It is well known that general body hypothermia results in a decrease in heart rate and an associated increase in ventricular contractile force. Previous studies of this temperature effect on myocardial activity have been restricted to isolated heart preparations or intact animals in which the entire body was cooled.1-4 The present studies were designed to investigate the site of action of hypothermia on myocardial contraction in intact animals utilizing separate cardiopulmonary and systemic perfusion systems.
Materials and Methods
Adult fasting mongrel dogs weighing 16 to 22 kg were used. After induction of anesthesia with intravenous thiopental (20 mg/kg) a cuffed endotracheal tube was inserted and ventilation was maintained by a positive pressure respirator supplying room air. A bilateral anterior thoracotomy was performed through the fourth intercostal spaces and the sternum transected. Heparin was administered intravenously.Cardiopulmonary bypass was then effected by draining venous blood from the venae cavae to