The auscultatory method of measuring arterial blood pressure involves first increasing the air pressure in a broad cloth or plastic cuff placed around the arm (sometimes the leg). The cuff pressure is then gradually allowed to decrease, as one listens with the bell of a stethoscope placed over the brachial artery at a point immediately below the cuff (Figure 1.08).
A sharp slapping sound is initially heard. This is called the appearance. Then follows the loudening phase which is heard as an increase in sound intensity, then a muffling phase, and finally a disappearance of all sound. These Korotkow or Korotkoff sounds are produced by turbulence resulting from high blood flow velocity in the vessel, as the cuff pressure is decreased below the systolic pressure. Blood flows at high velocity through the small opening which forms for an instant; the increased velocity prediposes to turbulence. It is the turbulence which is heard through the stethoscope.
Systolic blood pressure is most closely correlated with the appearance. Please note that this is an indirect method of measuring blood pressure, not a direct one such as where a catheter placed in an artery is attached to a pressure transducer. This approach produces more accurate measurements, but involves increased risk to the subject and is far less convenient. With the auscultatory method the appearance will usually predict the systolic pressure plus or minus a few mmHg when carried out by an experienced individual. Diastolic pressure is less accurately measured by this method. Some investigators believe the diastolic pressure is indicated by the muffling, some by the disappearance.
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