained pacemaker inhibition with first-degree AV block and a PR interval significantly longer than the programmed PV interval. This situation is created by a P wave coinciding with the PVARP and hence not being sensed. However, the P wave is conducted and the sensed R wave is treated as a PVC by the pacemaker, extending the PVARP and again precluding sensing of the next P wave. Although disconcerting when encountered, it is not dangerous because the patient has an intact native rhythm. To prevent ELT by using a long PVARP yet still allow tracking to high atrial rates some devices employ a rate variable PVARP that automatically shortens at higher rates. Another option is rate responsive AV delay allowing the PVARP to be kept at a longer interval while the sensed AV delay shortens as the rate increases.

ELT may be automatically terminated by specialized algorithms (Fig. 7.24). These algorithms typically extend the PVARP or withhold a ventricular output for one cycle after tracking at the upper rate limit for a number of cycles and intermittently thereafter. If the tachycardia is due to ELT, it is terminated by the failure to track the retrograde P wave. If tachycardia is due to sinus rhythm, the tracking will continue after a single missed beat.65

Endless Loop Tachycardia

Figure 7.23. This figure shows both a pacemaker endless loop tachycardia (ELT) and a repetitive non-reentrant ventriculoatrial synchronous (RNRVAS) rhythm. These were recorded from the same patient at the time of a follow-up evaluation. Both rhythms were induced by careful programming of the pacemaker. The base rate was increased to 90ppm during the atrial capture threshold test. A: To allow an ELT to occur in this patient with known retrograde conduction, the PVARP was reduced to 125 milliseconds. Upon loss of atrial capture, there was full ventricular capture at the end of the AV delay followed by retrograde conduction. The retrograde P wave was detected by the atrial sensing circuit and triggered the next ventricular output after the sensed AV delay was extended so as to not violate the programmed maximum tracking rate. This rhythm would persist until the pacemaker was reprogrammed or an automatic ELT termination algorithm intervened (see Fig. 7.24). B: To demonstrate the RNRVAS rhythm, the AV delay was increased to 300 milliseconds and the PVARP increased to preclude detection of the retrograde P wave. While the long PVARP will prevent an ELT, each subsequent atrial output pulse, even at full output, will be delivered at a time when the atrial myocardium is physiologically refractory as schematically represented by a heavy solid line starting with the native (retrograde) P wave as shown on the AEGM. The result is functional loss of capture. The top horizontal line within the event markers identifies the duration of the atrial refractory period. As each retrograde P wave coincides with the PVARP, there is also functional undersensing. This alignment is identified by the thin double arrow.

A unique ELT has recently been described when a dual-chamber pacemaker is used to provide biventricular pacing where the CS lead is connected to the atrial port and the RV lead is connected to the ventricular port. A PVC arising in the right ventricle will be labeled an R wave. This will trigger the PVARP but if conduction to the LV is delayed such that detection occurs after

Pacing Ventricular Undersensing

Figure 7.24. Stored EGM from a Guidant Pulsar Max II demonstrating the recognition and automatic termination of an endless loop tachycardia (ELT). Top tracing: atrial electrogram (A-EGM). Middle tracing: ventricular EGM. Bottom tracing: marker channel. Guidant's algorithm extends the PVARP after a number of cycles of atrial sense-ventricle pace (AS-VP) at the maximum tracking (MT) rate. If this rhythm were an ELT, the extended PVARP will preclude responding to the retrograde P wave and prompt termination of the rhythm as occurred. AS denotes atrial event in the refractory period. If the etiology of the atrial events was sinus tachycardia or an intrinsic atrial tachycardia in the setting of high-grade AV block, the extended PVARP would result in a pause due to the lack of tracking of the "blocked" P wave followed by prompt resumption of the rapid rhythm.

Figure 7.24. Stored EGM from a Guidant Pulsar Max II demonstrating the recognition and automatic termination of an endless loop tachycardia (ELT). Top tracing: atrial electrogram (A-EGM). Middle tracing: ventricular EGM. Bottom tracing: marker channel. Guidant's algorithm extends the PVARP after a number of cycles of atrial sense-ventricle pace (AS-VP) at the maximum tracking (MT) rate. If this rhythm were an ELT, the extended PVARP will preclude responding to the retrograde P wave and prompt termination of the rhythm as occurred. AS denotes atrial event in the refractory period. If the etiology of the atrial events was sinus tachycardia or an intrinsic atrial tachycardia in the setting of high-grade AV block, the extended PVARP would result in a pause due to the lack of tracking of the "blocked" P wave followed by prompt resumption of the rapid rhythm.

the PVARP, the detected LV depolarization will be labeled a P wave and trigger an output in the RV. This is an example of an intraventricular PMT.66

Another pacemaker-mediated rhythm resulting from AV dyssynchrony is repetitive non-reentrant ventriculoatrial synchronous rhythm. This rhythm represents a mismatch between the alert and refractory periods of the pacemaker and the heart.67 It is functionally equivalent to VVI pacing with intact retrograde conduction, but it occurs in the setting of AV pacing. The trigger for this rhythm is identical to the initiating factors for an endless-loop tachycardia (i.e., AV dissociation), that allow retrograde conduction to develop. In this rhythm, the PVARP is programmed to a sufficiently long interval to preclude the retrograde P wave from being detected; hence, a PMT does not begin. If the AV paced rate is sufficiently rapid, whether this is due to a high programmed base rate or sensor-drive, the P wave occurring within the PVARP is not detected (functional undersensing) thus allowing the atrial escape interval to complete, resulting in the delivery of an atrial output pulse. At the high rate, the atrial escape interval is relatively short and may result in the delivery of an atrial output pulse at a time when the atrial myocardium is still refractory on a physiologic basis. This pattern may then be repetitive. A new algorithm called non-competitive atrial pacing has been introduced in an attempt to preclude such a rhythm by extending atrial output to at least 300 milliseconds after an atrial sensed event (Fig. 7.23B).

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