250heart. The present study examines mechanisms which are brought into play when, during a period of constant cardiac function, coronary flow rates are suddenly increased or decreased by the imposition of sudden changes in coronary perfusion pressure. A pressure-independent autoregulatory mechanism was observed. The transient characteristics of this mechanism, the pressure range over which it operates, the effects of alterations in cardiac function, and the instantaneous pressure-flow relationships at various levels of coronary vascular tone were examined and are the subject of the present report. These studies have been reported previously in preliminary form. 13 ' 14 MethodsStudies were conducted on 22 mongrel dogs weighing between 14 kg and 34 kg. After induction of anesthesia (see below), the animals were intubated and respired with intermittent positive pressure from a Harvard respiratory pump. A left thoracotomy was performed, the heart was suspended in a pericardial cradle, and the circumflex coronary artery was dissected free close to its origin. Heparin (10 mg/kg) was administered and the dose repeated every half hour during the experiment. The vessels were then cannulated and a complete exchange transfusion with mixed donor blood was performed.The extracorporeal circuit used to permit variation of coronary perfusion pressure independent of aortic pressure is shown in figure 1. The coronary artery was perfused either from the femoral artery or, by turning a stopcock, from a reservoir from which nonphasic perfusions were carried out for periods up to thirty minutes. No difference in the rate of coronary blood flow was noted when sequential perfusions from the femoral artery and from the bottle were performed at identical mean pressures. The reservoir was attached to a compressed air system; a 20-liter buffer bottle prevented decay of pressure during perfusion.The reservoir was filled between runs from one femoral artery while mixed donor venous blood was simultaneously transfused at a matched rate.

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