Triggered rhythms in atrial muscle

Intracellular and extracellular recording techniques were used to study triggered activity in tissues isolated from the inferior right atrium of the dog. In the presence of norepinephrine (greater than 10(-7) M) single stimuli or short rapid pacing elicited action potentials with delayed after-depolarizations, leading to single or repetitive non-driven beats. Most commonly, during sustained triggered activity action potential and electrogram configurations remained constant. In addition, cycle length changed gradually, initially decreasing and then increasing before activity stopped. However in some preparations, once sustained triggered activity was initiated there were abrupt spontaneous changes in action potential and electrogram configurations that were coincident with abrupt spontaneous changes in cycle length. When quiescent, short rapid pacing initiated triggered activity with a relatively short cycle length that abruptly shifted to a longer cycle length before stopping. During sustained triggered activity, short rapid pacing caused an abrupt decrease in cycle length that was coincident with changes in action potential and electrogram configurations. The present results indicate that abrupt changes in cycle length can occur during bouts of triggered activity resulting in unique patterns of arrhythmias. These rhythms may be due to interactions and shifts between multiple sites of triggered pacemaker activity.