Vigilant perioperative monitoring of the cardiovascular system is one of the primary duties of anesthesia providers. This chapter focuses on the specific monitoring devices and techniques used by anesthesiologists to monitor cardiac function and circulation in healthy and nonhealthy patients alike.
The rhythmic contraction of the left ventricle, ejecting blood into the vascular system, results in pulsatile arterial pressures. The peak pressure generated during systolic contraction (in the absence of aortic valve stenosis) approximates the systolic arterial blood pressure (SBP); the lowest arterial pressure during diastolic relaxation is the diastolic blood pressure (DBP). Pulse pressure is the difference between the systolic and diastolic pressures. The time-weighted average of arterial pressures during a pulse cycle is the mean arterial pressure (MAP). MAP can be estimated by application of the following formula:
Arterial blood pressure is greatly affected by where the pressure is measured. As a pulse moves peripherally through the arterial tree, wave reflection distorts the pressure waveform, leading to an exaggeration of systolic and pulse pressures (Figure 5-1). For example, radial artery systolic pressure is usually greater than aortic systolic pressure because of its more distal location. In contrast, radial artery systolic pressures often underestimate more “central” pressures following hypothermic cardiopulmonary bypass because of changes in hand vascular resistance. Vasodilating drugs may accentuate this discrepancy. The level of the sampling site relative to the heart affects the measurement of blood pressure because of the effect of gravity (Figure 5-2). In patients with severe peripheral vascular disease, there may be a significant difference in blood pressure measurements among the extremities. The higher value should be used in these patients.
Changes in configuration as a waveform moves peripherally. (Reproduced, with permission, from Shah N, Bedford RF: Invasive and noninvasive blood pressuring monitoring. In: Clinical Monitoring: Practical Applications in Anesthesia and Critical Care Medicine. Lake CL, Hines RL, Blitt CD [editors]. WB Saunders, Philadelphia, 2001.)