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8.2 NIBP Monitoring
8.2.1 Measuring Principle
Blood pressure may be measured in an invasive way (whereby the sensor will be inserted into blood vessel
directly) or a non-invasive way. The non-invasive way includes several methodologies, such as the Korotkoff
Sound Method and oscillating method. The Korotkoff Sound 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
air, and release it slowly. A computer will record change of the cuff pressure when the air is released. With
this record, the blood pressure value will be determined. First of all, make sure the signal quality judgment by
computer meets the requirements of accurate calculation (such as sudden limb movement or cuff being hit
during the measurement). If the answer is negative, give up the calculation. If the answer is positive, proceed
with 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 ar-
terial pressure and systolic pressure from the Korotkoff Sound Method. When the oscillating method is used,
the circuit in the measuring apparatus will separate the amplitude of the cuff pressure from its change with pul-
sation. 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 according
to proper proportion is defined as systolic pressure, while the blood pressure at amplitude of cuff pressure
backward reduced according to proper proportion 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 Sound Method.
When the risk of invasive monitoring method outweighs its advantage of accuracy, non-invasive monitoring
method shall be used.
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 Sound 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 encounte-
red, such as why the measures taken by the oscillating method is lower or higher than those taken by Korotkoff
Sound 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 ca-
ses? Why the SpO
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waveforms may disappear suddenly? ...and so on. The following explanations are devised
to give the answers.
The Oscillating method vs. the Korotkoff Sound Method
Blood pressure measurement by the oscillating method and Korotkoff Sound Method has good correlation
with theinvasive measurement. Notwithstanding, any of the non-invasive blood pressure measurements has
its one-sidedness when it is compared to the invasive measurement. The oscillating method has its advan-
tages over the Korotkoff Sound Method in less error, higher reliability and stability. Their differences may be
reflected in the following aspects.
1. The measures by the Korotkoff Sound Method are liable to effect of human factors. For example, different
people may have different sound judging ability, or different reactivity when listening to heart sound and re-
ading mercury meter. The air release speed and subjectivity may also affect the judgment. By the oscillating
method, the computation is accomplished by the computer, thus relieving the possibility of effect due to
human factor.
2. With the Korotkoff Sound Method, the measure is taken on the basis of appearance and disappearance of
heart sound. The air release speed and heart rate may have direct effect on the measurement accuracy. It
also has the disadvantages of rapid air release and poor accuracy. In the contrast, with the oscillating me-
thod, the determination is calculated on the basis of cuff pressure oscillatory waveform envelope, and the
air release speed and heart rate has little effect on the measurement accuracy.
3. Statistics show that, when measuring the hypertension, the measure taken by the oscillating method is likely
to be lower than that taken by the Korotkoff Sound Method. When measuring the hypotension, the measure
taken by the oscillating method is likely to be higher than that by the Korotkoff Sound Method. But, it doesn’t
mean the advantages or disadvantages between the oscillating method and the Korotkoff Sound Method.
Comparison with the results taken by more accurate method, let’s say comparison of the invasive pressure
result with the output value by the blood pressure measuring simulator, will show which method has more
accurate results. In addition, higher or lower value should be a statistical concept. It is recommended those
used to adopt the Korotkoff Sound Method use different physiological calibration for values determined by
the oscillating method.