Critical closing and critical opening phenomena in the coronary vasculature of the dog.

Direct measurements of coronary venous outflow in a right-heart bypass preparation were used to study the critical closing (CC) and critical opening (CO) phenomena in the anesthetized dog heart. Arterial pressure was lowered rapidly by hemorrhage, and CC pressure was the pressure at which coronary flow stopped completely. Shed blood was returned immediately or after 3 min of ischemia. The pressure at which coronary flow resumed (CO pressure) was measured. Coronary flow stopped at a mean aortic pressure of 14.3 mmHg. Vasodilator drugs and ischemic lowered CC pressure to 10.5 and 10.2 mmHg, respectively. Aortic pressure of 47.3 mmHg was needed to resume coronary flow following perfusion arrest. A brief period of ischemia did not significantly change the CO pressure. These results confirm the presence of CC phenomenon in dog heart and show that CC pressure can be altered by ischemia or vasodilators. It is suggested that the high value of CO pressure may be responsible for myocardial injury associated with ischemic cardiac arrest during open-heart surgery.