The instrument used to measure blood pressure is called a sphygmomanometer. It measures the air pressure needed to raise a column of mercury (Hg). The instrument consists of an inflatable cuff connected to a small bulb pump and a pressure gauge. By means of the inflatable cuff, which encircles the limb (usually the upper arm), air pressure within the cuff is balanced against the pressure in the artery (usually the brachial artery at the elbow). The pressure is estimated by means of a mercury or aneroid manometer. The mercury manometer is the most accurate pressure gauge. The aneroid gauge is frequently used instead of the mercury manometer because it is more compact and is convenient as a portable instrument. Some electronic instruments may give falsely high diastolic readings, but manufacturers will improve these to meet market demands.
The cuff size of the sphygmomanometer is of great importance. If the cuff is too small for the patient’s arm, the blood pressure reading may be falsely high. In this case some moderately obese patients may be falsely classified as hypertensives if a normal cuff is used. A regular cuff may be used for arm circumference of less than 33 cm. A large cuff should be used whenever the mid-upper arm circumference exceeds 33 cm. The cuff must be applied snugly. To measure blood pressure, the cuff is wrapped around the arm about one inch above the elbow crease. Ask your doctor or nurse to show you how to take your own blood pressure. Occasionally both the radial pulse at the wrist and the brachial pulse at the elbow over which the stethoscope is placed are difficult to feel.

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