Chapter 4: Monitoring

CARDIOVASCULAR MONITORING

ARTERIAL BLOOD PRESSURE

The peak left ventricular end-systolic pressure (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:

As a pulse moves peripherally through the arterial tree, wave reflection distorts the pressure waveform, leading to an exaggeration of systolic and pulse pressures. 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 immediately following hypothermic cardiopulmonary bypass because of changes in hand vascular resistance. In patients with severe peripheral vascular disease, there may be significant differences in blood pressure measurements among the extremities. The greater value should be used in these patients.

1. Noninvasive Arterial Blood Pressure Monitoring

Indications

The use of any anesthetic is an indication for arterial blood pressure measurement. The techniques and frequency of pressure determination will depend on the patient’s condition and the type of surgical procedure. A noninvasive blood pressure measurement every 3–5 minutes is adequate in most cases.

Contraindications

Although some method of blood pressure measurement is mandatory, techniques that rely on a blood pressure cuff are best avoided in extremities with vascular abnormalities (eg, dialysis shunts) or with intravenous lines. It rarely may prove impossible to monitor blood pressure in patients (eg, those who have burns) who have no accessible site from which the blood pressure can be safely recorded.

Techniques & Complications

A. PALPATION

SBP can be determined by (1) locating a palpable peripheral pulse, (2) inflating a blood pressure cuff proximal to the pulse until flow is occluded, (3) releasing cuff pressure by 2 or 3 mm Hg per heartbeat, and (4) measuring the cuff pressure at which pulsations are again palpable. This method tends to underestimate systolic pressure, however, because of the insensitivity of touch and the delay between flow under the cuff and distal pulsations. Palpation does not provide a diastolic pressure or MAP.

B. DOPPLER PROBE

When a Doppler probe is substituted for the anesthesiologist’s finger, arterial blood pressure measurement becomes sensitive enough to be useful in obese patients, pediatric patients, and patients in shock. Note that only systolic pressures can be reliably determined with the Doppler technique.

C. AUSCULTATION

Inflation of a blood pressure cuff to a ...