Always consider that following the initiation of CPB, it is important to discontinue the administration of all exogenous inotropes so they are not mixed into the cardioplegia because these drugs can potentially increase myocardial energy consumption. Potassium cardioplegia is the most common method of myocardial protection during CPB. Traditionally, cardioplegia is delivered in an anterograde intermittent fashion through the cross-clamped aortic root with a cold, high-potassium, crystalloid solution. The increase in extracellular potassium concentration eventually inactivates the sodium channels, abolishing action potentials and arresting the heart in diastole, thus stopping the energy expenditure associated with electrical and mechanical activity. The composition of cardioplegia varies on an institutional basis; however, all solutions contain 10 to 40 mEq/L. In addition, small amounts of calcium and magnesium and a buffer (usually bicarbonate) are needed in the cardioplegia. Alkaline buffers improve myocardial protection by preventing acid metabolite buildup. With cold cardioplegia, multiple doses are given for the effects of washout, rewarming, and prevention of metabolite buildup. Washout of high-potassium cardioplegia occurs due to the presence of non-coronary collaterals. Standard practice has been to employ systemic and topical cardiac hypothermia to reduce basal metabolic oxygen consumption. O₂ consumption decreases by 50% for every 10°C decrease in myocardial temperature, and myocardial cooling to 25°C allows the interruption of coronary blood supply (aortic cross-clamping) for longer periods of time. However, hypothermia itself may elicit myocardial injury by affecting cellular fluidity, transmembrane gradients, and these changes may result in cellular edema. Recently, it has been suggested that continuous and/or warm cardioplegia provides better protection, although severe, diffuse CAD may preclude even distribution of the cardioplegia and lead to poor protection. So far, studies have not consistently demonstrated the superiority of one method over the other. Although only anterograde cardioplegia was initially used, retrograde perfusion via the coronary sinus while carefully monitoring coronary sinus perfusion pressure (at <40 mm Hg) has become popular, with many centers using a combination of both methods and showing superior protection. Retrograde cardioplegia is particularly advantageous in valve surgery and in patients with critical CAD or total coronary occlusions. This technique is contraindicated in patients with a persistent left superior vena cava draining into the coronary sinus because of the risk of retrograde cardioplegia perfusion to the brain. Most centers currently use blood cardioplegia, a mixture of blood in crystalloids with a final hematocrit of 16% to 20%. In addition to
the O₂-carrying capacity of hemoglobin, blood contains buffers, free radical scavengers, colloids, and other components that protect the myocardium.