Measurement principle of sphygmomanometer scheme:

The sphygmomanometer plans to measure the blood pressure using the oscillometric method, which is also called the pressure oscillation method.

Its working process is to inflate the cuff to block the arterial blood flow, and then detect the gas pressure in the cuff and extract the tiny pulse wave during the deflation process. As shown in the figure below, when the cuff pressure P is much higher than the systolic pressure, the pulse wave disappears, and as the cuff pressure decreases, the pulse begins to appear. When the cuff pressure increases from above the systolic pressure drop to below the systolic pressure Ps, the pulse wave will suddenly increase, reaching its maximum amplitude when uniformly pressing Pm. Then the pulse wave decays with the decrease in cuff pressure. Oscillographic blood pressure measurement is based on the relationship between pulse wave amplitude and cuff pressure to estimate blood pressure. The maximum value of the pulse wave corresponds to the uniform pressure Pm, and the systolic pressure Ps and diastolic pressure Pd are determined by the proportion of the corresponding maximum amplitude of the pulse wave.

When measuring blood pressure, as the pressure on the cuff decreases, the blood vessels change from blockage to conduction, which generates a series of small pulses in the cuff. Pick up the small pulse, connect its peak value into a curve, and get the envelope,. There are many detailed methods for using oscillography to determine systolic and diastolic blood pressure, which can be mainly summarized into two types: waveform characteristic method and amplitude coefficient method. The fundamental principle of the waveform feature method is to measure blood pressure by using the inflection point of the envelope of the pulse wave. The static pressure corresponding to the inflection point when rising is systolic pressure, and the static pressure corresponding to the inflection point when falling is diastolic pressure. This method has poor individual compliance and unstable measurement accuracy, and has gradually been replaced by the amplitude coefficient method.

The amplitude coefficient method is also known as the "normalization method". It is to normalize the amplitude of the pulse wave vibration signal compared to the maximum amplitude of the signal, and identify systolic and diastolic blood pressure by confirming the normalization coefficients of systolic and diastolic blood pressure, as shown in Figure 3. Among them, As is the amplitude of the pulse wave corresponding to systolic blood pressure, Am is the amplitude of the pulse wave corresponding to uniform blood pressure, Ad is the amplitude of the pulse wave corresponding to diastolic blood pressure, As/Am is the normalized value of systolic blood pressure Ps, Ad/Am is the normalized value of diastolic blood pressure Pd, Pc is the cuff pressure, and the horizontal axis represents the occasional decrease in pressure inside the cuff during the deflation process. As/Am=C1, Ad/Am=C2, corresponding to the positions of systolic and diastolic blood pressure, respectively. Based on the measured pulse amplitude and corresponding static pressure, systolic blood pressure Ps, diastolic blood pressure Pd, and mean blood pressure Pm can be obtained. The amplitude coefficient of normal systolic blood pressure is 0.46~0.64, and the amplitude coefficient of diastolic blood pressure is 0.43~0.73.