CARDIOPULMONARY BYPASS FOR CARDIAC SURGERY

PUMP OXYGENATOR APPARATUS

The precise apparatus available for cardiopulmonary by¬pass (CPB) changes frequently, but the basic components remain constant.
A venous reservoir is usually present and is positioned to provide adequate siphonage of blood by grav-ity. This provides storage of excess volume and allows escape of any air bubbles returning with venous blood.
The oxygenator provides oxygen to the blood and allows elimination of carbon dioxide. Currently, ?bubble oxygenators, ?membrane oxygenators, and ?microporous hollow-fiber oxy¬genators are available for clinical use.
An efficient heat exchanger is necessary for control of the perfusate temperature to achieve systemic cooling and re¬warming during CPB. Most heat-exchanging devices are in¬cluded as an integral part of the pump oxygenator.
The arterial pump is usually a ?roller pump, which should be adjusted before each perfusion to be slightly nonocclusive. The pump tubing is either Silastic or latex, which, unlike the Tygon tubing, does not become stiff at low temperatures. ?nonocclusive vortex pumps are also available, their use is generally restricted to extracorporeal membrane oxy¬genator (ECMO) or ventricular assist device (VAD) circuits.


PHYSIOLOGIC RESPONSE TO CARDIOPULMONARY BYPASS


Many complex physiologic changes occur when a patient is temporarily supported by means of an oxygenator system. The term total CPB indicates that nearly all of the systemic venous blood is returned to the oxygenator. Partial CPB im¬plies that some of the systemic venous blood returns to the heart and is ejected into the aorta.
Two main types of physio¬logic variables exist during CPB: externally controlled variables, which are controlled by the surgeon and perfusionist, and patient variables, which are less easily regulated.


Externally Controlled Variables

? perfusion flow rate is controlled by the perfusionist but should be actively established by the surgeon. normothermic perfusion flow rates of 1.7 liters per minute per sq. m. or greater are usually acceptable, but flow rates of 2.0 to 2.5 liters per minute per sq. m. provide a more secure margin of safety for organ perfusion.
? Temperature of the perfusate, and secondarily of the pa¬tient, is controlled by the perfusionist by means of the heat exchanger. This is one of the most important decisions to be made about each patient during CPB. Particularly in small infants, moderate and profound hypothermic perfusion allows the safe, temporary reduction in flow that is advanta¬geous for the accurate intracardiac repair of many malforma-tions
? Pulmonary venous pressure (left atrial pressure) should also be maintained near 0 mm. Hg during total CPB. High pulmo¬nary venous pressure during CPB may result from ? excessive pulmonary collateral blood flow, an ? unrecognized extracar¬diac left-to-right shunt (e.g., patent ductus arteriosus), or ? incomplete venous drainage leading to increased pulmonary blood flow. Marked and prolonged elevation in pulmonary venous pressure during CPB promotes accumulation of extra¬vascular lung water and probably contributes to postopera¬tive pulmonary dysfunction.
? Hematocrit of the patient-oxygenator system is deter¬mined by the ?body size and ?hematocrit of the patient, the ?composition and amount of the initial priming volume of the pump oxygenator, the ?amount of blood lost, and the ?composition and volume of solutions infused during CPB.