Intermittent noninvasive blood pressure monitoring

Published on 07/02/2015 by admin

Filed under Anesthesiology

Last modified 07/02/2015

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Intermittent noninvasive blood pressure monitoring

Clint Grant Humpherys, MD and Michael J. Murray, MD, PhD

Obtaining frequent blood pressure (BP) measurements is an essential component to delivering a safe anesthetic. The American Society of Anesthesiologists (ASA) Standards for Basic Anesthetic Monitoring states that “every patient receiving anesthesia shall have arterial blood pressure and heart rate determined and evaluated at least every five minutes.” A variety of ways exist by which to noninvasively measure BP, such as palpation, auscultation, Doppler, finger plethysmography with pulse oximetry, tonometry, and oscillometry.

Methods of obtaining blood pressure

Palpation

Systolic BP, but not diastolic BP, can be measured by palpating a peripheral pulse while inflating a BP cuff until flow is occluded and then slowly releasing the cuff pressure until a pulse is palpable. A similar technique can be used by substituting a pulse oximeter for palpation and placing it on one digit of the extremity, noting the pressure at which the Spo₂ measurement returns.

Auscultation

When measuring BP by auscultation, the BP cuff is connected to a sphygmomanometer, the cuff is inflated above the systolic pressure, and the cuff pressure is then gradually released while simultaneously auscultating with a stethoscope in the antecubital fossa for Korotkoff sounds. The first and last audible Korotkoff sounds represent the systolic and diastolic BPs, respectively.

Doppler

When utilizing the Doppler technique to ascertain BP, a small probe that emits ultrasound waves is placed above a peripheral artery. A BP cuff is inflated above systolic pressure and then slowly deflated. When blood begins to flow, it causes the characteristic Doppler shift in the ultrasound wave, which is created by the difference in the transmitted and received frequencies; this shift is recorded as the systolic pressure. Following the now-continuous blood flow in the artery, another shift in frequency occurs; this second shift is recorded as diastole.

Finger plethysmography

With finger plethysmography, a probe that emits infrared light is placed on the patient’s finger. With the probe in place, the cuff pressure is increased to above the systolic BP and the volume of blood in the finger decreases, which is measured as a change in the waveform of the light transmitted across the finger. As the cuff is subsequently deflated and systolic BP is reached, the volume of blood changes again. The subsequent change in wavelength is an indication of systolic BP, until the volume of blood in the finger is stable (i.e., diastolic pressure).

Tonometry

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