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Photoplethysmography-Based Continuous Systolic Blood Pressure Estimation Method for Low Processing Power Wearable Devices

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Photoplethysmography-Based Continuous Systolic Blood Pressure Estimation Method for Low Processing Power Wearable Devices applied sciences Article Photoplethysmography-Based Continuous Systolic Blood Pressure Estimation Method for Low Processing Power Wearable Devices Rolandas Gircys 1, Agnius Liutkevicius 1,* , Egidijus Kazanavicius 1, Vita Lesauskaite 2, Gyte Damuleviciene 2 and Audrone Janaviciute 1 1 Centre of Real Time Computer Systems, Kaunas University of Technology, Barsausko str. 59-A316, LT-51423 Kaunas, Lithuania; [email protected] (R.G.); [email protected] (E.K.); [email protected] (A.J.) 2 Clinical Department of Geriatrics, Lithuanian University of Health Sciences, Josvainiu str. 2, LT-47144 Kaunas, Lithuania; [email protected] (V.L.); [email protected] (G.D.) * Correspondence: [email protected] Received: 13 April 2019; Accepted: 28 May 2019; Published: 30 May 2019 Abstract: Regardless of age, it is always important to detect deviations in long-term blood pressure from normal levels. Continuous monitoring of blood pressure throughout the day is even more important for elderly people with cardiovascular diseases or a high risk of stroke. The traditional cuff-based method for blood pressure measurements is not suitable for continuous real-time applications and is very uncomfortable. To address this problem, continuous blood pressure measurement methods based on photoplethysmogram (PPG) have been developed. However, these methods use specialized high-performance hardware and sensors, which are not available for common users. This paper proposes the continuous systolic blood pressure (SBP) estimation method based on PPG pulse wave steepness for low processing power wearable devices and evaluates its suitability using the commercially available CMS50FW Pulse Oximeter. The SBP estimation is done based on the PPG pulse wave steepness (rising edge angle) because it is highly correlated with systolic blood pressure. The SBP estimation based on this single feature allows us to significantly reduce the amount of data processed and avoid errors, due to PPG pulse wave amplitude changes resulting from physiological or external factors. The experimental evaluation shows that the proposed SBP estimation method allows the use of off-the-shelf wearable PPG measurement devices with a low sampling rate (up to 60 Hz) and low resolution (up to 8-bit) for precise SBP measurements (mean difference MD = 0.043 and standard deviation SD = 6.79). In contrast, the known methods for − continuous SBP estimation are based on equipment with a much higher sampling rate and better resolution characteristics. Keywords: blood pressure estimation; photoplethysmogram; pulse wave; pulse oximeter; wearable device 1. Introduction It is very important to be able to determine the moment when physiological parameters deviate from normal levels in order to prevent chronic diseases. The dynamics of observed physiological parameters are more important over the long-run than accurate instant values, which are used in an emergency or cases of critical health disorders. It is important to detect the degree of a physiological parameter’s deviation from normal during a day, week, month or even a year, as well as the length of such a deviation. If deviation persists for a long time, this can suggest the risk of irreversible changes. Appl. Sci. 2019, 9, 2236; doi:10.3390/app9112236 www.mdpi.com/journal/applsci Appl. Sci. 2019, 9, 2236 2 of 16 Appl. Sci. 2019, 9, x FOR PEER REVIEW 2 of 16 Timely detection of such dangerous deviations from normal and an immediate start of treatment can helpchanges. to avoid Timely negative detection and irreversible of such dangerous health changes. deviations from normal and an immediate start of treatmentTherefore, can the help long-term to avoid negative constant and observation irreversible of physiological health changes. parameters, such as blood pressure allowsTherefore, us to reduce the the long healthcare‐term constant costs, asobservation discussed inof Referencephysiological [1], minimizesparameters, the such risk as of blood visiting secondarypressure allows healthcare us to institutions,reduce the healthcare and ensures costs, a as better discussed quality in Reference of life for [1], a minimizes longer period. the risk Blood of pressurevisiting measurements secondary healthcare made atinstitutions, home indicate and ensures the state a better of the quality cardiovascular of life for systema longer better period. than thoseBlood obtained pressure during measurements regular visitsmade toat ahome doctor, indicate as discussed the state of in the Reference cardiovascular [2]. A verysystem significant better proportionthan those of obtained chronic during diseases regular are cardiovascular visits to a doctor, diseases. as discussed Atherosclerosis in Reference and [2]. endothelial A very significant function changesproportion are accompaniedof chronic diseases by a are rise cardiovascular in blood pressure. diseases. As Atherosclerosis suggested in and Reference endothelial [3], function increasing bloodchanges pressure are accompanied values, measured by a rise at in home, blood canpressure. be considered As suggested as a in prognostic Reference sign[3], increasing of atherosclerosis blood orpressure endothelial values, dysfunction. measured Long-term at home, can blood be pressureconsidered variability, as a prognostic obtained sign during of atherosclerosis daily home blood or pressureendothelial measurement dysfunction. (HBP), Long‐ canterm show blood internal pressure organ variability, damage obtained or risk during of cardiovascular daily home blood events, aspressure reported measurement in Reference (HBP), [4]. The can authors show internal of [5] showorgan thatdamage increased or risk bloodof cardiovascular pressure is events, a potential as dementiareported illness in Reference risk factor; [4]. thus,The authors registering of [5] the show dynamics that increased of blood pressureblood pressure for elderly is a potential people can dementia illness risk factor; thus, registering the dynamics of blood pressure for elderly people can serve as a preventive mental health disorder measure. Hence, the continuous monitoring of blood serve as a preventive mental health disorder measure. Hence, the continuous monitoring of blood pressure is critical for the prevention and diagnosis of hypertension and cardiovascular diseases. pressure is critical for the prevention and diagnosis of hypertension and cardiovascular diseases. Both SBP and diastolic blood pressure (DBP) are important to assess the state of the cardiovascular Both SBP and diastolic blood pressure (DBP) are important to assess the state of the system, but according to References [6–8], systolic hypertension is much more common than diastolic cardiovascular system, but according to References [6–8], systolic hypertension is much more hypertension,common than and diastolic systolic hypertension, blood pressure and systolic contributes blood pressure more tocontributes the huge more global to the disease huge global burden attributabledisease burden to hypertension attributable thanto hypertension diastolic pressure. than diastolic Systolic pressure. blood pressureSystolic blood is much pressure more is important much formore elderly important people for and elderly their healthpeople state and their evaluation, health state as discussed evaluation, in Referencesas discussed [ 9in–12 References]. Since elderly [9– people12]. Since are the elderly major people group are of the geriatrics major group patients, of geriatrics which are patients, subjects which of this are study,subjects the of estimationthis study, of systolicthe estimation blood pressure of systolic is the blood main pressure focus ofis the this main paper. focus of this paper. TheThe traditional traditional auscultatory auscultatory blood blood pressurepressure measurement method method is is not not continuous continuous and and is quite is quite uncomfortable,uncomfortable, because because it usesit uses a cu aff cuffwrapped wrapped around around the armthe arm which which is inflated is inflated repeatedly. repeatedly. To address To thisaddress problem, this continuous problem, continuous blood pressure blood measurement pressure measurement methods have methods been developedhave been recently.developed The mostrecently. popular The methodsmost popular for continuousmethods for andcontinuous cuffless and blood cuffless pressure blood monitoringpressure monitoring are based are on based pulse transiton pulse time (PTT)transit [ 13time–18 ].(PTT) However, [13–18]. PTT However, is calculated PTT using is calculated two signals using (usually two signals electrocardiogram (usually andelectrocardiogram pulse wave), which and ispulse quite wave), difficult which to implement is quite difficult in wearable to implement devices forin wearable daily use. devices for dailyBlood use. pressure estimation methods which are based on one-channel pulse wave measurements are lessBlood complex pressure and allow estimation us to createmethods wearable which are devices based which on one are‐channel more comfortable pulse wave measurements for the end users. A pulseare less wave complex is often and obtained allow us using to create the wearable photoplethysmogram devices which (PPG) are more optical comfortable measurement for the method. end Bloodusers. pressure A pulse measurement wave is often methods obtained basedusing onthe one-channel photoplethysmogram PPG are implemented (PPG) optical usingmeasurement stationary method. Blood pressure measurement methods based on one‐channel PPG
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