CARDIOPULMONARY BYPASS

CPB Circuitry

By definition, CPB diverts blood away from both the left and right sides of the heart and the lungs. This objective is accomplished by cannulas that drain blood from the right side of the heart and a cannula that allows perfusion of the arterial circulation. However, this system alone does not effectively drain all blood that enters the heart during surgery. Systemic-to-pulmonary collaterals, systemic-to-systemic collaterals, and surgical site bleeding are routes of blood return to the left side of the heart and into the surgical field that require specific drainage mechanisms (cannulas) in the CPB pump to safely conduct CPB. Current CPB pumps can also be used to deliver cardioplegia through an independent pump and tubing mechanism. In addition to the pumping/suction mechanisms and the associated tubing connections to the patient, the CPB pump has several features that can be broadly categorized into three groups: (1) oxygenating-ventilating system that supplies oxygen and removes carbon dioxide, (2) cooling-warming system, and (3) several in-line monitors and safety devices ( Fig. 50-31 ).

Venous drainage from the patient to the pump is usually effected by gravity and is thus dependent on the height of the patient above the venous reservoir. This passive mechanism of venous return to the pump is subject to the risk of airlock. Suction mechanisms can also be applied to the venous lines to promote venous drainage. However, this approach requires close monitoring since excessive suctioning can paradoxically reduce venous outflow by causing collapse of the vena cava/right atrium over the tips of the venous cannula. Patient circumstances and surgical preference determine the type and location of venous cannula placement. Bicaval, single atrial, single cavoatrial, and femoral venous cannulation can each be used to effect venous drainage. Each approach has its advantages and disadvantages. For example, bicaval cannulation of the inferior and superior vena cava with tourniquet placement is necessary for mitral valve surgery, ensures adequate venous return, and minimizes return of blood to the right side of the heart and the consequential myocardial rewarming, but it is technically more difficult. Femoral venous cannulation can be used as a temporizing measure if it is deemed necessary to decompress the heart during high-risk repeat surgery. Arterial cannulas are usually placed in the ascending aorta, but they can be placed in the femoral or axillary arteries, as dictated by circumstances.

Non-vena caval blood return to the right side of the heart (through the coronary sinus before aortic cross-clamping, through the thebesian veins, and via systemic-to-systemic collaterals) is usually effectively drained by the atrial or cavoatrial cannula. Bicaval cannulation is often performed in circumstances in which the right atrium is opened surgically (e.g., closure of atrial septal defects) to minimize the risk of right ventricular distention. Distention of the left ventricle is an ever-present risk during CPB with aortic cross-clamping. It occurs as a result of bronchopulmonary collateral flow, systemic-to-systemic pericardial collateral flow, and thebesian venous return to the ventricular cavity, especially when the aorta is unclamped and there is the potential for coronary flow. Venous return to the left ventricle results not only in left ventricular distention and potentially subendocardial myocardial ischemia, but also in left ventricular warming (CPB with hypothermia of ∼30°C still delivers blood to the heart that is warm relative to myocardial temperatures during anoxic arrest induced by cardioplegic solutions with temperatures between 17°C and 20°C). Thus, effective venting of the left heart is an essential feature of CPB. The left side of the heart can be effectively vented by cannulas placed in either the left ventricle, left atrium, right superior pulmonary vein, or pulmonary artery. The patient's specific circumstances and practitioner preferences

1972

Figure 50-31 Illustration outlining the essential features of a typical cardiopulmonary bypass circuit. Venous blood is drained into a venous reservoir by gravity (although vacuum assist devices can be added) from the right atrium by using either a single cannula or bicaval cannulation. Blood can also be salvaged from the surgical field and cardiopulmonary cavities and returned to the venous reservoir by cardiotomy suckers and appropriately placed vents, respectively, thus avoiding increases in left ventricular pressure and left ventricular distention resulting from collateral flow in the presence of aortic cross-clamping. Blood from the venous reservoir is pumped by a nonocclusive roller-head pump through the oxygenator to the patient. The content of the gas flow to the oxygenator determines the arterial blood gas and acid-base content. Various monitoring and safety devices are shown for these circuits. Cardioplegia can also be delivered by a roller-head pump and is monitored as shown. Both cardioplegia and the patient's blood can be heated-cooled by heat exchangers connected to a water-based heater-cooler device.

Today, membrane oxygenators are used almost exclusively and usually incorporate a venous reservoir, a heat exchanger, and gas flow conduits. Monitors and safety devices are somewhat variable but include in-line filters and bubble traps, in-line blood gas monitors, pressure and flow measurement mechanisms, oxygen and gas flow measurement devices, temperature monitoring, and low-level sensor detectors on the venous reservoir. CPB circuits can also incorporate an anesthetic vaporizer. As previously mentioned, cardioplegia can be delivered by the CPB system ( Fig. 50-31 ).