View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by UWL Repository Pervasive blood pressure monitoring using Photoplethysmogram (PPG) Sensor Farhan Riaza, Muhammad Ajmal Azadb, Junaid Arshadc, Muhammad Imrand, Ali Hassana, Saad Rehmana aDepartment of Computer and Software Engineering, National University of Science and Technology, Islamabad, Pakistan bDepartment of Computer Science and Mathematics, The University of Derby, Derby, United Kingdom cSchool of Computing and Engineering, University of West London, London, United Kingdom dCollege of Applied Computer Science, King Saud University, Saudi Arabia Abstract Preventive healthcare requires continuous monitoring of the blood pressure (BP) of patients, which is not feasible using conventional methods. Photoplethysmogram (PPG) signals can be effectively used for this purpose as there is a physiological relation between the pulse width and BP and can be easily acquired using a wearable PPG sensor. However, developing real-time algorithms for wearable technology is a significant challenge due to various conflicting requirements such as high accuracy, computationally constrained devices, and limited power supply. In this paper, we propose a novel feature set for continuous, real-time identification of abnormal BP. This feature set is obtained by identifying the peaks and valleys in a PPG signal (using a peak detection algorithm), followed by the calculation of rising time, falling time and peak-to-peak distance. The histograms of these times are calculated to form a feature set that can be used for classification of PPG signals into one of the two classes: normal or abnormal BP. No public dataset is available for such study and therefore a prototype is developed to collect PPG signals alongside BP measurements. The proposed feature set shows very good performance with an overall accuracy of approximately 95%. Although the proposed feature set is effective, the significance of individual features varies greatly (validated using significance testing) which led us to perform weighted voting of features for classification by performing autoregressive modeling. Our experiments show that the simplest linear classifiers produce very good results indicating the strength of the proposed feature set. The weighted voting improves the results significantly, producing an overall accuracy of about 98%. Conclusively, the PPG signals can be effectively used to identify BP, and the proposed feature set is efficient and computationally feasible for implementation on standalone devices. Keywords: Butterworth filters, Averaging filters, Classification, Photoplethysmogram (PPG). 1. Introduction One way to measure the BP is through arterial tonome- ter [5, 6]. The heart-to-beat waveform can be measured Wearable health monitoring technologies (smart watches, from arterial tonometer but is strongly subject to mo- fitness tracker, smart wristbands) are becoming widely tion artifacts caused by high sensitivity, sensor placement used by consumers for tracking and monitoring their health and movement [7]. Cuff sphygmomanometer is the most conditions. It is estimated that more than 240 million standard non-invasive technique that is used to measure wearable devices will be shipped annually by the year BP [8]. Continuous measurement of BP using this tech- 2021 [1] with an estimated revenue to cross US$ 30 Bil- nique requires periodic cuff inflation and deflation, thus is lion. Today, modern wearable devices can perform so- not suitable for the continuous measurement of BP. The phisticated tasks and constantly monitor health indica- correlation between BP and Pulse Transit Time (PTT) has tors, such as glucose measurements in the blood, heart rate also been used to estimate the BP [9, 10, 11]. Specifically, variability, blood pressure readings, calories burned, steps PTT is the time interval for the arterial pulse pressure walked etc. Heart rate variability (HRV) is the most im- wave to travel from the aortic valve to the peripheral site. portant parameter for predicting major cardiovascular dis- It is widely known that the PTT varies inversely with BP eases in both the general population and patients. Nowa- changes and can be used for the cuffless and continuous es- days, non-invasive detective methods have become a ba- timation of BP. Usually, the time interval between R-wave sic requirement for the wearable medical devices, espe- in the electrocardiography (ECG) signal and the peak of cially in the scenario, when devices are used for the per- the peripheral pulse is selected as the PTT [12]. The pe- sonal health monitoring without assistance from the nurs- ripheral pulses are recorded at the fingertips using PPG ing staff [2, 3, 4]. The bio-signal is highly related to the signals sensors (Fig. 1). cardiovascular conditions and thus can be an efficient way The risk of having hypertension (BP issues) is no longer to detect the heart diseases by estimating the Blood Pres- imminent only in aged patients but is also the younger peo- sure (BP). Preprint submitted to Journal Name February 26, 2019 ple are also having the risk of cardiovascular diseases [13]. To identify cardiovascular events, the Electrocardiogram signal has been widely used for tracking abnormalities in patients. However, to collect the ECG parameters, it is necessary that several bio-electrodes are placed at various parts of the human body which might become complex in emergency conditions and also requires the assistance of technical personnel. Alternatively, Photoplethysmogra- phy (PPG) can be used as a heart monitoring technique. Specifically, PPG technique uses infrared light to measure the blood volume changes in the microvascular bed of tis- sues by using a pulse oximeter [14]. To effectively calculate BP, most of the previous works are focused on the calculation of PTT from both the PPG and the ECG signals. This requires affixing of sensors on the chest of the patients due to which, the calculation of BP on the fly is not feasible for the continuous seamless Figure 1: A PPG signal obtained after preprocessing followed by monitoring. More recently, researchers have found that butterworth filtering. PPG signals can be used for estimating BP levels. This presents two major gaps in this area of research: 1) Given that the aim of this work is to use wearable sensors for icance of each feature set by calculating their vari- preventive healthcare, it is imperative that the algorithms ations for the normal and abnormal cases and cal- developed can work in real-time on devices which are com- culate their individual and collective performances putationally less powerful, and 2) The system should be using different machine learning methods. able to raise alarms for commercial usage of the devices • Weighted voting: We have proposed a feature vot- in a more reliable manner. In this context, it is possible ing strategy based on autoregressive modeling to cal- to further simplify the research problem: since the work culate weights assigned to individual features in the is mainly focused on preventive healthcare [15], the aim of least square manner which significantly improves the systems is more inclined towards the detection of abnor- classification results. mal BP rather than being able to measure the exact value of BP as already achieved in [16]. In this paper, we aim to Rest of the paper is organized as follows: We review ex- propose a novel method to detect an abnormality in the isting work (Section 2) followed by a description of dataset BP using PPG signals acquired from the pulse oximeter (Section 3) . Later, we present the proposed approach sensor. The method is based on de-noising of the signal, (Section 4) followed by a description of the experimental followed by the peak detection in PPG signals. After peak setup (Section 5). The classification results are presented detection, different features are extracted which are then in Section 6 and detailed discussion is provided in Sec- passed through the machine learning module for detect- tion 7. ing abnormalities in BP. The major contributions of this paper are as follows. 2. Background and State of the Art • Data collection: We have devised an experimental This paper is focused on using PPG signals to achieve protocol that can be used to collect PPG dataset. continuous measurement of BP for elderly and vulnerable The proposed system uses an embedded system (Ar- patients by exploiting the physiological relation between duino board) and the pulse sensor to collect the data. pulse width and BP. In particular, we model this chal- Along with the PPG signals, the BP readings of the lenge as a general classification problem leveraging features subjects are simultaneously recorded to validate the which can be used to identify if a patient has a normal or correlation between the physiology of PPG signals abnormal BP. Within this context, the research presented and BP. in this paper is relevant to multiple domains such as anal- • Feature extraction: We have proposed a novel tem- ysis of PPG signals and classification. In this section, we poral feature set that can be used for the identifica- present background on the PPG signal, the motivation of tion of BP. Prior to feature extraction, de-noising of this work and state of the art on the use of estimating the the signals is performed for detecting the peaks and blood pressure through sensor devices. valleys in a PPG signal. This is followed by the fea- 2.1. Photoplethysmogram (PPG) tures that have been proposed i.e. the rising time, falling time and peak-to-peak distance. PPG is a pulsating wave produced by a pulse oximeter and has been traditionally used to measure oxygen satu- • Feature analysis: We analyzed the statistical signif- ration, blood pressure, and other vital functions. A pulse 2 oximeter is often used to have a PPG which measures present a comparative analysis of different signal process- light changing and absorption through the skin. With ing techniques to extract breathing rate from the PPG.In each cardiac cycle, the heart pumps blood to the periph- addition to this, PPG signal has been also been used to ery [17],[18].