loss of atrial capture
76-year-old man implanted with an Accent MRITM pacemaker for complete atrioventricular block; recording of this tracing during the consultation.
The atrial EGM reveals the presence of regular intrinsic atrial activity unaffected by atrial pacing; there is a clear failure of atrial capture; P waves are sensed when they fall within the PVARP (AR) or outside of any refractory period (AS); in the latter case, atrial sensing triggers an AV delay and ventricular pacing; the atrial activities are not sensed when they fall during the AV delay (on the AP-VP cycles) or during post-ventricular atrial blanking which consists of 2 absolute atrial refractory periods.
This tracing reveals a failure of atrial capture. The atrial output amplitude was programmed in this patient at 2.5 Volts for a pulse duration of 0.5 ms. The pacing threshold had passed beyond this value (threshold at 3 Volts for 0.5 ms) thus explaining the loss of capture. The temporary increase in output amplitude to 4 Volts for 0.4 ms allowed resolving the problem. This tracing raises the question of programming the automatic adjustment of the atrial pacing amplitude (ACapTM Confirm).
There are a number of commonalities as well as differences in the functioning of atrial ACapTM Confirm and ventricular AutoCaptureTM. The algorithm uses the morphology of the analysis of the evoked atrial response between the atrial distal electrode and the pulse generator to automatically measure the atrial threshold. The output amplitude is adjusted automatically but without cycle-to-cycle verification and with a non-programmable and variable margin depending on the value of the threshold. Atrial pacing must be programmed in bipolar mode.
Different parameters are programmable: 1) ON (periodic threshold measurements, indexed data and automatic adjustment of the atrial pacing amplitude), MONITOR (periodic threshold measurements, indexed data but no amplitude adjustment), OFF; 2) Search interval: 8h or 24h; the pacing amplitude is hence adjusted for the next 8 or 24 hours.
The safety margin varies according to the threshold: if the atrial threshold is less than 1.5V, the margin is 1.0V; if the threshold is between 1.625V and 2.25V, the margin is + 1.5V; if the threshold is between 2.375V and 3.0V, the margin is 2.0V; if the threshold is greater than 3V, the amplitude delivered is 5V.