The present status of pump-oxygenators and some problems related to their use.

Summary During the past decade the pump-oxygenator has been employed extensively for the support of circulation and respiration during open heart operations. It has proven to be a reliable substitute for the heart and lung within certain limits. There are three pump-oxygenator systems in general use today, namely, the Gibbon-Mayo vertical screen unit with roller pumps, the rotating disc oxygenator with roller pumps, the DeWall bubble oxygenator with Sigmamotor pumps. In each of these units oxygenation is accomplished by the creation of a blood-gas interface and the pumps are internally valved producing unidirectional, essentially non-pulsatile flow. Each system is capable of maintaining normal tensions of oxygen and carbon dioxide in arterial blood returning to the patient at flows approaching basal cardiac output. Trauma to the blood is minimal except for prolonged perfusion. The parameters measured during cardiopulmonary bypass are usually arterial and central venous blood pressure, the electroencephalogram and electrocardiogram. Arterial blood pH and total blood volume are frequently measured at the termination of perfusion. Procurement of blood for priming extracorporeal circuits has been a major problem. A number of studies have been made to determine the feasibility of using preserved blood for this purpose. Among these are the use of edglugate magnesium which permits the utilization of blood up to five days after procurement, glycerolized frozen blood and modification of blood preserved in ACD solution. Although all of these methods have promise, it would appear at the present time that the use of freshly drawn heparinized blood is desirable. The physiologic and pathologic alterations occurring during and after extracorporeal circulation have been extensively investigated. Metabolic acidosis is one of the hazards of cardiopulmonary bypass and most often results from inadequate perfusion. For this reason blood flow rates approaching basal cardiac output should be maintained at normal temperatures and reduced flow rates should only be used in association with general body hypothermia. Severe metabolic acidosis in the presence of inadequate cardiac output should be corrected promptly by administration of sodium bicarbonate and by correction of factors leading to its development. Hemolysis may result from trauma to erythrocytes occurring in the pumping system, the oxygenator or from the intracardiac suction apparatus. The ability of the reticuloendothelial system to remove free hemoglobin reduces the hazard of hemolysis since rarely does the free hemoglobin rise above 150 to 200 mg. per cent. Cellular elements undergo a moderate reduction in numbers during and in the first few days following perfusion. This is due not only to hemolysis during perfusion but to reduction in the survival of erythrocytes following perfusion. The phenomenon of sludging has been demonstrated to occur during and after extracorporeal circulation and is thought to be due to inadequate perfusion of certain vascular beds. The addition of low molecular weight dextran to the priming solution of the extracorporeal circuit appears to minimize its development. The most important recent observation concerning changes occurring in blood as a result of extracorporeal circulation concerns the denaturation of plasma proteins during the passage of blood through oxygenators. This is attributed to intermolecular energy known to exist at the blood-gas interface in the system. The reinfusion of the supernatant of plasma following passage through an oxygenator is capable of producing sludging of blood, a toxic reaction, and death in experimental animals. Furthermore, serological incompatibilities may develop as a result of these changes. Experimental studies have confirmed the clinical observations that potassium citrate arrest of the heart is poorly tolerated, because of certain metabolic changes occurring in the myocardium. Cardiac arrest should be avoided whenever possible but if necessary for the proper conduct for the operation it should be induced by hypothermia. Dilatation of the heart, particularly the left ventricle, during cardiopulmonary bypass with cardiac arrest, is poorly tolerated because of the increase in diastolic length of the myocardial fibers. Under these circumstances resuscitation is difficult. To avoid distention of the left ventricle, it should be vented during bypass whenever arrest is induced or for operations on the aortic and mitral values. Cerebral symptoms have been encountered after extracorporeal circulation and are due largely to hypoxia as a result of inadequate perfusion. In some instances cerebral embolization may be the cause of neurologic manifestations. Treatment consist of the induction of general body hypothermia to minimize cerebral edema. Pulmonary complications are frequent following extracorporeal circulation and are responsible for a large proportion of the morbidity and mortality. Problems may arise as a result of inadequate ventilation and impaired diffusion. The use of tracheostomy and a mechanical respirator are invaluable in the treatment of pulmonary complications. Renal function during extracorporeal circulation is proportional to the flow rates maintained during perfusion. Acute renal failure following operation may be due to excessive hemolysis from inadequate perfusion. The maintenance of flow rates approximating basal cardiac output minimize the development of renal insufficiency. Embolization has often been implicated as the cause of a variety of complications arising following extracorporeal circulation but in most instances the evidence is not conclusive. In conclusion, it should be emphasized that any of several types of pumpoxygenator systems will permit intracardiac operations, but only the most careful surgical technique will insure good results. In fact, too much, perhaps, has been said about the apparatus and to little about the men who use it. Certainly, there is no other surgical field which so completely separates the professional from the amateur.