Monitoring of Heart Rate from Photoplethysmographic Signals Using a Samsung Galaxy Note8 in Underwater Environments

Abstract

Photoplethysmography (PPG) is a commonly used in determining heart rate and oxygen saturation (SpO₂). However, PPG measurements and its accuracy are heavily affected by the measurement procedure and environmental factors such as light, temperature, and medium. In this paper, we analyzed the effects of different mediums (water vs. air) and temperature on the PPG signal quality and heart rate estimation. To evaluate the accuracy, we compared our measurement output with a gold-standard PPG device (NeXus-10 MKII). The experimental results show that the average PPG signal amplitude values of the underwater environment decreased considerably (22% decrease) compared to PPG signals of dry environments, and the heart rate measurement deviated 7% (5 beats per minute on average. The experimental results also show that the signal to noise ratio (SNR) and signal amplitude decrease as temperature decreases. Paired t-test which compares amplitude and heart rate values between the underwater and dry environments was performed and the test results show statistically significant differences for both amplitude and heart rate values (p < 0.05). Moreover, experimental results indicate that decreasing the temperature from 45 °C to 5 °C or changing the medium from air to water decreases PPG signal quality, (e.g., PPG signal amplitude decreases from 0.560 to 0.112). The heart rate is estimated within 5.06 bpm deviation at 18 °C in underwater environment, while estimation accuracy decreases as temperature goes down.

Keywords: PPG; heart rate variability; photoplethysmography; temperature; underwater.

Figure 1

Our measurement and calibration devices used for data acquisition: (a) FLUKE device which is an emulation device to calibrate other measurement devices, (b) NeXus-10 MKII is FDA approved physiological signal measurement device which we use as our gold standard device in this paper, and (c) Samsung Galaxy Note8, a water-resistant device which is capable of measuring PPG in both dry and underwater environments.

Figure 2

Data acquisition in dry and underwater environments. (a) Dry environment: the Samsung Galaxy Note8 PPG sensor is covered with the index finger of the right hand while the PPG sensor of the NeXus-10 MKII device encloses the index finger of the left hand; and (b) underwater environment: the Samsung Galaxy Note8 PPG sensor is covered with the index finger of the right hand while the hand is immersed in the transparent bucket filled with water. The PPG sensor of the NeXus-10 MKII device encloses to the index finger of the left hand.

Figure 3

The flowchart of our proposed data processing procedure which consists of acquiring screenshot during the measurement, cropping acquired images, converting RGB image to HSV color space, obtaining a binary image after denoising background from HSV images, and scaling a binary image to values in x-y axis. The heart rate is extracted from peak-to-peak (or pulse) interval from the constructed PPG signal (left). The acquired smartphone PPG signal is compared to gold-standard PPG signal acquired by NeXus-10 MKII (right).

Figure 4

An example of output result at each step of our proposed data processing procedure. (a) Screenshot from the smartphone built-in application, (b) its cropped image, (c) the binary image derived by the RGB to HSV transformation and background removal, and (d) extracted signal by scaling the binary image to the x-y axis.

Figure 5

Refraction effect between water and air with the smartphone configuration with an LED emitter and the photodiode (PD) receiver sensor.

Figure 6

Systolic peaks and peak-to-peak intervals of a PPG signal measured by the NeXus-10 MKII device signal in the dry environment.

Figure 7

Representative PPG signal examples measured in dry and underwater environments. (a) PPG signals measured in dry environment and (b) in underwater environment using Samsung Galaxy Note8, and (c) a reference PPG signal measured using NeXus-10 MKII.

Figure 8

Amplitude plot: left plot shows the amplitude of the dry environment from NeXus-10 MKII and right plot shows the amplitude of underwater environment from Samsung Galaxy Note8.

Figure 9

Bland–Altman plots of heart rate measurements after signal processing. (a) Heart rate from Samsung Galaxy Note8 and NeXus-10 MKII in dry environment, and (b) Heart rate from NeXus-10 MKII in dry and Samsung Galaxy Note8 in wet environment.

Figure 9

Bland–Altman plots of heart rate measurements after signal processing. (a) Heart rate from Samsung Galaxy Note8 and NeXus-10 MKII in dry environment, and (b) Heart rate from NeXus-10 MKII in dry and Samsung Galaxy Note8 in wet environment.

Figure 10

Signal amplitude and heart rate accuracy acquired from smartphone PPG signals for varying temperature in dry (red) and underwater (blue) environments. (a) signal amplitude, and (b) heart rate accuracy.

Figure 10

Signal amplitude and heart rate accuracy acquired from smartphone PPG signals for varying temperature in dry (red) and underwater (blue) environments. (a) signal amplitude, and (b) heart rate accuracy.