Blood pressure measurement is an indirect measurement of cardiovascular function. Blood pressure measurement is taken using a standard stethoscope with a diaphragm head (since the sounds will be high pitched) and a cuff with an inflatable bladder (a sphygmomanometer).
Selection of cuff size is important because a cuff with a bladder too small will yield an elevated reading, and vice-versa. The bladder inside the cuff should almost completely encircle the arm. A simple way to check for correct size is to lay the cuff lengthwise over the upper arm. If properly sized, the bladder should be 20% wider than the diameter of the arm. For example, the average adult would require a bladder (inside the cuff) 12-14 cm wide. A child 6-7 years of age would require a bladder width of 6-7 cm and an obese person could require a very large cuff such as one used to measure blood pressure in the thigh.
Attached to the bladder of the sphygmomanometer cuff is a squeeze bulb with a thumb screw and a pressure gauge. Both mercury and aneroid manometers are calibrated in millimeters of mercury pressure.
The patient should be seated upright with the forearm about level with the heart. Next the brachial artery should be located on the medial aspect of the antecubital fossa. The cuff should then be placed so that the bladder (marked by an arrow on the cuff) is centered over the brachial artery just medial to the biceps. The lower portion of the cuff should be one inch above the antecubital fossa and no clothing or other material should be under the cuff. Fasten the cuff firmly, not tightly, around the arm. Hold the diaphragm of your stethoscope over the brachial artery at the medial antecubital fossa. With the other hand, inflate the cuff until the brachial pulse sounds disappear and then 20-30 mm higher. Deflate the cuff 2-3 mm per heartbeat and record the first sound heard as the systolic blood pressure and the last sound heard as the diastolic blood pressure.