Pacing protocols

Hearts were stimulated at an amplitude of twice diastolic threshold voltage plus 0.5 mV. Hearts underwent two separate pacing protocols during each experiment. First, a standardized S1S2 protocol was used to determine the atrial effective refractory period (AERP) from the ECG recordings. This delivered successive trains of eight S1 stimuli separated by an interval of 125 ms, followed by a solitary extrasystolic stimulus (S2) initially delivered 89 ms after the preceding S1 stimulus. This pattern of stimulation was repeated with the S1S2 interval decremented by 1 ms for each successive cycle until failure of stimulus capture. Incremental pacing protocols then began after achieving stable microelectrode impalement. These consisted of cycles of regular pacing, each of 100 stimulations. They began with a BCL of 130 ms that was then decremented by 5 ms for each subsequent cycle. These were repeated until the heart entered into 2:1 block or sustained arrhythmia.