Cardiovascular diseases (CVDs) are considered one of the main reasons of deaths causing more than 30% of deaths around the world [1
]. It is reported that 2200 out of 801,000 deaths in the U.S are caused by CVDs. Almost 47% of sudden CVD deaths occur outside the hospital, which indicates that most individuals may not manage warning signs of heart disease [2
]. Accordingly, continuous heart rate monitoring is highly demanded to prevent the complications of CVDs. Recently, smartphone applications for healthcare, e.g., mobile heart disease detection, heart rhythm analysis, remote home care monitoring, eye disease diagnosis, have become highlighted [4
]. Moreover, smartwatches, wrist bands, and activity trackers based on electrocardiography (ECG) or photoplethysmography (PPG) are also widely used for continuous measurement of physiological signs such as heart rate and peripheral oxygen saturation (SpO2
) in daily life [11
]. Among these technologies, PPG using a smartphone camera or smartwatches is highlighted as continuous and comfortable method for heart rate measurement since it does not require additional electrodes or skin preparation. Moreover, PPG measurement using smart devices was shown to be effective in detecting arrhythmia [12
PPG signals can reflects changes in blood volume below underlying body tissues in a simple, noninvasive, and low-cost manner [15
]. PPG sensor technology facilitates the development of several wearable devices such as smartwatches, Fitbits, and activity trackers for easy monitoring of heart rate in rest state, during daily activities, or exercises [13
]. Since PPG measurement is sensitive to any disturbance and is affected by changes in the reflected light intensity induced by the variations in the volume of blood [13
], motion and noise artifacts (MNA) caused by any movement or physical activity, displacement of the finger, finger pressure, and temperature can cause disturbance and discrepancy in PPG measurements, which yields to inaccurate heart rate measurements [16
]. Therefore, there have been studies improving PPG signals acquired in the presence of MNA [17
]. The effects of skin and room temperature on PPG signals have been studied in [19
], which shows that cold temperature significantly reduces the amplitude of PPG signals and the accuracy of the SpO2
measurement while those are improved in warm temperature.
Representative examples of constant heart rate monitoring during underwater activities are (1) heart rate monitoring of divers [20
], and (2) heart rate monitoring of subjects during water walking or jogging, which is suggested for rehabilitation and fitness enhancement. Underwater electrodes have been proposed using carbon black powder (CB) and polydimethylsiloxane (PDMS) for underwater ECG measurements [21
]. Cold water is observed to influence both heart rate and blood pressure in [22
]. Changes in blood pressure of trained divers during cold water immersion was analyzed by asking the divers to hold their breath during the cold water submersion in [20
]. The diver’s vital signs were analyzed using an ECG unit and the study focused on the biological changes in the body that effects the changes in blood pressure. The influence of cold water immersion on blood flow has been investigated [22
] while The effects of underwater activities (water immersion, submersion, and scuba diving) on heart rate variability has been studied [23
]. Comparison of surface electromyography (sEMG) electrodes for measuring muscle movement in water and land has been studied [24
] and Electromyography (EMG) sensors measuring muscle movement in rehabilitation treatment in exercise pools were studied in [25
]. However, these studies do not use PPG but ECG which requires additional electrodes.
In this paper, we evaluate the performance of PPG sensors embedded in the Samsung Galaxy Note8 in dry and underwater environments. As performance metrics, we consider the accuracy of the heart measurements, signal amplitude, and signal to noise ratio (SNR). Specifically, we compared the acquired information from PPG signals to a gold-standard reference which is obtained from NeXus-10 MKII (Mind Media, Herten, Germany) device. The NeXus-10 MKII has a Food and Drug Administration (FDA) approval. The effect of cold temperature on PPG signals was studied in the viewpoints of (1) physiology and (2) sensor hardware. In the viewpoint of physiology, the effect of cold temperature on PPG signals was analyzed in terms of blood vessels in human body [27
]. In this study, a temperature drop from 20 °C to 3 °C was observed to increase heart rate from 82 to 98 beats per minute (bpm) [28
], and increase oxygen saturation level from 97% to 99%. Moreover, the temperature drop caused blood viscosity to increase, which decreased the PPG signal amplitude. On the other hand, the effect of cold temperature on PPG signal amplitude was studied in the viewpoint of sensor hardware including its photodiodes [19
]. For example, temperature change from 25 °C to 4 °C shifted the wavelength by 18 nm, decreased the voltage by 0.1 volts, and decreased the current by 0.05 amps. However, cold temperature was observed not to change the pattern (or shape) of PPG signals in both physiology and sensor hardware viewpoints [30
]. The rest of this paper is organized as follows. Section 2
describes data collection, measurement devices, and study protocol. In Section 3
, our proposed image processing method consisting of signal extraction, data processing, feature extraction, and heart rate measurement is explained. Experimental results are presented in Section 4
. Finally, Section 5
concludes this paper.
The PPG signals extracted from the smartphone and the NeXus device for each environment were analyzed in terms of signal quality (signal amplitude and SNR). To eliminate the effect of temperatures and only focus on medium effect, we fixed the temperature at 18 °C. The experimental environment was an indoor environment with regular ambient light condition and air-conditioned room temperature maintained at 18 °C. PPG signal examples acquired by the smartphone in the dry and underwater environments are shown in Figure 7
a,b, respectively while a reference PPG signal example acquired by the NeXus device is shown in Figure 7
c. The reference PPG signal is always measured in dry environment for both dry and underwater environments since it is required to be gold-standard. As shown in Figure 7
a,b, the wet environment changed smartphone PPG signals in terms of shape as well as peak-to-peak interval (or heart rate).
The smartphone application has artifact correction algorithm that operates before giving an estimation of the heart rate [31
]. Whenever the signal is affected by any type of artifact, the measurement time increases until the application gets the usable signal. Therefore, the moment when the smartphone shows the final estimated heart rate value is ≈10 s in the dry environment but it increases to ≈20 s in the underwater environment as shown in Table 1
shows signal amplitude, SNR, and heart rate values derived from the PPG signals of each volunteer in dry and underwater environments. As shown in Table 2
, average SNR decreases from 12.84 into 4.96 on average and signal amplitude decreases from 0.4 to 0.2 on average. Moreover, heart rate acquired by the smartphone in the dry environment is shown to have 0.05 bpm difference on average compared to the gold-standard while the average difference is 4.8 bpm in underwater environment.
Comparison of measurements of the two environments from the smartphone with the NeXus as a reference device was performed using the two-way repeated measures ANOVA using SPSS (IBM Corp. Released 2017. IBM SPSS Statistics for Windows, Version 25.0. Armonk, NY: IBM Corp). The results revealed that there was a significant interaction effect between Device and Environment (F = 69.6, p < 0.05). The interaction effect indicates that the heart rates from smartphone (M = 80.52) were significantly higher than those from NeXus (M = 76.39) in the underwater environment (Mean Difference = 4.13, SE = 1.84, p < 0.05) while the heart rates from the smartphone (M = 76.35) and NeXus (M = 75.70) were not statistically different in the dry environment (Mean Difference = 0.652, SE = 0.59, p = 0.29).
shows the plot of each measurement’s amplitude for all participants between the dry and underwater environments. A paired-samples t
-test was conducted to compare the measurement’s amplitude of heart rates in the dry and underwater environments. There was a significant difference in amplitude for the dry environment (M = 0.37, SD = 0.04) and the underwater environment (M = 0.18, SD = 0.04); t (22) = 17.9, p
The Bland–Altman plots in Figure 9
show the agreement between the heart rates obtained from the smartphone and NeXus in dry and underwater conditions. Figure 9
a shows a bias between the mean differences of 0.65 in heart rates of smartphone and NeXus in dry condition with 95% limits of agreement interval of the mean differences. On the other hand, Figure 9
b shows a bias between the mean difference of 4.13 from smartphone in underwater and NeXus in dry condition, as well as an agreement interval with 95% of the mean differences. This implies that the bias between heart rates with smartphone and NeXus are larger in underwater condition than in dry condition.
To evaluate the effect of temperature on the PPG signal measurement in underwater environment, we measured signal amplitude and heart rate accuracy at 45 °C, 18 °C, and 5 °C. Here, the heart rate accuracy is calculated by Accuracy
(%) = 100% − PE
is the percentage of error and is derived by
. Figure 10
shows the amplitude and heart rate accuracy of the measured PPG signal for varying temperature in underwater and dry environments. The signal amplitude in the underwater environment was on average 32%, 18%, and 14% lower at 45 °C, 18 °C, and 5 °C, respectively, compared those in the dry environment. The accuracy in the underwater environment was on average 5%, 18%, and 14% lower at 45 °C, 18 °C, and 5 °C, respectively, compared those in the dry environment.
There have been studies investigating the effect of different conditions and factors including motion noise artifact, skin tone, nail polishes, age, and ambient light on PPG signal amplitude [8
]. For heart rate measurements in underwater condition, several approaches have been proposed [10
]. Schipke et al. found that 5 min immersions for divers in 6 °C cold ocean water resulted in 10% decrease in SpO2
measurement accuracy and 40% decrease in heart rate measurement accuracy. Reyes et al. developed a compound of carbon black powder and polydimethylsiloxane (CB/PDMS) and tested their electrode for underwater ECG measurements which had an average of 6 bpm deviation in the heart rate measurement while in our underwater PPG measurement the average heart rate deviation was 5 bpm. Their methods use ECG electrodes for heart rate measurements, and it remains to be seen if their accuracy remains in salty and non-salty water conditions. Moreover, these studies provide good estimates of heart rate measurements in cold water while the effect of high temperatures (higher than 6 °C) on heart rate measurement was not studied in [21
Khan et al. studied the effects of cold and warm temperatures (5 °C and 45 °C) on PPG signal amplitude and heart rate measurements and concluded that temperature drop from 45 °C to 5 °C decreases the PPG signal from 0.4 V to 0.1 V [12
]. In particular, our experiment found that decreasing the water temperature decreases the performance. Moreover, Khan’s study was only conducted in dry conditions and they did not study the effect of different mediums (e.g., water) on the PPG signal amplitude. Our results indicate that changing the medium from air to water and decreasing the temperature from 45 °C to 5 °C decreased the signal amplitude from 0.561 to 0.091. To the authors’ knowledge, no research has studied the effects of different mediums (e.g., water) on PPG sensors signals amplitude and SNR.
As it was expected, our results indicate that there was a significant interaction effect between device and environment (F = 69.6, p < 0.05). The interaction effect indicates that the heart rates from dry environment (M = 80.52) were significantly higher than those from underwater environment (M = 76.39) (mean difference = 4.13, SE = 1.84, p < 0.05). Our experimental result indicates that PPG measurement gives acceptable accuracy in normal water pressure, normal light condition and temperature maintained at 18 °C. When the temperature decreases, however, a noise resilient peak detection is needed to acquire reliable heart rate information while it is not in the dry environment.