# PPG-Based Systolic Blood Pressure Estimation Method Using PLS and Level-Crossing Feature
## 5 Conclusions  
In this study, we proposed a BP estimation method based on PPG waveforms to realize stress-free BP measurements. In this method, LCFs were extracted from SDPPG waveforms, and PLS regression was used to establish a model to estimate the SBP on the basis of these features. We compared the estimation accuracy of our method with those of HRFs + MRA and LCFs + MRA. The results confirmed that our method has a superior estimation accuracy. Although the SBP estimation method proposed in this study has not reached an accuracy sufficient for clinical use, only the LCFs are used, and we did not use any other information (gender, body characteristics, etc.). In order to improve the accuracy of the proposed method, it is conceivable to use the information of subjects other than LCFs. [PERSON] and [PERSON] conceived this study. [PERSON] programmed the experiments and wrote the manuscript. [PERSON] measured the PPG and BP data of the experiments and improved the manuscript from a medical point of view. Finally, [PERSON] supervised the experiments, reviewed the manuscript, and contributed to the final version. This research was funded by JSPS KAKENHI Grant Number JP16K00393. The authors declare no conflict of interest. * _[PERSON] et al. (2014)_ [PERSON]; [PERSON]; [PERSON]; [PERSON]; [PERSON] Noninvasive cuff'less estimation of blood pressure using photoplethysmography without electrocardiogram measurement. In Proceedings of the IEEE TENSYMP, Kuala Lumpur, Malaysia, 14-16 April 2014; pp. 254-257. * _[PERSON] (2007)_ [PERSON] Photoplethysmography and its application in clinical physiological measurement. _Physiol. Meas._**2007**, _28_, 1-39. [CrossRef] [PubMed]
* _[PERSON] et al. (1998)_ [PERSON] [PERSON]; [PERSON] [PERSON]; [PERSON] [PERSON]; [PERSON] [PERSON]; [PERSON] [PERSON]; [PERSON] [PERSON]; [PERSON] [PERSON]; [PERSON] [PERSON] Assessment of vasoactive agents and vascular aging by the second derivative of photoplethysmogram waveform. _Hypertension_**1998**, _32_, 365-370. [CrossRef] [PubMed]
* _[PERSON] et al. (2002)_ [PERSON]; [PERSON]; [PERSON] [PERSON]; [PERSON]; [PERSON]; [PERSON] [PERSON]; [PERSON] [PERSON]; [PERSON] [PERSON]; [PERSON] [PERSON]; [PERSON]; et al. Pulse wave velocity and the second derivative of the finger photoplethysmogram in treated hypertensive patients: Their relationship and associating factors. _J. Hypertens._**2002**, _20_, 2415-2422. [CrossRef] [PubMed]
* [PERSON] and [PERSON] (2017) [PERSON]; [PERSON] Feasibility study for the non-invasive blood pressure estimation based on ppg morphology: Normotensive subject study. _Biomed. Eng. Online_**2017**, _16_, 1-14. [CrossRef] [PubMed]
* _[PERSON] et al. (2018)_ [PERSON]; [PERSON]; [PERSON]; [PERSON]; [PERSON]; [PERSON] New photoplethysmogram indicators for improving cuffless and continuous blood pressure estimation accuracy. _Physiol. Meas._**2018**, _39_, 025005. [CrossRef] [PubMed]
* _[PERSON] et al. (2014)_ [PERSON]; [PERSON]; [PERSON]; [PERSON] Estimating blood pressure using Windkessel model on photoplethysmogram. In Proceedings of the IEEE EMBC, Chicago, IL, USA, 26-30 August 2014; pp. 4567-4570. * _[PERSON] (2012)_ [PERSON] On the analysis of fingertip photoplethysmogram signals. _Curr. Cardiol. Rev._**2012**, \(8\), 14-25. [CrossRef]
* _[PERSON] et al. (2007)_ [PERSON]; [PERSON]; [PERSON]; [PERSON]; [PERSON] Second derivative of photoplethysmography for estimating vascular aging. In Proceedings of the IEEE ITAB, Tokyo, Japan, 8-11 November 2007; pp. 70-72. * _[PERSON] (2014)_ [PERSON] Detection of c, d and e waves in the acceleration photoplethysmogram. _Comput. Methods Progr. Biomed._**2014**, _117_, 125-136. [CrossRef]
* _[PERSON] (2009)_ [PERSON]; [PERSON] Multiclass MTS for saxophone timbre quality inspection using waveform-shape-based features. _IEEE Trans. Syst. Man Cybern._**2009**, _39_, 690-704. [CrossRef] [PubMed]
* _[PERSON] et al. (2016)_ [PERSON]; [PERSON]; [PERSON]; [PERSON] Analyzing the relationship between contour-line-based feature and blood pressure. _IEEE Trans. Electr._**2016**, _137_, 652-657.