Cardiac pacemakers in absence of the SA node: responses to exercise and autonomic blockade.

Surgical excision of the sinoatrial node, verified histologically, is followed by a supraventricular (atrial) rhythm in both resting and exercising dog whether anesthetized or conscious. The ECG is characterized by a definitive P wave and associated atrial electrograms. Waxing and waning in heart rate, generally referred to as sinus arrhythmia continues in the awake animal after complete excision of the SA node. This arrhythmia is responsive to both adrenergic and cholinergic interventions, thus indicating the presence of both sympathetic and parasympathetic regulation of subsidiary atrial pacemaking tissues.