Intermittent noninvasive blood pressure monitoring

Published on 07/02/2015 by admin

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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.

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

With tonometry, a pressure transducer is applied to the skin above a peripheral artery. The transducer measures the contact stress between the transducer over the artery and the skin, reflecting the intraluminal pressure. Tonometry provides the capability of measuring beat-to-beat arterial BP.

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.

Figure 17-1 An oscillometer measures the oscillations in the pressure within the blood pressure cuff. The point at which the oscillations have a maximum amplitude is the mean arterial pressure (MAP). Once oscillations are barely perceptible, the device releases the air remaining in the cuff and then calculates the systolic and diastolic blood pressure, using an algorithm unique to each manufacturer.

Factors affecting the accuracy of oscillometric blood pressure monitoring

Relationship between the size of the cuff and the size of the arm

It is recommended that the width of the cuff be 40% to 50% of the arm’s circumference (or 125% to 150% of the arm’s diameter). The bladder should encircle the arm’s circumference by 50% or more. If the upper arm is too large or conically shaped, it may be better to place the cuff at the forearm, ankle, or thigh. Placing the cuff over superficial nerves, bony prominences, or joints should be avoided. A cuff that is too loose or too large may underestimate the BP; conversely, a cuff that is too tight or too small may overestimate the BP.

Site of monitoring

As the cuff moves more peripherally, the systolic pressure increases, whereas the diastolic pressure decreases secondary to changes in the amplitude of the pulse of blood being conducted down the underlying artery. Vascular disease and peripheral vasoconstriction may reduce pressure values at distal locations. As a general rule, venous engorgement has little effect on accuracy.

Arm positioning

The cuff should remain at the level of the patient’s heart. If the cuff is placed 10 cm above or below heart level, a correction of 7.4 mm Hg needs to be added or subtracted from the measured BP.

Arrhythmias

Oscillometric noninvasive BP monitoring is often inaccurate in patients with arrhythmias. If the patient has a history of an arrhythmia, the provider should consider placing an arterial line catheter for direct BP monitoring.

Potential adverse events during oscillometric blood pressure monitoring

Artifacts from intrinsic and extrinsic motion of the patient and staff, mechanical problems, and equipment failure may occur during oscillometric monitoring and may lead to inaccurate measurement of the patient’s BP. Several complications may arise during oscillometric monitoring of BP. Damage to the skin and underlying tissue may occur when the cuff is placed improperly. The patient may also develop neuropathies of the median, ulnar, or radial nerves, or a combination thereof. Compartment syndrome is most likely to develop in patients in whom the use of oscillometric monitoring of BP is prolonged or if the device malfunctions and the malfunction is undetected if the patient’s extremity is covered.

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