47
Appendix A: Measuring Principle
A-1 Blood Pressure
Measuring principle
Wrap the cuff around the upper arm. Inflate the cuff
to a pressure exceeding the systolic blood pressure.
Then, exhaust the air from the cuff gradually. While
the pressure is detected in the cuff in the air
exhaustion stage, the pulse waveform appears in
synchronization with the heart rate.
The pulse waveform suddenly increases near the
systolic blood pressure. It increases further with
exhaustion until it reaches the highest in amplitude,
then decreases gradually. The changes in the pulse
waveform are illustrated to the right.
The oscillometric blood pressure measurement
evaluates the point where the amplitude increases
suddenly after the pulse in the cuff pressure is
detected, as the systolic blood pressure, the point
where the amplitude reaches the highest, as the
mean blood pressure, and the point where the amplitude decreases gradually and becomes small, as the
diastolic blood pressure.
Actually, the pressure sensor detects the subtle changes in the cuff pressure with time, stores the pulse
waveform in memory, and evaluates the systolic and diastolic blood pressures according to the
oscillometric measurement algorithm.
The details in the algorithm vary with the blood pressure monitor. Blood pressure values of adults and
infants, measured by the oscillometric method, compare with those measured by the auscultatory method.
Diastolic blood pressure is defined to be the end point of phase 4 in the auscultatory method.
The pulse waveform of the cuff pressure depends on the characteristics of the cuff material. Therefore,
using the specified cuff, the measurement accuracy is maintained according to the measurement
algorithm.
The length of the hose is to be within 3.5 m, in consideration of the damping characteristics due to pulse
wave propagation.
