Studies on the mechanisms of secondary pause after high rate atrial pacing

Atrial pacing up to 400 beats per minute (bpm) was performed in 20 open chest dogs. Measurements were made on the first five return cycles. Instances with the first cycle shorter than the basic cycle length were excluded. Secondary pauses (SP) were frequently observed at pacing rates equal to or faster than 280 bpm. Intravenous application of propranolol and atropine did not diminish SP, nor did bilateral vagotomy. Additional subthreshold stimuli, as well as stimulation with electrodes with large interpolar distance influenced neither the length of sinus node recovery time (SNRT) nor the SP phenomenon. Simultaneous pacing of the right atrium and right ventricle had no significant effects on SNRT and SP. Pacemaker shifts occurring frequently after high rate pacing were not necessarily accompanied by SP. The velocity of activation spread over the sinoatrial region was significantly slower (46.9±7.3 cm/s) in cases with SP than in those with primary pauses (PP) (72.5±8.1 cm/s). It was concluded that high rate induced SP was a feature indicating suppression of sinoatrial conduction.