Spontaneous heart rate, propranolol, and ischaemia-

Abrupt coronary occlusion was performed in anaesthetised, open-chest dogs with pre-occlusion arterial blood Po, ranging from 8.0 to 10.7 kPa (60 to 80 mmHg). Ventricular fibrillation occurred in 15/19 dogs with high spontaneous heart rate (greater than 155 beats.min-') and in 0/8 dogs with low spontaneous heart rate (less than 155 beatSamin-'). Dogs with low spontaneous heart rate fibrillated when paced a t a fast rate (200-min-9 (4;s). I n dogs with high spontaneous heart rate fibrillation occurred in O j 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. I t has been suggested that there is an association between elevated heart rate and development of primary ventricular fibrillation in man (Lie et ul,, 1975). Administration of atropine with the resultant increase in sinus rate has been reported to precipi-tate ventricular fibrillation in the clinical setting ( Massumi et a/., 1972). Pacing-induced tachycardia during acute myocardial ischaemia in dogs has bee