# PPG-Based Systolic Blood Pressure Estimation Method Using PLS and Level-Crossing Feature
## 1 Introduction  
Hypertension is known as a risk factor of serious diseases such as stroke, myocardial infarction, and chronic kidney diseases [1]. Management of the blood pressure (BP) by daily monitoring can help improve an individual's lifestyle. However, a cuff-based BP measurement method is inconvenient because the measurement requires compression of the artery by the cuff. Hence, cuffless BP estimation has recently attracted attention [2; 3; 4]. There has been considerable effort to develop cuffless BP estimation methods over the years, and one of them is based on the photoplethysmography (PCG) and electrocardiography (ECG) signals. However, these BP estimation methods require two types of sensors to measure the ECG and PPG sensors, which increases the manufacturing costs of these devices. As the simplest method, studies have been carried out on developing cuffless BP estimation methods based only on PPG waveforms [5]. In PPG, the skin is irradiated by light from a light-emitting diode (LED) source. The PPG waveforms are obtained by converting the light transmitted or reflected to the PPG sensor, and the absorption characteristics of the PPG waveforms correspond to the blood volume in the blood vessels, particularly changes in the amount of hemoglobin [6]. PPG is considered as a useful physiological signal for cuffless BP measurement due to its inexpensiveness and simplicity of measurement. [PERSON] et al. reported two cases of the change in the second-derivative PPG waveform. One is the change in the second-derivative PPG waveform by medication with an antihypertensive agent and a vasopressor. The second is based on epidemiological studies of the presence or absence of several cardiovascular diseases including hypertension and the second-derivative PPG waveform [7]. Therefore, the BP can be estimated by extracting various features from the waveform [8; 9; 10; 11]. However, it is difficult to extract features from PPG waveforms owing to variations in the baseline and poor undulation of the waveform [12]. Hence, the first derivative of the photoplethysmography (FDPPG) and the second derivative of the photoplethysmography (SDPPG) are often used. Since the SDPPG has several characteristic peaks, it is often used instead of PPG. Figure 1 shows examples of (a) PPG, (b) FDPPG, and (c) SDPPG waveforms. The relative heights of the peaks of the SDPPG waveform are typically used as features for estimating the BP [8]; this set of features is called the height ratio feature (HRF). However, the peaks of the SDPPG waveform are prone to noise, and it is particularly difficult to extract these features for low peak heights [13; 14]. Figure 2a shows a typical SDPPG waveform, which consists of five waves, named successively as waves a, b, c, d, and e. These are used as the HRFs. Although clear peaks appear in the SDPPG waveforms of young adults with flexible blood vessels (such as that shown in Figure 2a), this is not the case for older adults who have lost flexibility in their blood vessels because of aging. In the latter case, there may be no clear undulations in the SDPPG waveforms, and/or the successive downstream peaks (waves c, d, and e) may not be present in the waveforms after filtering for noise reduction. Figure 2b shows an example of a loss of waves c and d. The disappearance of these peaks causes loss of the corresponding HRFs. Figure 1: Photoplethysmography (PPG) waveform and its derivative waveforms: (**a**) PPG waveform, (**b**) first-derivative PPG waveform, and (**c**) second-derivative PPG waveform. Thus, in this study, we used level-crossing features (LCFs) [15; 16] because they can be reliably extracted from the SDPPG waveforms unlike the HRFs, which are dependent on the peaks of the SDPPG waveforms. In our proposed method, contour lines are drawn at arbitrary intervals on the SDPPG waveform, and the number of crossings between each contour line and the waveform and the total length of each contour line lying within the waveform are extracted as the LCFs.