Regarding Article 'Multiple Episodes of Presyncope in a Pacemaker Dependent Patient: What is the Diagnosis?'

NCAP algorithm is designed to prevent competitive atrial pacing in Medtronic dual-chamber pacemakers and is nominally 'on'. It is activated on sensing atrial activity (AR) in the postventricular atrial refractory period. AR event does not trigger a ventricular pace. Also, NCAP algorithm delays the A pace for 300 ms (non-programmable in pacemakers) after AR even though, lower rate or sensor indicated rate may have required atrial pace to occur earlier. This prevents atrial pacing when the atrial tissue is refractory and thus may reduce atrial arrhythmias. The interval 300 ms is chosen based on the observations that an atrial stimulus delivered earlier than 300 ms may find the atrial myocardium refractory. But the present case and another report [2] suggest that the atrial myocardium especially if diseased may not recover excitability in this period. In the NCAP algorithm, the pacemaker also attempts to maintain the ventricular pacing rate at the sensor indicated rate (or the lower rate). This is done by shortening the PAV as is seen in the present example.