Loss of ventricular capture

ECG 7A: This tracing shows correct sensing of sinus activity followed by ventricular pacing with capture; on one beat, ineffective ventricular pacing without capture;

ECG 7B: Tracing recorded in continuity with the first tracing; increased number of ineffective ventricular pacings;

ECG 7C: The EGM confirms the absence of ventricular capture with, on one beat, a junctional escape rhythm;

ECG 7D: EGM interrogation confirms the elevation of the right ventricular threshold; this tracing is recorded after increasing the pacing amplitude and shows an effective permanent ventricular capture;

Comments: These tracings reveal an intermittent loss of ventricular capture in a patient implanted with a dual-chamber pacemaker in the setting of complete atrioventricular block and therefore dependent on his pacemaker. Intermittent loss of capture by elevation of the ventricular pacing threshold results in the occurrence of syncope with a high risk of sudden death.

The pacing threshold corresponds to the smallest electrical pulse, delivered outside of all natural refractory periods, capable of generating the propagation of a depolarization. It can be measured in voltage (volts) or in pulse width (milliseconds). The determination of the pacing threshold is of major importance since the programming of both voltage and pulse duration is a prerequisite for establishing the safety margin and determines the energy consumption of the prosthesis and hence the speed of wear of the batteries. It is generally recommended to set a safety margin of 100% which corresponds to a double threshold voltage. This margin of safety is intended to take into account the circadian variations of the pacing threshold which is variably influenced from one subject to another by sleep, taking of meals, physical activity, fever, etc.

It is now possible in all modern pacemakers to program an automatic measurement function of the ventricular threshold associated in varying degrees with i) an automatic adaptation of the pacing amplitude with cycle-to-cycle verification of the efficiency of the capture (Autocapture model allowing to deliver amplitudes close to the threshold with high-amplitude safety pacing in the event of  loss of capture) or ii) adaptation for extended durations after planned implementation of threshold controls but without cycle-to-cycle verification (Autothreshold model requiring higher margins). For the 5 largest manufacturers, the assessment of ventricular capture is based on the analysis of the evoked response (differentiation between polarization and evoked response). For the Biotronik, Boston Scientific and Abbott pacemakers, there is a cycle-to-cycle control of ventricular capture efficiency allowing to deliver amplitudes very near the measured threshold. For Medtronic and Livanova pacemakers, the threshold is measured periodically with amplitude adapted as a function of this measurement without cycle-to-cycle verification of the capture, requiring greater safety margins.

This patient underwent an increase in pacing amplitude (increase from 2.5 to 4 Volts for a threshold of 2.6 V) which helped suppress the symptoms.

Take-home message: In a patient implanted with a pacemaker, a ventricular pause may occur in 1) a specific algorithm to reduce the percentage of ventricular pacing able to tolerate the presence of ventricular pauses and blocked P waves; 2) ventricular oversensing (myopotentials, rupture potentials, interferences, etc.) inhibiting pacing (absence of pacing artifact); 3) inefficient pacing in relation to a threshold elevation (pacing artifact not followed by ventricular depolarization).