A New Wearable Device for Blood Pressure Estimation Using Photoplethysmogram

Abstract

We present a novel smartwatch, CareUp ® , for estimating the Blood Pressure (BP) in real time. It consists of two pulse oximeters: one placed on the back and one on the front of the device. Placing the index finger on the front oximeter starts the acquisition of two photoplethysmograms (PPG); the signals are then filtered and cross-correlated to obtain a Time Delay between them, called Pulse Transit Time (PTT). The Heart Rate (HR) (estimated from the finger PPG) and the PTT are then input in a linear model to give an estimation of the Systolic and Diastolic BP. The performance of the smartwatch in measuring BP have been validated in the Institut Coeur Paris Centre Turin (ICPC), using a sphygmomanometer, on 44 subjects. During the validation, the measures of the CareUp ® were compared to those of two oscillometry-based devices already available on the market: Thuasne ® and Magnien ® . The results showed an accuracy comparable to the oscillometry-based devices and they almost agreed with the American Association for the Advancement of Medical Instrumentation standard for non-automated sphygmomanometers. The integration of the BP estimation algorithm in the smartwatch makes the CareUp ® an easy-to-use, wearable device for monitoring the BP in real time.

Keywords: PPG; PTT; blood pressure; photoplethysmogram; pulse transit time.

Figure A1

CareUp${}^{\circledR }$ smartwatch home screen and watch panels for starting the BP measure and the calibration. In addition, the instruction manual to guide the user in the calibration and measurement procedure.

Figure 1

Principles of photoplethysmograms (PPG) measurement. (a) Schematics of the LED and the receiver: the LED emits light at the specific wavelength at which the absorption of the oxy-hemoglobin is maximum; the receiver collects the back scattered light [36]. (b) The PPG signal is composed by a DC component, due to the not-changing part in the tissue and the AC component due to the blood whose concentration of oxy- and deoxy–hemoglobin changes as the pulse [37].

Figure 2

The CareUp${}^{\circledR }$ smartwatch, where the algorithm for detecting the blood pressure (BP) has been embedded. One PPG waveform is taken from the back sensor of the watch in contact with the wrist skin and the second one is acquired by positioning the index finger of the other hand on the front oximeter sensor.

Figure 3

Hardware architecture of CareUp${}^{\circledR }$ for BP estimation.

Figure 4

Diagram showing the BP estimation algorithm. SBP = systolic blood pressure; DBP = diastolic blood pressure; HR = heart rate.

Figure 5

Two PPG signals: one from the wrist and one from the finger, used to perform the cross-correlation between the two signals.

Figure 6

Time Delay boxplots for the Learning Database (LDB) and Test Database (TDB).

Figure 7

Bland–Altman plots showing the results comparison for SBP and DBP estimation on 126 measures from 44 subjects, taken in the Institut Coeur Paris Centre Turin (ICPC) in Paris. The acquisitions have been performed using a sphygmomanometer as reference device and the CareUp${}^{\circledR }$ smartwatch.

Figure 8

Bland–Altman plots showing the results comparison for SBP and DBP pressure estimation on 78 measures from 44 subjects, taken in the ICPC clinic in Paris. The acquisitions have been performed using a sphygmomanometer as reference device, Magnien${}^{\circledR }$ and the CareUp${}^{\circledR }$ smartwatch.

Figure 9

Bland–Altman plots showing the results comparison for SBP and DBP pressure estimation on 76 measures from 44 subjects, taken in the ICPC clinic in Paris. The acquisitions have been performed using a sphygmomanometer as reference device, Thuasne${}^{\circledR }$ and the CareUp${}^{\circledR }$ smartwatch.