User Manual for Patient Monitor
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7.2 The Principle for Measurement of the Blood Pressure
Blood pressure may be measured in an invasive manner (whereby the sensor will be inserted into blood
vessel
directly) or a non-invasive manner. The non-invasive methods include several methodologies, the
Korotkoff-souna method and oscillating method. The Korotkoff-souna method is used as a conventional way,
whereby stethoscope is used to measure the blood pressure. By the oscillating method, an inflation pump will fill
the cuff, and release the air slowly. A computer will record change of the cuff pressure when the air is released.
With this record, the blood pressure valve will be determined. First of all, make sure the signal quality obtained by
computer meets the requirements of accurate calculation (sudden limb movement or cuff being hit during the
measurement may invalidate the data). If the answer is negative, give up the calculation. If the answer is positive,
carry on the calculation of the blood pressure value.
As change of the blood pressure is recorded by electric sensor, which sensitivity is much higher than that of
human ears, the oscillating method uses different definitions for measurement of diastolic pressure, mean arterial
pressure and systolic pressure from the Korotkoff-souna method. When the oscillating method is used, the circuit
in the measuring equipment will separate the amplitude of the cuff pressure from its change with pulsation. With
the oscillating method, the blood pressure at the maximum amplitude of cuff pressure is defined as the mean
arterial pressure. The blood pressure at amplitude of cuff pressure forward reduced by 50% is defined as systolic
pressure, while the blood pressure at amplitude of cuff pressure backward reduced by 80% is defined as diastolic
pressure. The maximum change of pulse pressure occurs at these two points. They are equivalent to the point
with pulse sound and the point without pulse sound respectively in the Korotkoff-souna method.
When the risk of invasive monitoring method outweighs its advantage of accuracy, non-invasive monitoring
method shall be used.
7.2.1 Comparison between Blood Pressure Measuring Methods
To overcome the effect of human hearing variation and air release speed on measurement accuracy when
the
conventional Korotkoff-souna method is used to take measure of blood pressure, people have been dedicated to
study of automatic measurement of blood pressure. By now, automatic blood pressure measuring system based
on the principle of oscillating method is mature. In practice, however, various problems are encountered, why the
measures taken by the oscillating method is lower or higher than those taken by Korotkoff-souna method? Why
the measures are inclined to decline? Why, in some cases, no result is obtained in spite of the inflation actions?
Why the measure values have big discreteness, and even abnormal data in some cases? Why the pulse oxygen
waves may disappear suddenly? The following explanations are devised to give the answers.
The oscillating method vs. the Korotkoff-souna method
Blood pressure measurement by the oscillating method and Korotkoff-souna method has good correlation
with the invasive measurement. Notwithstanding, any of the non-invasive blood pressure measurements has its
weakness when it is compared to the invasive measurement. The oscillating method has its advantages over the
Korotkoff-souna method for its less error, higher reliability and stability. Their differences may be reflected in the
following aspects.
