Ⅴ. Implementation of continuous blood pressure monitoring systems
5.1 Portable continuous BP monitoring system with minimized DSP
5.1.2 Experimental results
As shown in figure 5.3, The pulse transit time and blood pressure were measured by four subjects under various conditions such as rest and exercise conditions in order to find out the relationship between the PTT and BP, where the BP was measured using a commercial device and PTT was measured using the developed sensor module. Blue points are the PTT results which are obtained from the ECG and PPG peaks measured from the analog peak detectors, and red points are the PTT results which are obtained from the ECG and PPG peaks obtained using a digital processing for peak extraction from the ECG and PPG waveforms.
The measured relationship between the PTT and BP are different because the four subjects have different physical characteristics. For example, subject 1 has a PTT of about 0.14s when the measured systolic BP is around 120mmHg, but subject 2 has a PTT of about 0.17s when the measured SBP is around 120mmHg. On the other hand, as mentioned in the chapter 4.5, since the peak detector outputs the peak signal slightly before the peak point of the input signal, the PTT value obtained using the peak detector is slightly shorter than the PTT obtained using the DSP. Therefore, in the results of the four measurements, there is a tendency that the blue points have a smaller value than the red points.
Figure 5.3. Relationship between the measured systolic blood pressure and pulse transit time using the portable BP monitoring system for four subjects.
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Figure 5.4. Relationship between the measured SBP and estimated SBP, and their Bland-Altman plots for four subjects using the portable BP monitoring system.
[Correlation plot : Subject 1]
(a)
(b)
(c)
(d)
[Bland-Altman plots : Subject 1]
[Correlation plot : Subject 2] [Bland-Altman plots : Subject 2]
[Correlation plot : Subject 3] [Bland-Altman plots : Subject 3]
[Correlation plot : Subject 4] [Bland-Altman plots : Subject 4]
STD = 4.94mmHg STD = 4.72mmHg
R = 0.9302 R = 0.9364
STD = 2.50mmHg STD = 2.30mmHg R = 0.8295
R = 0.8585
STD = 8.01mmHg STD = 5.87mmHg
R = 0.8625 R = 0.9288
STD = 4.92mmHg STD = 5.19mmHg R = 0.9215
R = 0.9125
+2· STD
-2· STD
+2· STD
-2· STD
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As mentioned in the chapter 2.4.2, various mathematical models were studied for blood pressure estimation, and equation (5.1) shows one of them.
BP = A· ln(PTT) + B (5.1)
The subject dependent coefficients A and B which are optimized for each subject were obtained using the measurement results in the figure 5.3 and the equation (5.1), then estimated SBP values were obtained. Figure 5.4(a) show a correlation plot and its Bland-Altman plots between the measured and estimated systolic blood pressure for the subject 1, where the correlation coefficient (R) is calculated as 0.9302 and 0.9364 with the calculated PTT from the peak detector and DBP respectively. Bland-Altman plots show the difference between measured SBP and estimated SBP, and each plot obtained by the two methods has a similar graph. It is believed that the peak obtained from the analog peak detector was measured with a similar to the peak obtained by DSP, and it means that even in a portable continuous BP monitoring system using an analog peak detector, the BP estimation results have satisfactory accuracy without peak extraction using DSP. Also, it can be seen that the results measured by the two methods are quite similar in figure 5.4(b).
However, in figure 5.4(c), the result obtained from the peak detector has significantly lower correlation and higher standard deviation (STD) than the result obtained from the DSP, which is thought that the peaks obtained from the analog peak detectors have significant errors due to factors such as signal fluctuation, noise, and small peak amplitude. In figure 5.4(d), the result obtained from the peak detector has slightly higher correlation and lower STD than the result obtained from DSP, but it is not considered a significant result because the difference is small.
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Figure 5.5. Relationship between the measured SBP and estimated SBP, and their Bland-Altman plots for four PTT-BP equations using the portable BP monitoring system.
[Correlation plot : Equation 1]
(a)
(b)
(c)
(d)
[Bland-Altman plots : Equation 1]
[Correlation plot : Equation 2] [Bland-Altman plots : Equation 2]
[Correlation plot : Equation 3] [Bland-Altman plots : Equation 3]
[Correlation plot : Equation 4] [Bland-Altman plots : Equation 4]
STD = 4.94mmHg STD = 4.72mmHg
R = 0.9302 R = 0.9364
STD = 5.34mmHg STD = 4.94mmHg
R = 0.9179 R = 0.9301
STD = 5.12mmHg STD = 4.83mmHg
R = 0.9248 R = 0.9333
STD = 5.19mmHg STD = 4.86mmHg
R = 0.9225 R = 0.9326
+2· STD
-2· STD
+2· STD
-2· STD
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Figure 5.5 shows a correlation plots and their Bland-Altman plots between the measured and estimated systolic blood pressure for four PTT-BP equations, and all results were analyzed using the measured results of subject 1 in figure 5.3. The result in figure 5.5(a) is obtained using the equation (5.1), and each result in figure 5.5(b, c, d) are obtained using the equations (5.2, 5.3, 5.4) as follows.
BP = A
PTT2+ B (5.2)
BP = A·PTT + B (5.3)
BP =SBP0
3 +2·DBP0
3 + A·ln (PTT0
PTT) +2(SBP0− DBP0)
3 (PTT0
PTT)
2 (5.4)
From the whole results in figure 5.5, it is confirmed that all the results using the four PTT-BP equations had fairly similar correlation coefficients and standard deviations. It means that there is no significant difference of the BP calibration accuracy even if the BP calibration is performed using various PTT-BP equations when using the same PTT-BP measurement data. The table Ⅴ shows a summary of the BP estimation using the portable BP monitoring system.
Table Ⅴ
Summary of BP estimation results using the portable BP monitoring system.
PTT calculation Subject PTT-BP equation
(4.1) (4.2) (4.3) (4.4)
Using the peaks obtained by peak detector
Subject 1 R = 0.9302 STD = 4.94
R = 0.9179 STD = 5.34
R = 0.9248 STD = 5.12
R = 0.9225 STD = 5.19 Subject 2 R = 0.8295
STD = 2.50
R = 0.8433 STD = 2.41
R = 0.8389 STD = 2.44
R = 0.8391 STD = 2.44 Subject 3 R = 0.8625
STD = 8.01
R = 0.8592 STD = 8.10
R = 0.8670 STD = 7.89
R = 0.8529 STD = 8.28 Subject 4 R = 0.9215
STD = 4.92
R = 0.9183 STD = 5.02
R = 0.9213 STD = 4.93
R = 0.919 STD = 5.00
Using the peaks obtained by
DSP
Subject 1 R = 0.9364 STD = 4.72
R = 0.9301 STD = 4.94
R = 0.9333 STD = 4.83
R = 0.9326 STD = 4.86 Subject 2 R = 0.8585
STD = 2.30
R = 0.8612 STD = 2.28
R = 0.8605 STD = 2.28
R = 0.8605 STD = 2.28 Subject 3 R = 0.9288
STD = 5.87
R = 0.9375 STD = 5.51
R = 0.9380 STD = 5.48
R = 0.9380 STD = 5.48 Subject 4 R = 0.9125
STD = 5.19
R = 0.9330 STD = 4.56
R = 0.9276 STD = 4.74
R = 0.9300 STD = 4.66
* Correlation coefficient has no dimension.
** Standard deviation (STD) has a dimension of mmHg.
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