Impaired activity rate responsiveness of an atrial activity-triggered pacemaker: the role of differential atrial sensing in its prevention.

The physiological benefit of rate responsive, single-chamber cardiac pacing is well documented. We studied the activity response of nine atrially placed Activitrax II pacemakers. Seven patients were noted to have an inadequate activity-rate response with maximal pacing rates of 85 to 101 beats/min. Marker Channel analysis revealed that the upper rate timeout was reset by far-field R wave sensing, even when sensing occurred in the atrial refractory period. These 9 pacemakers were tested by atrial sensitivity adjustment for ability to exclude far-field R wave sensing, while preserving P wave sensing. Unipolar implantation data were then examined for predictors of this differential far-R and P-wave sensing. Differential atrial sensing occurred in 4/9 pacemakers (2/2 bipolar in the right atrial appendage; 0/1 bipolar in the coronary sinus; and 4/9 unipolar). An empirically developed index utilizing unipolar implant parameters discriminated outcomes for 8/9 unipolar pacemakers. We conclude that: (1) the rate responsiveness of the atrial Activitrax II pacemaker is limited by far-field R wave sensing even when this occurs during atrial channel refractoriness; (2) reprogramming atrial sensitivity to differentially sensed P and far-field R waves may restore appropriate rate responsiveness; and (3) although a unipolar implant discriminant index may correctly identify adequacy of future rate responsiveness, the atrial application of the Activitrax II pacemaker is cautioned until further validation is forthcoming, particularly when used in unipolar and coronary sinus applications.