sphygmomanometer
Chemistry in Everyday Life Measuring Blood Pressure Blood pressure is measured using a device called a sphygmomanometer Greek sphygmos = pulse . It consists of an inflatable cuff to restrict blood flow, a manometer to measure the pressure, and a method of determining when blood flow begins and when it becomes impeded Figure 9.6 . Since its invention in 1881, it has been an essential medical device. There are many types of sphygmomanometers: manual ones that require a stethoscope and are used by medical professionals; mercury ones, used when the most accuracy is required; less accurate mechanical ones; and digital ones that can be used with little training but that have limitations. When using a sphygmomanometer, the cuff is placed around the upper arm and inflated until blood flow is completely blocked, then slowly released. As the heart beats, blood forced through the arteries causes a rise in pressure. This rise in pressure at which blood flow begins is the systolic pressurethe peak pressure in the cardiac cycle. When the cuffs pressure equals the arterial systolic pressure, blood flows past the cuff, creating audible sounds that can be heard using a stethoscope. This is followed by a decrease in pressure as the hearts ventricles prepare for another beat. As cuff pressure continues to decrease, eventually sound is no longer heard; this is the diastolic pressurethe lowest pressure resting phase in the cardiac cycle. Blood pressure units from a sphygmomanometer are in terms of millimeters of mercury mm Hg .