Effect of Ventricular Rate on the Cardiac Output in the Dog with Chronic Heart Block

• There is no general agreement as to the relative importance of factors controlling cardiac output.1"3 In order to study the de-terminants of the cardiac output, attempts have been made to alter independently, se-lected variables. The present investigation was designed to evaluate the effect of controlled augmentation of heart rate on the cardiac output. Utiliza-tion of dogs with chronic heart block allowed the study of changes in cardiac output over a wide range of heart rates. Methods Six medium-sized mongrel dogs with complete atrioventricular (A-V) block of 4- to 24-months duration were studied. A-V block was produced surgically by tightening a ligature around the common bundle or the right and left bundle branches high in the ventricular septum, according to a modification of the method of Taufic et al.4 Each dog was premedicated for study with 45 nig. of morphine sulfate subcutaneously, fol-lowed by intravenous injection of approximately 3 ml./Kg. body weight of a solution containing 1.6 Gm. of chloralose and 16 Gin. of urethane per 100 ml. An endotracheal tube was inserted, and constant ventilation was maintained by a Harvard pump at a respiratory rate of 12 t