Noninvasive Arterial Blood Pressure Monitoring Indications
The use of any anesthetic, no matter how “trivial,” is an indication for arterial blood pressure measure-ment. The techniques and frequency of pressure determination depend on the patient’s condition and the type of surgical procedure. An oscillometric blood pressure measurement every 3–5 min is ade-quate in most cases.
Although some method of blood pressure measure-ment 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. Rarely, it may prove impossible to monitor blood pressure in cases (eg, burns) in which there may be no accessible site from which the blood pressure can be safely recorded.
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 heart-beat, 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. The equip-ment required is simple and inexpensive.
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 (Figure 5–3). The Doppler effect is the shift in the frequency of sound waves when their source moves relative to the observer. For example, the pitch of a train’s whistle increases as a train approaches and decreases as it departs. Similarly, the reflection of sound waves off of a moving object causes a fre-quency shift. A Doppler probe transmits an ultra-sonic signal that is reflected by underlying tissue. As red blood cells move through an artery, a Doppler frequency shift will be detected by the probe. The dif-ference between transmitted and received frequency causes the characteristic swishing sound, which indicates blood flow. Because air reflects ultrasound, a coupling gel (but not corrosive electrode jelly) is applied between the probe and the skin. Positioning the probe directly above an artery is crucial, since the beam must pass through the vessel wall. Interference from probe movement or electrocautery is an annoy-ing distraction. Note that only systolic pressures can be reliably determined with the Doppler technique.
A variation of Doppler technology uses a piezo-electric crystal to detect lateral arterial wall move-ment to the intermittent opening and closing of vessels between systolic and diastolic pressure. This instrument thus detects both systolic and diastolic pressures. The Doppler effect is routinely employed by perioperative echocardiographers to discern both the directionality and velocity of both blood flow within the heart and the movement of the heart’s muscle tissue (tissue Doppler).
Inflation of a blood pressure cuff to a pressure between systolic and diastolic pressures will par-tially collapse an underlying artery, producing tur-bulent flow and the characteristic Korotkoff sounds. These sounds are audible through a stethoscope placed under—or just distal to—the distal third of the blood pressure cuff. The clinician measures pres-sure with an aneroid or mercury manometer.
Occasionally, Korotkoff sounds cannot be heard through part of the range from systolic to diastolic pressure. This auscultatory gap is most common in hypertensive patients and can lead to an inaccurate diastolic pressure measurement. Korotkoff sounds are often difficult to auscultate during episodes of hypotension or marked periph-eral vasoconstriction. In these situations, the sub-sonic frequencies associated with the sounds can be detected by a microphone and amplified to indicate systolic and diastolic pressures. Motion artifact and electrocautery interference limit the usefulness of this method.
Arterial pulsations cause oscillations in cuff pres-sure. These oscillations are small if the cuff is inflated above systolic pressure. When the cuff pressure decreases to systolic pressure, the pulsa-tions are transmitted to the entire cuff, and the oscillations markedly increase. Maximal oscilla-tion occurs at the MAP, after which oscillations decrease. Because some oscillations are present above and below arterial blood pressure, a mer-cury or aneroid manometer provides an inaccurate and unreliable measurement. Automated blood pressure monitors electronically measure the pres-sures at which the oscillation amplitudes change (Figure 5–4). A microprocessor derives systolic, mean, and diastolic pressures using an algorithm. Machines that require identical consecutive pulse waves for measurement confirmation may be unre-liable during arrhythmias (eg, atrial fibrillation). Oscillometric monitors should not be used on patients on cardiopulmonary bypass. Nonetheless, the speed, accuracy, and versatility of oscillomet-ric devices have greatly improved, and they have become the preferred noninvasive blood pressure monitors in the United States and worldwide.
Arterial tonometry measures beat-to-beat arterial blood pressure by sensing the pressure required to partially flatten a superficial artery that is supported by a bony structure (eg, radial artery). A tonometer consisting of several independent pressure trans-ducers is applied to the skin overlying the artery (Figure 5–5). The contact stress between the trans-ducer directly over the artery and the skin reflects intraluminal pressure. Continuous pulse recordings produce a tracing very similar to an invasive arterial blood pressure waveform. Limitations to this tech-nology include sensitivity to movement artifact and the need for frequent calibration.
Adequate oxygen delivery to vital organs must be maintained during anesthesia. Unfortunately, instruments to monitor specific organ perfusion and oxygenation are complex, expensive, and often unreliable, and, for that reason, an adequate arterial blood pressure is assumed to predict adequate organ blood flow. However, flow also depends on vascular resistance:
Even if the pressure is high, when the resistance is also high, flow can be low. T hus, arterial blood pressure should be viewed as an indicator—but not a measure—of organ perfusion.
The accuracy of any method of blood pressure measurement that involves a blood pressure cuff depends on proper cuff size (Figure 5–6). The cuff ’s bladder should extend at least halfway around the extremity, and the width of the cuff should be 20% to 50% greater than the diameter of the extremity.
Automated blood pressure monitors, using one or a combination of the methods described above, are frequently used in anesthesiology. A self-con-tained air pump inflates the cuff at set intervals. Incorrect or too frequent use of these automated devices has resulted in nerve palsies and extensive extravasation of intravenously administered fluids. In case of equipment failure, an alternative method of blood pressure determination must be immedi-ately available.
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