The photoelectric blood pressure measurement technology combined with ECG and PPG is to estimate blood pressure by using the time interval between the transmission of the same pulse wave from ECG R wave to PPG feature points as PTT. As long as the elastic modulus of the vascular wall remains constant, the change of systolic blood pressure (SBP) can be expressed by the high-frequency component, in which the high-frequency component is calculated by the ECG R wave as the time interval PTT from the PPG characteristic point, and the low-frequency component is the systolic blood pressure data measured intermittently by auscultation or oscillography. The systolic blood pressure is calculated by combining the high-frequency component with the low-frequency component
The calculation formula of blood pressure can be deduced
V represents the pulse wave velocity, K represents the distance, and T represents the propagation time. Since the thickness a and internal diameter D of the arterial wall can be ignored, and the elastic E0 of the arterial wall changes slowly enough, it can be obtained by deriving from both sides of equation (10)
Therefore, the high frequency component can be expressed as
The systolic blood pressure is calculated as PE = Pb+ Δ P. Where Pb represents the blood pressure measured by Oscillographic method or auscultation method. However, the low-frequency component will change slowly. This method needs to recalibrate the low-frequency component after a period of time. Twenty cardiovascular patients were tested and compared with invasive methods. The results showed that the correlation coefficient of systolic blood pressure obtained by the two methods was 0.97 ± 0.02 (mean difference ± standard deviation), and the monitoring data with an error range of 10% accounted for 97% of the total data (ECG R wave is the time interval from the starting point to the bottom of PPG waveform, as shown in Figure 1 (a)) and systolic and diastolic blood pressure (DBP). Finally, it is concluded that the correlation between pttf and SBP and PBP (the difference between systolic and diastolic blood pressure) (as shown in Figure 1 (b)) is better than PTTP
According to the bramble Hill model, heard et al. Deduced the relationship between pulse wave propagation time CDX and systolic and diastolic blood pressure. In order to improve the calculation accuracy of diastolic blood pressure, human heart rate signal was also introduced into the diastolic blood pressure calculation formula as a parameter , i.e
In the formula, psi and PDI represent systolic blood pressure and diastolic blood pressure respectively, IHRI represents the instantaneous heart rate of the ith pulse wave, KS, KD and KHR are fixed constants, and K sys_cal and kdis_cal are calibration parameters. The blood pressure values obtained by this method are compared with those measured by oscillometric method. The results of mean difference and standard deviation are: SBP, 4.0 ± 9.6 mmHg; DBP, less than 1.5 ± 6.4 mmHg; MB P. Less than 1.5 ± 6.3 mmHg. Based on this method, huichenghe technology has designed a wearable medical device "heart rate and blood pressure watch" , it can monitor ECG and blood pressure in real time. Because ECG and PPG sensors are designed on the watch, the mobility of the monitoring device is improved. In addition, the device can collect and establish a database of physiological parameters of the tested object, and manage and test blood pressure at any time through smartphone app. Compared with electronic sphygmomanometer, the accuracy of systolic blood pressure estimated by the device is 94.6%, The accuracy of diastolic blood pressure was 92.35%, and the accuracy of blood pressure measurement reached ± 3ppm, which completely reached the medical level.
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