SPHYGMOMANOMETER

Typical Number in Hospital: 300

Cost Bands: 1

References: 3

This is the commonest form of blood pressure measuring apparatus used in every ward, theatre and clinic of the hospital. The measurement is indirect (i.e. there is no sensor inside the body) and is subjective, and therefore can result in large errors, particularly if the operator is inexperienced. The pressure is usually measured by a mercury-in-glass manometer but an aneroid gauge is sometimes used.

A rubber bag is attached to the upper arm under a cuff which is wrapped around the arm and secured by Velcro tape. The bag is connected by tubing to the manometer and to an inflating device in the form of a bulb. The bag is inflated by squeezing the bulb until the pressure exceeds the arterial pressure. This condition is detected by a stethoscope placed over the brachial artery just below the elbow since no sound is heard from the closed artery. A valve adjacent to the bulb is then partially opened so that the bag deflates slowly. Sounds from the artery are first heard when the applied pressure just fails to occlude the artery at the peak of the arterial pressure cycle (the systolic pressure). This pressure is noted and the applied pressure allowed to continue falling until the artery fails to occlude even at the lowest point of the arterial pressure cycle (the diastolic pressure). This point is identified from characteristic sounds (the Korotkoff sounds) at this point, which the operator learns to recognize. The two pressures, systolic and diastolic, are recorded as the patient's blood pressure and are typically around 120 and 80 mmHg respectively.

Automatic indirect blood pressure recording devices exist which operate the pressurizing and depressurizing cycle and which detect the systolic and diastolic pressures from pulsations in the cuff pressure, sounds detected by a microphone under the cuff or from the arterial movements as detected by an ultrasonic doppler transducer under the cuff. Such devices have proved useful when the patient is immobile (in intensive care and under anaesthesia) but may be subject to patient movement artefacts in other situations.