The McGill Physiology Virtual Lab

Cardiovascular Laboratory

Blood Pressure> Auscultatory Method
  The relaxed subject sits on a chair with the lower arm supported as before.  The blood pressure cuff is placed on the subject's right arm, allowing 1 inch between the bottom of the cuff and the crease of the elbow. 
  The brachial pulse is palpated just above the angle of the elbow (the "antecubital fossa"). 

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.

The diaphragm is placed over the brachial artery in the space between the bottom of the cuff and the crease of the elbow. At this point no sounds should be heard. 

The cuff pressure is inflated quickly to a pressure about 30 mm Hg higher than the systolic pressure determined by the method of palpation.  Then the air is let out of the cuff at a rate such that cuff pressure falls at a rate of about 5 mm Hg/sec. 

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.



As the pressure is lowered further, the character of the Korotkoff sounds should change.  At some point, the sounds will disappear. 

The pressure reading at this point gives the diastolic pressure.

The subject should now lie on his or her back for five minutes.  The systolic pressure and diastolic pressure are recorded.   Then the subject stands up, and the pressures are immediately recorded once more.
Explanatory Notes

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. 

When measuring blood pressure using the auscultation method, turbulent blood flow will occur when the cuff pressure is greater than the diastolic pressure and less than the systolic pressure.  The   "tapping" sounds associated with the turbulent flow are known as Korotkoff sounds. Remember that these sounds are not to be confused with the heart sounds produced by the opening and closing of the heart valves.
Summary of the auscultatory method: 
Initially the cuff is inflated to a level higher than the systolic pressure.  Thus the artery is completely compressed, there is no blood flow, and no sounds are heard.  The cuff pressure is slowly decreased.  At the point where the systolic pressure exceeds the cuff pressure, the Korotkoff sounds are first heard and blood passes in turbulent flow through the partially constricted artery. Korotkoff sounds will continue to be heard as the cuff pressure is further lowered.  However,  when the cuff pressure reaches diastolic pressure, the sounds disappear.  Now at all points in time during the cardiac cycle, the blood pressure is greater than the cuff pressure, and the artery remains open.


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JNC Classification of blood pressure in adults


BP (mm Hg)

Normal systolic: less than 120
diastolic: less than 80
Pre-hypertension 120-139/80-89
Stage 1 hypertension 140-159 (systolic)
90-99 (diastolic)
Stage 2 hypertension equal 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

Errors in blood pressure readings:

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