Blood pressure is measured by instrument called sphygmomanometer also called as blood pressure meter, or blood pressure gauge sphygmometer. It consists of an inflatable cuff, a measuring unit, and a mechanism for inflation which may be a manually operated bulb and valve or a pump operated electrically.
Manual and digital sphygmomanometers are two types of sphygmomanometers.
Manual sphygmomanometers are of two types – mercury and aneroid. Mercury sphygmomanometers are considered to be the gold standard. They measure blood pressure by observing the height of a column of mercury.
Aneroid sphygmomanometers are mechanical types and come with a dial. They require regular calibrations.
Digital sphygmomanometers use oscillometric measurements and electronic calculations rather than auscultation. . They also require frequent calibrations.
How to Measure Blood Pressure
Clothing should be removed from the arm. If it cannot be removed, it is better to leave it as it is, rather than fold the clothing into tight constricting bands.
The cuff should be encircled around the arm. If the bladder of the cuff does not encircle the arm completely, the centre of the bladder should be over the brachial artery. The rubber tubes from the bladder are usually placed inferiorly at the site of the brachial artery, but it would be better to place it superiorly or posteriorly so that the antecubital fossa is easily accessible for auscultation.
The bell of the stethoscope gives better sound reproduction but a diaphragm is easier to secure with the finger of one’s hand and covers a large area.
To measure BP in the legs a thigh cuff containing a large bladder (18 x 24 cms) for adults should be wrapped around the thigh of the prone patient and the Korotkoff sounds auscultated in the popliteal fossa in the usual way.
BP in the legs is equal to that in the arms provided the bladder is adequate in size.
- To begin blood pressure measurement, use a properly sized blood pressure cuff. The length of the cuff’s bladder should be at least equal to 80% of the circumference of the upper arm.
- Wrap the cuff around the upper arm with the cuff’s lower edge one inch above the antecubital fossa.
- Lightly press the stethoscope’s bell over the brachial artery just below the cuff’s edge.
- Rapidly inflate the cuff to 180mmHg. Release air from the cuff at a moderate rate (3mm/sec).
- Listen with the stethoscope and simultaneously observe the sphygmomanometer. The first knocking sound (Korotkoff) is the subject’s systolic pressure. When the knocking sound disappears, that is the diastolic pressure (such as 120/80).
- Record the pressure in both arms and note the difference; also record the subject’s position (supine), which arm was used, and the cuff size (small, standard or large adult cuff).
- If the subject’s pressure is elevated, measure blood pressure two additional times, waiting a few minutes between measurements.
- Explain the procedure to the patient to allay anxiety.
- Avoid exertion, meals or smoking for 30 minutes before BP is measured. The patient must be allowed to rest 5 minutes before BP is measured.
- High BP may be erroneously recorded in an obese person because the inflatable rubber bladder may be too short for the obese arm (Recommended dimensions are 12 x 35 cms). When the bladder does not completely encircle the arm the centre of the bladder must be placed directly over the brachial artery.
- The arm must be supported to the heart level. In the supine position the arm is usually at the heart level. In sitting and standing positions the arm must be horizontal with fourth intercostals space at the sternum. In normal people there is no significant difference in BP between supine, sitting and standing positions provided the arm is supported at the heart level. Some antihypertensive agents cause postural hypotension and when this is expected, BP must be measured in both lying and standing position.
- If the arm is unsupported, the patient will perform isometric exercises which may elevate the diastolic BP by 10 percent. This is especially so in hypertensive patients on beta-adrenergic blocking agents. To avoid this the arm must be supported.
- The BP may be higher in right arm by 2-10 mm Hg. Most pressures in practice are measured on the right arm. However if the BP is higher by 10 mm Hg in one arm further measurements should be made in that arm.
- The cuff should be snugly fitted to the arm. A cuff which is too tight may give a false lower blood pressure and a loose cuff may give a false higher BP.
- Repeated inflation of the cuff may cause venous congestion of the limb and elevate both systolic and diastolic BP. To avoid this the cuff should be inflated as rapidly as possible and deflated completely between successive readings. At least 15 seconds should be allowed between successive measurements.
Conditions Diagnosed by Measuring BP
- Pulsus paradoxus
- Pulsus alternates
- Coarctation of aorta (Hypertension in upper limb, hypotension in lower limb)
- Aortic incompetence (Hill’s sign)
Reason for Unequal Blood Pressure in Two Arms of Same Individual
- In normal individuals, BP may vary due to recording at different times in two arms.
- Supravalvular aortic stenosis (right sided higher BP)
- Preductal coarctation of aorta (right sided higher BP)
- Unilateral occlusive disease of the arteries-Atherosclerosis, embolism, aortoarteritis, thoracic outlet syndrome, etc. (BP will be low on the affected side)