The noise sampling period: a new cause of apparent sensing malfunction of demand pacemakers.

Two patients with Omni-Stanicor pulse generators presented an apparent sensing problem characterized by intermittent reversion to fixed-rate pacing only during atrial fibrillation with a very rapid ventricular rate. Every fixed-rate cycle contained two unsensed beats. The first unsensed beat fell in the noise sampling period (the last 1/6 of the pacemaker refractory period) and, therefore, disabled the demand function of the pulse generator for a single timing cycle. The presence of two consecutively unsensed beats within one timing cycle (automatic or escape interval) during tachycardia suggests normal function of the noise sampling period of this particular pulse generator, rather than a true sensing problem. The diagnosis becomes evident if the sensing problem disappears when abbreviation of the refractory period occurs by reprogramming the pulse generator at a higher rate.