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What is the Principle of a Sphygmomanometer?

The measurement method of sphygmomanometer is divided into direct measurement method and indirect measurement method, and the principle will be different depending on the measurement method.


The direct measurement method is to place a catheter in the blood vessel through a puncture to obtain the blood pressure value. This measurement method will vary according to the measured part, and it cannot fully reflect the blood pressure of the human body. It can only be used to monitor blood pressure during the diagnosis and treatment of some diseases.


The indirect measurement method is subdivided into auscultation method and oscillometric method.


The auscultation method is our common method of measuring blood pressure, which is to measure it with a pressure gauge and a stethoscope.


The sphygmomanometer used in the auscultation method to measure blood pressure is composed of a balloon, a cuff, and a pressure gauge. The two rubber bladder tubes of the cuff are respectively connected with the balloon and the pressure gauge, and the three form a closed pipeline system.


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The blood pressure measured by the auscultation method can only be an approximate value, and its accuracy has a certain relationship with the measurement technology. When measuring, the binding cuff should be flat, so that the zero point of the upper arm, the heart and the calibrator should be kept at the same level as much as possible, and the deflation should not be too fast, otherwise a larger error will occur.


The oscillometric method is a relatively advanced electronic measurement method developed in the 1990s.


The oscillometric method is a relatively advanced electronic measurement method developed in the 1990s.


First, tie the cuff to the arm and inflate the cuff automatically. After reaching a certain pressure, stop the pressure and start to deflate. When the air pressure reaches a certain level, the blood flow can pass through the blood vessels, and there is a certain oscillating wave.


The oscillating wave propagates to the pressure sensor through the trachea, and the pressure sensor can detect the pressure and fluctuations in the measured cuff in real time. Gradually deflate, the oscillation wave gets bigger and bigger. As the contact between the cuff and the arm becomes looser, the pressure and fluctuations detected by the pressure sensor become smaller and smaller.

Choose the moment with the largest fluctuation as the reference point. Based on this point, look forward to the peak 0.45 fluctuation point, which is the systolic blood pressure; look backward to find the peak 0.75 fluctuation point, and the pressure corresponding to this point is the diastolic blood pressure. And the pressure corresponding to the point with the highest fluctuation is the average pressure.


However, the constants of 0.45 and 0.75 will vary depending on the instrument, and should be based on clinical test results.


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