Spontaneous heart rate, propranolol, and ischaemia-induced ventricular fibrillation in the dog

Abrupt coronary occlusion was performed in anaesthetised, open-chest dogs with pre-occlusion arterial blood PO2 ranging from 8.0 to 10.7 kPa (60 to mmHg). Ventricular fibrillation occurred in 15/19 dogs with high spontaneous heart rate (less than 155 beats.min-1) and in 0/8 dogs with low spontaneous heart rate (less than 155 beats.min-1). Dogs with low spontaneous heart rate fibrillated when paced at a fast rate (200.min-1) (4/5). In dogs with high spontaneous heart rate fibrillation occurred in 0/5 dogs when propranolol reduced spontaneous heart rate, but in 3/5 dogs when reduction in spontaneous heart rate in response to propranolol was prevented by pacing.