What intraoperative monitoring techniques can be used for the patient undergoing cardiac surgery using cardiopulmonary bypass (CPB)?
Intraoperative monitoring should include intra-arterial blood pressure monitoring. This is necessary due to the nature of the disease, the nature of the surgery, the need for frequent arterial blood gas sampling, and the necessity of monitoring mean arterial blood pressure directly while on CPB. Additional monitors include a central venous catheter, which is needed to administer vasoactive medica-tions directly into the central circulation. If patients have severe left ventricular dysfunction or valvular heart disease, a pulmonary artery catheter would be preferable. Pulmonary arterial catheterization allows for the direct measure of pulmonary capillary wedge pressure (PCWP), which is often directly related to LVEDP. Monitoring the PCWP gives the anesthesiologist a close approximation of intravascular volume status assuming that the left ventric-ular compliance is normal. Intravascular volume is more sensitively measured using transesophageal echocardiogra-phy (TEE) because the end-diastolic area of the left ventricle can be directly visualized.
The CPB pump uses nonpulsatile flow and the heat exchanger system is sometimes used to lower the patient’s body temperature. Vasoconstriction and vasodilation are common and do not represent an alteration in the anesthetic depth of the patient. Also, the addition of crystalloid fluid to the pump prime increases the blood volume of the patient thereby diluting the blood concentrations of anesthetic agents and other drugs. For these reasons, many practition-ers choose to use a monitor of anesthetic depth during and after CPB.
The heat exchanger as a part of CPB helps to lower and to raise the patient’s body temperature to facilitate the conduct of CPB. The rate or speed of temperature change should be small, and the gradient between body temperature and blood temperature should not be excessive. For these reasons, it is important to monitor at least two temperatures during CPB: one temperature reflects blood temperature (esophageal), and a second reflects core body temperature (rectal or bladder).