The McGill Physiology Virtual Lab

Cardiovascular Laboratory

Blood Pressure> Background Information
  Blood pressure can be measured by several techniques.  The direct method (refer to the diagram below) involves directly inserting a tube or catheter into a blood vessel.  The catheter is connected to a blood pressure transducer, which generates an electrical signal.
In this experiment, we measure the arterial blood pressure using two different methods.  Both of these methods are indirect, in that they do not involve inserting a catheter directly into the artery and connecting that catheter to a blood pressure transducer.
The first method uses the sense of touch:  it is thus called the palpatory method.
The second method uses the sense of hearing: it is thus called the auscultatory method.

The figure to the left shows a typical tracing of the blood pressure recorded from an artery using the direct method.  The maximum pressure is called the systolic pressure; the minimum pressure is called the diastolic pressure.  The pulse pressure is the difference between the systolic and diastolic pressures.  The mean pressure is given approximately by the sum of the diastolic pressure and one third of the pulse pressure.
A sphygmomanometer, an instrument that measures pressure, is needed in both methods.  Each sphygmomanometer consists of a cuff (containing a "bladder") which is connected by lengths of tubing to an inflating bulb with a needle valve and to an aneroid pressure gauge.

An appropriately sized cuff covers 2/3 of the biceps; the bladder is long enough to encircle >80% of the upper arm, and bladder width equals at least 40% of the upper arm's circumference.
Correct positioning of the cuff
The centre of the sphygmomanometer bladder should be placed over the brachial artery. Many cuffs have some sort of marking scheme so that placement over the brachial artery - under, or just medial to, the biceps tendon - is facilitated.  The lower border of the cuff should be ~2cm proximal to the antecubital fossa and the cuff should be firmly wrapped around the arm.

In addition, a stethoscope is needed for the auscultatory method.   Note that the chestpiece of the stethoscope has both a bell and a diaphragm.
To continue with the next section: Palpation Method, click here