Encircling endocardial incision interrupts efferent vagal-induced prolongation of endocardial and epicardial refractoriness in the dog.

The purpose of this study was to test the hypothesis that vagal efferent nerves travel in the left ventricular subendocardium in a base to apex direction. Efferent vagal stimulation during constant background isoproterenol infusion prolonged left ventricular endocardial and epicardial effective refractory periods in a control state and after a left ventriculotomy performed while dogs were supported by cardiopulmonary bypass. After a 2 mm deep endocardial circumferential incision, efferent vagal stimulation still prolonged the effective refractory period at an endocardial site basal to the encircling endocardial incision, but no longer prolonged the effective refractory period at the endocardium or immediately overlying epicardium apical to the incision. Interpretation of these data suggests that efferent vagal fibers travel in the superficial subendocardium of the canine left ventricle in a base to apex direction, penetrating upward to innervate the epicardium. Conceivably, a lesion such as a subendocardial myocardial infarction could selectively interrupt efferent vagal innervation, leaving sympathetic innervation unopposed. This may be a source of some arrhythmias.