27-06-2013, 04:48 PM
Sphygmomanometer
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INTRODUCTION
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 in 1881.[1] Scipione Riva-Rocci
introduced a more easily used version in 1896. Harvey Cushing
discovered this device in 1901 and popularized it.
A sphygmomanometer consists of an inflatable cuff, a measuring unit
(the mercury manometer, or aneroid gauge), and inflation bulb and
valve, for manual instruments.
Operation
In humans, the cuff is normally placed smoothly and snugly around an
upper arm, at roughly the same vertical height as the heart while the
subject is seated with the arm supported. Other sites of placement
depend on species, and may include the tongue, flipper, tail or teat. 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.
Significance
By observing the mercury in the column while releasing the air
pressure with a control valve, one can read 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).