One group member puts on a stethoscope, with the earpieces on the headpiece angled forward. The recording end of the stethoscope is twisted, so that the diaphragm and not the bell is activated. This can be tested by tapping lightly on the diaphragm.
At some point the person listening with the stethoscope will begin to hear sounds with each heartbeat. This point marks the systolic pressure.
The sounds are called Korotkoff sounds.
The pressure reading at this point gives the diastolic pressure.
The laminar flow that normally occurs in arteries produces little vibration of the arterial wall and therefore no sounds. However, when an artery is partially constricted, blood flow becomes turbulent, causing the artery to vibrate and produce sounds.
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JNC Classification of blood pressure in adults
Classification
BP [mm Hg]
Normalsystolic: less than 120diastolic: less than 80Pre-hypertension120-139/80-89Stage 1 hypertension140-159 [systolic]
or
90-99 [diastolic]Stage 2 hypertensionequal or more than 160 [systolic]
equal or more than 100 [diastolic]
JNC: Joint National Committee on Prevention, Detection, Evaluation,
and treatment of high blood pressure
The cuff is not of the proper size: if the cuff is too small the blood pressure readings may be artefactually high. If the cuff is too big, the readings may be artefactually low.
The cuff is positioned too loosely: the blood pressure may be artefactually high.
The centre of the cuff bladder is not positioned over the brachial artery.
The cuff is inflated slowly: a slow inflation causes venous congestion, which in turn causes the Korotkoff sounds to be faint; this results in false readings with the systolic value being too low and the diastolic reading too high.
If the cuff is re-inflated immediately after an initial reading [trying to re-check the reading]: a rapid re-inflation could cause venous distension, the Korotkoff sounds become more muffled. The initial Korotkoff sound may be missed so the systolic reading would be falsely low, and the diastolic reading would be falsely high because the last Korotkoff sounds could not be heard. To continue with the next section: the Electrocardiogram, click here