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INTRODUCTION

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The use of noninvasive blood pressure monitoring is critical in any anesthesiology practice. The standards of monitoring, as defined by the American Society of Anesthesiologists, require measurement of blood pressure at a minimum every 5 minutes during an anesthetic procedure. Two types of noninvasive method for arterial pressure measurement can be defined: periodic or continuous sampling using pulse waveform. Periodic sampling techniques provide systolic and diastolic information over a series of heart beats, whereas continuous monitoring provides beat-to-beat measurements and pulse pressure waveform in real time.

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PERIODIC SAMPLING

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Manual Techniques

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Scipione Riva-Rocci first created the occlusive cuff-based method in 1895. The vascular unloading principle was adapted using an external compression pressure against the limb to indirectly collapse the vessel. At this point, equilibrium exists between the external force and the vessel. The compression is released until tension on the wall of the vessel is zero, which equals the transmural pressure that unloads the vessel. Using the Riva-Rocci principle, the detection of opening and closing of artery can be clinically demonstrated by examining skin flushing or by palpating the pulse.

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In 1905, Korotkoff adopted the ascultatory method, which is currently the most common approach in clinical practice. The blood pressure cuff is inflated above the systolic blood pressure (SBP), a stethoscope is placed over the brachial artery, and the external compression is slowly decreased. There are five phases of the Korotkoff sounds but clinically only two are important for measurement. Phase 1 will begin as the initial “tapping” sounds correspond to SBP, whereas phase 5 is the end of the muffled sound corresponding to the diastolic blood pressure (DBP). In between, phases 2 and 3 produce progressively changing sound, whereas phase 4 is the beginning of the muffled sound. Although the mean arterial pressure (MAP) is not measured, it can be calculated using SBP and DBP (MAP = 2/3 DBP + 1/3 SBP).

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Oscillometric Measurements

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Within a pressure chamber, the pressure produced with each heartbeat contains pulsatile variations. The amplitude of each pulsation can vary by changing the chamber pressure. Even with manual measurement, pulsatile variation in an air gauge can be appreciated. Because of this oscillation effect during cuff deflation, it is possible to estimate SBP, DBP, and mean blood pressure. Oscillometry forms the basis of the automated noninvasive blood pressure cuff. The cuff contains an inflatable device with a sensor that measures oscillations electronically. A microprocessor initiates an inflation–deflation sequence, in which the cuff is inflated to a pressure above the previous SBP and then slowly deflated in an incremental manner. The start of rapidly increasing oscillations indicates SBP, whereas DBPs occur when the oscillations quickly slow down. The DBP can be difficult to measure directly because oscillations can still be present even when the cuff is below the actual diastolic value. The maximum oscillation amplitude occurs when the arterial ...

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