Intermittent noninvasive blood pressure monitoring

Published on 07/02/2015 by admin

Filed under Anesthesiology

Last modified 22/04/2025

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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.

Oscillometry

Oscillometry has become the most common method to measure arterial BP in the operating room.

Equipment for oscillometric blood pressure monitoring

Intermittent noninvasive BP monitoring with oscillometry requires several pieces of equipment: an inflatable cuff, pressure tubing, a cuff-inflation pump, and a bleed valve. Inflatable cuffs are available in varying sizes and are marked to indicate an appropriate limb circumference for the cuff size. Oscillometric devices have either one or two tubes that permit inflation and deflation of the cuff while simultaneously allowing measurement of the oscillations of the pressure in the cuff secondary to the impact of the pulse pressure in the underlying artery.

The cuff-inflation pump and bleed valve incorporate a sensing mechanism, a timing circuit, a control circuit, and alarms. The sensing mechanism is a pressure transducer containing a piezoelectric crystal that is distorted by the oscillations in the pressure of the cuff; this mechanical energy is converted to a measureable electric current that is amplified and recorded as the systolic, diastolic, and mean arterial pressures by the amplifier in the recording device. The timing circuit determines the frequency of BP measurement to be taken. A manual cycling option is also often available. The control circuit regulates the maximum pressure in the cuff, artifact rejection, deflation rate, and automatic cutoff. All monitors should be capable of triggering alarms when sensing abnormal BP measurements.

Oscillometry operation and function

Most intermittent noninvasive BP monitors use oscillometry to measure BP. In most situations, a BP cuff is applied above the patient’s elbow of either the left or right upper extremity. The cycle begins with the cuff inflating to a pressure above the measurable arterial systolic BP. The cuff is then deflated slowly in a stepwise or linear manner until oscillations (arterial pressure pulsations) begin to be detected. The oscillations initially increase to a maximum and then decrease to an immeasurable level. After the final oscillation is detected, the cuff quickly bleeds out the remaining air. The point of maximum amplitude of the oscillations corresponds with the mean arterial pressure (Figure 17-1). The device then calculates the systolic and diastolic BP by an empirically derived algorithm; because the algorithms are proprietary, the systolic and diastolic BP measurements may vary from one device to another, depending on the manufacturer’s algorithm. In contrast, direct arterial BP monitoring via arterial catheter measures both the systolic and diastolic pressures and calculates the mean.

Factors affecting the accuracy of oscillometric blood pressure monitoring