Blood pressure (BP) is measured by placing an inflatable
cuff around the upper arm, inflating the cuff to a pressure higher than the blood
pressure, then gradually deflating the cuff as one listens with a stethoscope to the
crook of the arm for the pulse. The pressures when the pulse first appears, and when
the pulse drops off or changes quality are recorded as the systolic and diastolic
pressures, respectively. The pulse can be determined by listening during the blood
pressure determination, or by feeling the pulse at the
wrist.
A machine can do these functions. The machine
inflates and deflates the cuff, and measures the pulses using a Doppler device. The
computer also keeps track of the pulse rate.
Since you are
using a machine in your study, you should read the manufacturer’s manual to determine
the pitfalls and inaccuracies expected. Follow the manufacturer’s instructions exactly
to obtain optimal results. Also, repeated measurements are
helpful.
An experienced examiner must teach the correct
manual technique for BP measurement. It is important to recognize the somewhat subtle
pulse change that marks the correct diastolic pressure. The rate and degree of cuff
inflation, as well as the bleed-off speed are
important.
Other important considerations (and therefore
sources of error) are as follows:
Cuff size: A larger
patient needs a larger cuff.
Patient position: The patient
should be seated comfortably with arm at side in a comfortable, loosely flexed position,
free of obstructing clothing.
Clothing: Do not place the
cuff over, or listen through clothing.
Multiple readings:
Always a good idea to obtain an accurate reading.
Compared
to BP, pulse measurement is far less subject to error. Again, repeated measurements
(either manually or by machine) are recommended.
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