Key result
The proposed diversity method increased the usable period of smartphone camera-based heart rhythm signals by 19.53% and 6.25% compared to using only the color intensity or fingertip movement signals, respectively.
Why the study?
Smartphone-based physiological signals are vulnerable to motion and noise artifacts and ambient light during heart rhythm detection compared to medical devices.
Effect estimate: 19.53% and 6.25% increase
A novel diversity method using dual signals from a single smartphone camera recording improves the usable period of heart rhythm detection by reducing motion and noise artifacts.
May extend smartphone PPG usability for rhythm detection; leaves open clinical validation before practice adoption.
The advent of smartphones has advanced the use of embedded sensors to acquire various physiological information. For example, smartphone camera sensors and accelerometers can provide heart rhythm signals to the subjects, while microphones can give respiratory signals. However, the acquired smartphone-based physiological signals are more vulnerable to motion and noise artifacts (MNAs) compared to using medical devices, since subjects need to hold the smartphone with proper contact to the smartphone camera and lens stably and tightly for a duration of time without any movement in the hand or finger. This results in more MNA than traditional methods, such as placing a finger inside a tightly enclosed pulse oximeter to get PPG signals, which provides stable contact between the sensor and the subject's finger. Moreover, a smartphone lens does not block ambient light in an effective way, while pulse oximeters are designed to block the ambient light effectively. In this paper, we propose a novel diversity method for smartphone signals that reduces the effect of MNAs during heart rhythm signal detection by 1) acquiring two heterogeneous signals from a color intensity signal and a fingertip movement signal, and 2) selecting the less MNA-corrupted signal of the two signals. The proposed method has advantages in that 1) diversity gain can be obtained from the two heterogeneous signals when one signal is clean while the other signal is corrupted, and 2) acquisition of the two heterogeneous signals does not double the acquisition procedure but maintains a single acquisition procedure, since two heterogeneous signals can be obtained from a single smartphone camera recording. In our diversity method, we propose to choose the better signal based on the signal quality indices (SQIs), i.e., standard deviation of instantaneous heart rate (STD-HR), root mean square of the successive differences of peak-to-peak time intervals (RMSSD-T), and standard deviation of peak values (STD-PV). As a performance metric evaluating the proposed diversity method, the ratio of usable period is considered. Experimental results show that our diversity method increases the usable period 19.53% and 6.25% compared to the color intensity or the fingertip movement signals only, respectively.
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Tabei et al. (2019) studied Healthy (n=15). Diversity method for smartphone signals vs. Color intensity or fingertip movement signals only was evaluated on Ratio of usable period (19.53% and 6.25% increase). The proposed diversity method increased the usable period of smartphone camera-based heart rhythm signals by 19.53% and 6.25% compared to using only the color intensity or fingertip movement signals, respectively.
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