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Prior to placing a patient on CPB, the bypass machine must be linked to the patient via arterial and venous cannulas. Placement of these cannulas is facilitated by skilled management of the patient’s blood pressure. The arterial cannula is placed in the ascending aorta and in some cases, the femoral artery or axillary artery. The venous cannula(s) are placed in the right atrium and IVC/SVC. During aortic cannulation it is the responsibility of the anesthesiologist to limit the systolic blood pressure to 100 mmHg to minimize the risk of aortic dissection during aortotomy. This can be achieved by either increasing the inhaled anesthetic concentration or using vasodilators such as nitroglycerin. Once the aortic cannula is placed, the pressure is allowed to rise as the risk of complications during venous cannulation with respect to systolic blood pressure is minimal. In addition, at this point, in many centers, the institution of retrograde autologous priming will commence.
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In an effort to decrease the hemodilution that occurs during CPB, a retrograde autologous prime can be performed in which the crystalloid prime of the CPB machine circuit is displaced by a retrograde flow of blood through the aortic cannula and into the CBP circuit. Roughly 1000 mL of blood is used to accomplish this prime. During this period, which usually lasts a few minutes, the patient will often require vasopressor support to maintain an adequate blood pressure. This is also confounded by the very recent need to lower SBP to facilitate aortic cannulation; thus, vasodilators may still be having an effect.
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Once the prime is complete and the venous cannula is placed, the patient is ready for the institution of cardiopulmonary bypass (CPB). In some cases, the cardioplegia cannulas are placed before the start of CPB and in others this will occur after.
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Once CPB is initiated, during full support, the arterial pressure tracing will lose its characteristic pulsatile waveform, and, once the heart is arrested, will be completely devoid of any pulsatile activity, although a mean arterial pressure will be visible on the flat pressure tracing. As long as the heart is arrested, the patient is cooled, and the perfusionist is dosing cardioplegia at regular intervals, the main concern for the anesthesiologist is cerebral perfusion and by default systemic perfusion. It is generally accepted that as long as the MAP is maintained at 50–100 mmHg, the cerebral blood flow is preserved in otherwise normal patients. This MAP target should be adjusted to 60 mmHg in patients with preexisting hypertension.
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The physiologic response to CPB is complex and mimics the Systemic Inflammatory Response Syndrome, causing decreased insulin release, increased insulin resistance, and hyperglycemia. Hyperglycemia has been shown to be detrimental to neurologic recovery in areas of the brain subjected to ischemia and these outcomes are worse in patients with blood glucose levels exceeding 180 mg/dL. Therefore, ...