Abstract
Recent progress in information and communication technologies has made it possible to measure heart rate (HR) and normalized pulse volume (NPV), which are important physiological indices, using only a smartphone. This has been achieved with reflection mode photoplethysmography (PPG), by using a smartphone’s embedded flash as a light source and the camera as a light sensor. Despite its widespread use, the method of PPG is susceptible to motion artifacts as physical displacements influence photon propagation phenomena and, thereby, the effective optical path length. Further, it is known that the wavelength of light used for PPG influences the photon penetration depth and we therefore hypothesized that influences of motion artifact could be wavelength-dependant. To test this hypothesis, we made measurements in 12 healthy volunteers of HR and NPV derived from reflection mode plethysmograms recorded simultaneously at three different spectral regions (red, green and blue) at the same physical location with a smartphone. We then assessed the accuracy of the HR and NPV measurements under the influence of motion artifacts. The analyses revealed that the accuracy of HR was acceptably high with all three wavelengths (all rs > 0.996, fixed biases: −0.12 to 0.10 beats per minute, proportional biases: r = −0.29 to 0.03), but that of NPV was the best with green light (r = 0.791, fixed biases: −0.01 arbitrary units, proportional bias: r = 0.11). Moreover, the signal-to-noise ratio obtained with green and blue light PPG was higher than that of red light PPG. These findings suggest that green is the most suitable color for measuring HR and NPV from the reflection mode photoplethysmogram under motion artifact conditions. We conclude that the use of green light PPG could be of particular benefit in ambulatory monitoring where motion artifacts are a significant issue.
Highlights
The measurement of heart rate (HR) and the recording of the peripheral pulse (PP) have long been established as fundamental components of both research and clinical investigation [1,2]
To address this, during baseline and while adding motion of the smart phone to create artifact in the PPG signal, we examined the agreement of HR and normalized pulse volume (NPV) measurements derived from the photoplethysmograms recorded with these three colors and from the reference methods, and the signal-to-noise (S/N) ratio of the PPGs from the three light colors
As already mentioned in the introduction, blue light PPG probes the shallowest arterioles, which are the most distal of the three vascular regions to be probed
Summary
The measurement of heart rate (HR) and the recording of the peripheral pulse (PP) have long been established as fundamental components of both research and clinical investigation [1,2]. PP detected optically has been further investigated and this has led to the establishment of normalized pulse volume (NPV) as a valuable index for a variety of applications [3,4]. The potential for physiological monitoring with a smartphone has been demonstrated by measuring the reflection mode finger-PPG, or pulse wave, with its embedded pseudo-white color flash light emitting diode (LED) as a light source and the complementary metal oxide semiconductor (CMOS) camera as a photo sensor [12,13,14]. The performance of the smartphone approach under the influence of motion artifact is yet to be examined [16]
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