# PPG-Based Systolic Blood Pressure Estimation Method Using PLS and Level-Crossing Feature
## 4 Discussion  
The results suggest that our method gives a better estimation accuracy. The explanatory ability of the BP estimation model in comparison method 1 is weak owing to the small number of HRFs (4), which we believe leads to the low estimation accuracy. Even though seven LCFs were selected in comparison method 2, these features comprise those with relatively strong correlations, which we believe reduces the explanatory ability of the SBP estimation model.  
In this study, we obtained PPG data for 20 s from 265 subjects. Among them, 20% of the HRFs could not be acquired in some cases. In contrast, all LCFs have been acquired. In other words, the proposed LCFs are considered to be superior to HRFs in both accuracy and robustness.  
We evaluated the BP estimation accuracy of the proposed method with the standard of the British Hypertension Society (BHS) [21]. Table 2 summarizes the BHS BP grading system. In the BHS standard, the BP value by auscultation is used as the reference BP value, but in this study, the SBP value by the automatic oscillometric BP monitor was used as an alternative. On this basis, we referred to the estimation accuracy of the SBP of the proposed method to the BHS standard. With the proposed method for the tested subjects, the cumulative percentage of subjects whose SBP absolute error was within 5 mmHg was 38%, at 10 mmHg was 78%, and within 15 mmHg was 91%. As a result, the grade of the proposed method is D. Therefore, the SBP estimation method proposed in this paper is not suitable for clinical use. However, the proposed LCFs show a better result than the conventional HRFs. LCFs are thought to be used instead of HRFs in future PPG-based cuffless BP estimation methods.  
From this viewpoint, we compare the proposed method with previous cuffless BP estimation methods. First, we will describe the features of our proposed method. Calibration is unnecessary before SBP estimation, and information other than PPG (height, weight, age, and sex) is unnecessary for estimating the SBP. In the BP estimation method of [PERSON] et al. [9], the correlation between the BP and the proposed index, which is a combination of the PPG features and the height of a subject, was analyzed. The correlation coefficient between the index proposed in [PERSON] et al. [9] and the SBP is 0.826, which is higher than the result of this study (0.78). However, in [PERSON] et al. [9], a correlation analysis was performed within a single dataset. On the other hand, in this study, the accuracy (correlation) was checked by dividing the training data and testing data. A cuffless BP estimation method that constructs an estimation model for each individual was proposed in [PERSON] et al. [10]. However, this method requires periodic BP calibration. In [PERSON] et al. [11], a cuffless BP estimation method based on the Windkessel model was proposed. In this method, the two parameters of Windkessel model are estimated using PPG features. The number of PPG waveform features used in this method was relatively small--four. In our proposed method using PLS regression, six scores (i.e., combined LCFs) related to the SBP were obtained. These scores may improve the estimation accuracy using a small number of features, as in [PERSON] et al. [11].  
As a limitation, the PPG data were acquired in this study in the sitting posture at rest. The acquisition site was the fingertip. Therefore, it cannot be used for acquisition during exercise. There is no study on implementations on smart watches etc. worn on the wrist.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Grade**} & \multicolumn{2}{c}{**Absolute Difference between Standard and Test Device (\%)**} \\ \cline{2-4}  & \(\leq\)**5 mmHg** & \(\leq\)**10 mmHg** & \(\leq\)**15 mmHg** \\ \hline A & 80 & 90 & 95 \\ B & 65 & 85 & 95 \\ C & 45 & 75 & 90 \\ D & & Worse than C & \\ \hline \hline \end{tabular}
\end{table}
Table 2: British Hypertension Society blood pressure measurement grading system.