ARTERIAL BLOOD PRESSURE
The rhythmic contraction of the left ventricle, eject-ing blood into the vascular system, results in pulsa-tile arterial pressures. The peak pressure generated during systolic contraction (in the absence of aor-tic 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 follow-ing formula:
Arterial blood pressure is greatly affected by where the pressure is measured. As a pulse moves peripherally through the arterial tree, wave reflec-tion distorts the pressure waveform, leading to
an exaggeration of systolic and pulse pressures (Figure 5–1). For example, radial artery systolicpressure is usually greater than aortic systolic pres-sure 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 sam-pling site relative to the heart affects the measure-ment of blood pressure because of the effect of gravity ( Figure 5–2). In patients with severe periph-eral vascular disease, there may be a significant dif-ference in blood pressure measurements among the extremities. The higher value should be used in these patients.
Because noninvasive (palpation, Doppler, aus-cultation, oscillometry, plethysmography) and inva-sive (arterial cannulation) methods of blood pressure determination differ greatly, they are discussed separately.