Perfusion science is a unique discipline unto itself, and a full discussion of its many intricacies is far beyond the scope of this introduction to cardiac anesthesia and echocardiography. Still, much that is unique to cardiac anesthesia care can be in some degree related to the use of cardiopulmonary bypass (CPB). At the start it is important for practitioners new to cardiac anesthesia to establish a close working partnership with their perfusionist colleagues. Perfusionists are certified healthcare professionals who devote their careers to the management of circulatory support. In most institutions they work under the direct authority of the attending surgeon; however, from time to time they are under the medical direction of the anesthesiologist. At no times must they be considered a substitute for an appropriately qualified anesthesia practitioner in the operating room. Hence, during the “bypass run” a member of the patient’s anesthesia team must be physically present in the operating room. During CPB, the anesthesiologist and the perfusionist work together to bypass the functions of the heart and the lungs so that cardiac surgery may proceed. The pump’s flow becomes the patient’s cardiac output (CO). The oxygenator of the CPB machine provides gas exchange. Simply put, the hemodynamic principles, which guide normal patient management, are operative when the bypass machine is in use. Blood pressure is still the product of CO and systemic vascular resistance (SVR)—except that the CPB machine now generates the CO in place of the heart’s pumping function.
CPB has been in use in cardiac surgery for more than 50 years. It is likely that over that time successive generations of anesthesia trainees have been initially overwhelmed by the complexity of the bypass machine. Nonetheless, the basic circuit is straightforward enough (Figure 17–1). Anticoagulated venous blood is drained from the right atrium or directly from the superior and inferior venae cavae through the venous cannula and associated tubing to the pump reservoir. The deoxygenated venous blood is returned to the patient after passing through the oxygenator as well as a heat exchanger to deliver oxygenated blood at the desired temperature back to the patient. In the oxygenator, carbon dioxide is swept away by the oxygen gas flow across a gas permeable membrane. Oxygenated blood is then returned to the patient through a cannula most often placed in the ascending aorta or occasionally in a femoral artery. Along the oxygenated blood’s course there are a number of filters and alarms to prevent the perfusionist from pumping air or clot into the aorta resulting in a perioperative embolic catastrophe.
Venous blood is drained from the patient, flows through the pump oxygenator, and is returned oxygenated to the arterial system. Other functions of a simple CPB circuit are to deliver cardioplegia solution to the heart as well as suction blood from the surgical field to be oxygenated, filtered, and returned to the patient.