Blood Pressure Monitor Measurement Methods
Blood pressure is the indication of life, blood pressure measurement device is the most used clinical medical equipment. As a result of advances in modern electronics, blood pressure measurement devices from the most simple sphygmomanometer, blood pressure meter to the development of today's various blood pressure monitor, in addition to measuring blood pressure, can still guard other various physiological majority. Digital blood pressure monitoring allows physicians to diagnose high blood pressure and help their patients to control high blood pressure. The portable blood pressure monitor allows patients to economically measure blood pressure without a doctor at home, thus contributing to early diagnosis and high blood pressure control. Family care can also help physicians to distinguish between white coat hypertension (diagnosed indoor blood pressure is always increased and the diagnosis of normal outdoor blood pressure) and true high blood pressure.
A blood pressure monitor is a device used to measure arterial pressure when the blood is pumped away from the heart. Typically, from a user's point of view, the monitor includes an inflatable cuff to limit blood flow and a pressure gauge to measure blood pressure. From a system designer's point of view, a blood pressure monitor is much more complicated. Its components include: power supply, motor, reservoir, pressure sensor and user interface.
A small change in the pressure in the cuff can be noticed when the cuffs around the patient's arms are slowly released. These fluctuations are caused by the patient's rhythm cycle, and then it is amplified and shifted by a 1 Hz high-pass filter to produce a blood pressure curve. This new signal is the heartbeat signal. Using the previously described heartbeat assay method, systolic blood pressure (SBP) and vasodilatation (DBP) can be measured by simple oscillometry, which is used by most automated noninvasive blood pressure monitoring devices. When the cuff is inflated to above the systolic pressure, and then slowly deflated, the amplitude of the pressure change in the cuff is measured. When the pressure is lower than the blood vessel systolic blood pressure, this amplitude will suddenly increase. When the pressure in the sleeve is further reduced, the pulse amplitude reaches the maximum and decreases rapidly. Diastolic blood pressure is obtained at the beginning of this rapid change. SBP and DBP are therefore obtained by defining the rapid rise area (SBP) and the falling area (DBP) of the pulse amplitude. Mean arterial blood pressure (MAP) is at the maximum rate. Measurement of SBP and DBP can help diagnose normal hypertension, but only by clinical monitoring can not distinguish between two types of hypertension.
Resolution of a bit up and down, through this method can avoid going rounded. The state of the least significant bit of the transition is randomly jittering between 0 and 1, rather than at a value. By introducing tiny noise, the extensible ADC can convert the effective range of the signal, rather than simply removing all the signals at this low level. Again, this introduces quantitative errors throughout the range. Jitter only increases resolution and improves linearity, but does not improve accuracy. However, by increasing the noise of the least significant bits of 1 to 2 bits in the signal and using oversampling techniques, the accuracy can be improved.
Oversampling is the process of collecting signals through a sampling rate that is significantly higher than the Nyquist sampling frequency. In fact, oversampling is used to obtain high-resolution ADC converters. For example, a 16-bit conversion can be performed using a 12-bit converter running at 256 times the target sample rate. For each additional resolution bit, the signal must be oversampled 4 times. Because real-world ADCs can not be uninterrupted, the input value should remain constant during the conversion of the converter.
The sample and hold circuit accomplishes the task by storing the analog voltage at the input with a capacitor and disconnecting the capacitor from the input with an electronic switch. Using an ADC that sets the sampling and hold time that best fits the input signal is helpful in improving the accuracy of the conversion results.