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Summary o 1. A canine irreversible hemorrhagic shock preparation is described. It is produced by lowering the blood pressure to 20 mm. Hg for 40 minutes in the anesthetized, ventilated animal, followed by retransfusion of the shed blood in 15 minutes. 2. Arterio-arterial counterpulsation in animals subjected to hemorrhagic shock caused increased diastolic and decreased systolic pressures. This is known to increase coronary perfusion and decrease the pressure work of the heart. Measurements of flow and cardiac function studied showed no significant differences in the control and counterpulsated groups. 3. It would seem that improvement in coronary perfusion and decreased pressure work of the heart failed to alter the course of the experimental animal with hemorrhagic shock.

1. 1. Synchronized arterial counterpulsation is an effective method for reducing mean systolic pressure and myocardial oxygen consumption while maintaining coronary perfusion. 2. 2. Further evidence is presented to emphasize the dominant role of pressure work as the determinant of myocardial oxygen consumption in the intact beating heart. 3. 3. Coronary flow can be altered by changes in diastolic pressure, irrespective of myocardial oxygen need or metabolism. 4. 4. Arterial counterpulsation appears to be an excellent method of assisting the failing myocardium and merits further investigation and application.

Assistance to the failing heart by extracorporeal circulation techniques may benefit a number of disease states involving the right or left ventricle or both. A partial list of such conditions would include: 1. Acute myocardial infarction 2. Shock due to hemorrhage or sepsis 3. Postoperative states (postcardiotomy) 4. Preoperative states (treatment of intractable failure prior to surgical correction of cardiac disease) 5. Pulmonary disease with right ventricular failure The goal of mechanical assistance to the heart is a decrease in the external work to be furnished by the failing myocardium and maintenance of adequate pulmonary and systemic flows and pressures. The following methods of mechanical assistance have been used clinically and/or experimentally. 1. Cardiopulmonary bypass, partial or total * 2. Veno-arterial pumping without oxygenator (partial bypass) 5,9,12,30 3. Right heart bypass 4. Left heart bypass 3,4,8,21 5. Counterpulsation (postsystolic augmentation) † A limitation inherent in all of these methods is the