Cardiovascular adaptation to partial heart-lung bypass.

P REVIOUS experiments indicated that during partial heart-lung bypass (also called partial perfusion), the cardiac output decreased as the extracorporeal flow increased. 1 In those experiments, the arterial tree was perfused from the heart-lung machine , using gravity instead of a pump. With greater amounts of blood passing through the extracorporeal circuit, there seemed to be an increase of the blood content stored in the heart-lung machine. It was suspected that an increase in extracorporeal flow might be accompanied by a shift of some of the blood volume of the organism into the extracorporeal circuit. In other experiments, 2 prolonged diminution of cardiac filling, by partial heart-lung bypass, did not decrease substantially the energy expenditure of the heart, as long as the aortic pressure and the body blood content were kept constant. Therefore, the thought arose that some of the benefit expected from partial heart-lung bypass might result from a decreased body blood volume. This hypothesis was quantitatively investigated under strictly controlled conditions, in Avhich the relationships between the cardiac output, the extracorporeal blood flow, and the blood volume contained in the extracorporeal circuit could be examined. Methods Tun open-chest dogs, ranging from 20 to 30 Kg. in weight, anesthetized with 25 mg./Kg. of pento-barbital and heparinized with 3 mg./Kg., were used. The experiments were designed in such a manner that cardiac output and blood volume changes could be recorded directly in conjunction with phasic changes in blood pressure (fig. 1). The cardiac output was directly recorded with a Shipley-Wilson rotameter(R), inserted into the tho-racic aorta during a temporary aortic bypass. All the thoracic branches of the aorta, distal to the coronary arteries, were tied. Under these conditions, all blood ejected by the left ventricle, except the coronary flow, had to pass through the rotameter before being distributed to the periphery. 3 The brain was perfused from the descending tube of the rotameter through a carotid artery. This arrangement provided a fairly constant head of pressure, since brain perfusion was secured by the extracorporeal flow from a constant level reservoir, regardless of the left ventricular output. At high extracorporeal flow, when cardiac output decreased to such an extent that the pressure in the aortic arch dropped below the pressure in the abdominal aorta, the rotameter became locked, and no cardiac output was recorded. In such instances, all blood ejected by the left ventricle passed through the coronary arteries. The extracorporeal …