Exercise basics

In a CRT patient, the main objectives of a differential programming during exercise are to maintain a permanent and effective biventricular capture at high heart rates, to ensure a good contribution of atrial systole to cardiac output while maintaining atrioventricular synchronism, and to allow an appropriate acceleration of the heart rate which is the fundamental adaptive mechanism of cardiac output during exercise, particularly in heart failure patients.
The verification of maintained pacing and a permanent biventricular capture during exercise must be part of the standard assessment of a CRT patient. The recording of ventricular sensing episodes when occurring at high sinus rates should lead to suspect a loss of pacing during exercise. There are various possible causes for loss of biventricular pacing during exercise: atrial undersensing, frequent PVCs, sustained atrial or ventricular arrhythmias, shortening of PR interval during exercise below the programmed AV delay, programmed maximum tracking rate too low compared to the patient’s capability. The conducting of an stress test allows to confirm the prerequisites of the proper functioning of the device during exercise, namely: good atrial sensing in patients without chronotropic incompetence, appropriate adaptive rate response in patients with chronotropic incompetence, that the 2:1 point is not reached during exercise (shortened AV delay and Auto PVARP), adequate setting of the maximum tracking rate and/or the maximum adaptive rate response, absence of arrhythmia during exercise (supraventricular arrhythmia, many PVCs, ventricular arrhythmia, pacemaker reentrant tachycardia).
The tracing recorded in this patient shows that during an exercise sinus tachycardia, the patient’s heart rate exceeds the programmed maximum tracking rate. In a complete atrioventricular block patient, this would result in Wenckebach behavior. In this patient with preserved atrioventricular conduction, with no possible ventricular pacing beyond the maximum tracking rate, the AV delay is prolonged and spontaneous conduction reappears with loss of biventricular pacing at the peak of the exertion. An increase in the maximum tracking rate to 140 beats per minute eliminated this type of episode. This new programming was validated by the conducting of a new stress test which confirmed the persistent pacing and a biventricular capture to the maximum of the patient’s capabilities. There is no reason to curb the programmed maximum tracking rate below the patient’s acceleration capabilities during exercise. This type of episode is however very rarely symptomatic since it is not associated with a sudden drop in heart rate.

2:1 block and Wenckebach behavior during exercise

The specific settings in young CRT patients with a congenital atrioventricular block present certain peculiarities to be aware of. Indeed, in the vast majority of cases, retrograde conduction, as with anterograde conduction, is interrupted. This specificity must be integrated in the decision process regarding optimal programming, the risk of PMT being zero. A suitable adjustment must therefore enable preventing the occurrence of the 2:1 point during exercise, which is often associated with very disabling symptoms. In this patient, the 2:1 point was set too low. The total atrial refractory period, which defines the 2:1 point, corresponds to the sum of the AV delay + PVARP.  In young patients, it is essential to push this 2:1 point beyond the capacities of sinus acceleration during exercise by programming a rate-adaptive AV delay with automatic shortening during exercise and the shortest possible PVARP, the majority of these patients not requiring protection from the risk of PMT, which can be considered nil. Setting a minimum AV delay to 70 ms and a PVARP to 180 ms allows pushing the 2:1 point well beyond 200 beats/minute. 
The setting must also allow synchronization of biventricular pacing until the actual maximum tracking rate is reached by the patient. The Wenckebach behavior is most often less symptomatic than the 2:1 behavior although it must be avoided by setting a sufficiently high maximum tracking rate. In a CRT patient with a complete atrioventricular block and preserved chronotropic function, when the sinus rate accelerates and exceeds the programmed maximum tracking rate, ventricular pacing at the end of the programmed AV delay would be associated with an overshoot of the maximum tracking rate which is impossible. The ventricular rhythm can no longer follow the atrial rate in a 1:1 mode and plateaus around this value. To overcome this limitation, the device prolongs the AV delay and a Wenckebach operation of the device occurs. As the sinus frequency increases beyond the maximum tracking rate, the ventricular pacing rate remains at the maximum tracking value and the observed sensed AV delay is prolonged with each pacing cycle. When a sensed atrial event occurs during the PVARP, it is not synchronized with ventricular pacing, resulting in a missing beat. The following P wave falls outside of the refractory period and again initiates a programmed AV delay. This pattern is repeated for as long as the sinus rate remains above the programmed maximum tracking rate. Missing beats occur less frequently when the sinus rate is only slightly higher than the maximum tracking rate and more frequently when the sinus rate well exceeds the maximum tracking rate. Once the sinus rate falls below the maximum tracking rate, the 1:1 AV association is restored. The Wenckebach behavior can be defined by the rate at which the missing beat occurs and by the ratio of sensed atrial events over paced ventricular events (e.g., 8/7, 7/6, 6/5 or 3/2). Again, there is thus no reason to constrain the programmed maximum tracking rate below the patient’s rhythm acceleration capacities during exercise.