Sphygmomanometer

A sphygmomanometer (IPA: /ˌsfɪgmoʊməˈnɒmɪtər/, SFIG-moe-mə-NO-mi-tər) or blood pressure meter is a device used to measure blood pressure, comprising an inflatable cuff to restrict blood flow, and a mercury or mechanical manometer to measure the pressure. It is always used in conjunction with a means to determine at what pressure blood flow is just starting, and at what pressure it is unimpeded. Manual sphygmomanometers are used in conjunction with a stethoscope.

The word comes from the Greek sphygmós (pulse), plus the scientific term manometer (pressure meter). The device was invented by Samuel Siegfried Karl Ritter von Basch. Scipione Riva-Rocci, an Italian physician, introduced a more easily used version in 1896. Harvey Cushing discovered this device in 1901 and popularized it.

A sphygmomanometer usually consists of an inflatable cuff, a measuring unit (the mercury manometer), a tube to connect the two, and (in models that don't inflate automatically) an inflation bulb also connected by a tube to the cuff. The inflation bulb contains a one-way valve to prevent inadvertent leak of pressure while there is an adjustable screw valve for the operator to allow the pressure in the system to drop in a controlled manner.

BP 126/70 mmHg as result on electronic sphygmomanometer

Operation

The cuff is normally placed around the left arm, at roughly the same vertical height as the heart while the subject seated with the arm supported.It is essential that the correct size of cuff is selected for the patient. Too small a cuff results in too high a pressure, whilst too large a cuff results in too low a pressure. The cuff is inflated until the artery is completely occluded. Listening with a stethoscope to the brachial artery at the elbow, the examiner slowly releases the pressure in the cuff. As the pressure in the cuffs falls, a "whooshing" or pounding sound is heard (see Korotkoff sounds) when blood flow first starts again in the artery. The pressure at which this sound began is noted and recorded as the systolic blood pressure. The cuff pressure is further released until the sound can no longer be heard. This is recorded as the diastolic blood pressure.

Significance

By observing the mercury in the column while releasing the air pressure with a control valve, one can read on the scale the values of the blood pressure in mm Hg. The peak pressure in the arteries during the cardiac cycle is the systolic pressure, and the lowest pressure (at the resting phase of the cardiac cycle) is the diastolic pressure. A stethoscope is used in the auscultatory method. Systolic pressure (first phase) is identified with the first of the continuous Korotkoff sounds. Diastolic is identified at the moment the Korotkoff sounds disappear (fifth phase).

Types

Conventional (mechanical) sphygmomanometer with aneroid manometer and stethoscope

There are three types of sphygmomanometers:

• Digital with manual or automatic inflation. These are electronic, easy to operate and practical in noisy environments. Many have not been validated for all patient groups, and can give very inaccurate readings.^{[citation needed]} They measure mean arterial pressure (MAP) and use algorithms to calculate systolic and diastolic values. In this sense, they do not actually measure the blood pressure, but derive the readings. Digital oscillometric monitors are also confronted with “special conditions” for which they are not designed to be used: arteriosclerosis, arrhythmia, preeclampsia, pulsus alternans, and pulsus paradoxus. Both large floor-standing and portable units are used. Portable units for the arm and wrist are available.

• Digital portable finger blood pressure monitors with automatic inflation. These are more portable and easy to operate, though less accurate. They are the smallest blood pressure monitors.

• Manual. Should be operated by a trained person. Mercury manometers are considered to be the "gold standard" of measurement because their measurement is absolute and does not require re-calibration. For this reason they are often required in clinical trials of pharmaceuticals and for clinical evaluations of determining blood pressure for high risk patients including pregnant women. Aneroid, mechanical, types are in common use, but should be calibrated against a mercury manometer. The unit of measurement of blood pressure is millimeters of mercury (mmHg). Blood pressures are usually given as an even number.

Manual sphygmomanometers require a stethoscope for auscultation.