Photoplethysmography(PPG), which is a non-invasive optical method used to measure blood volume changes in the microvascular bed of tissue. It works by shining a light through the skin and into the underlying blood vessels, and detecting the amount of light that is absorbed and scattered by the blood.
A PPG sensor typically consists of a light source (such as an LED), a photodetector, and a signal processing unit. The sensor is placed on the skin and the light is transmitted into the tissue. As blood volume changes in the underlying vessels, the amount of light absorbed and scattered changes as well, and this is detected by the photodetector. The resulting signal is then processed to extract information about heart rate, blood pressure, and other physiological parameters.
PPG signals can be obtained from various parts of the body, such as the earlobe, forehead, nail bed as well as the finger and wrist.
We were always aware of 3 main reasons why we did not want to go ahead with a wrist based wearable, the reasons being - a) anatomical artefacts b) motion artefacts c) skin pigmentation
a) Anatomically, the finger and wrist have several differences that can affect PPG readings.
Here are some of the key differences:
Figure 1: Transverse section of Finger. Source- https://slideplayer.com/slide/7364200/
Blood flow: The finger has a higher density of blood vessels and a smaller cross-sectional area compared to the wrist. This means that blood flow in the finger is faster and more pulsatile, which can result in a stronger PPG signal (see fig.1). On the other hand, the wrist has a larger cross-sectional area and a more complex tissue structure compared to the finger. This means that blood flow in the wrist is slower and less pulsatile, which can result in a weaker PPG signal (see fig 2).
Figure 2: Cross-section of the wrist. Source- https://radiopaedia.org/cases/wrist-cross-section-diagrams-grays-illustrations
Tissue thickness: The finger has thinner tissue layers compared to the wrist, which allows for better transmission of light through the tissue. This results in a clearer PPG signal from the finger.
b) Motion- it is widely known that motion plays a major role in the accuracy of readings from a PPG sensor-
Bone structure: The finger has a bony structure that is more compact and less prone to movement compared to the wrist. This stability can help ensure that the PPG sensor remains in place during measurements.
c) The melanin content in the skin-
Skin colour: The colour of the skin can affect the quality of PPG signals. It is known that the palm generally contains less melanin than the skin.
Hence we have designed the wearable in such a way that we can take the readings from the palm side of the finger, which can result in a stronger PPG signal due to better light transmission through the tissue.
In summary, the finger and wrist have different characteristics that can affect the quality of PPG signals and the finger provides a stronger and clearer PPG signal, due to higher blood flow, lesser movement and a comparatively lower concentration of melanin.
References:
Blood flow:
K. T. Lee, C. S. Kim, S. H. Kim, J. Y. Seo, J. H. Kim, J. H. Chang, J. W. Choi, "Comparison of pulse oximetry between the wrist and the ankle during peripheral vascular surgery," Annals of vascular surgery, vol. 26, no. 2, pp. 182-188, Feb. 2012.
D. H. Shin, H. R. Lim, S. Y. Cho, J. W. Kim, J. S. Kim, "Comparison of finger and forehead pulse oximeter in neonates," Pediatric Anesthesia, vol. 23, no. 2, pp. 111-115, Feb. 2013.
Tissue thickness:
L. Xie, A. Rovithis, N. Pappas, "Investigating the Performance of Photoplethysmography-Based Devices for Blood Pressure Measurement," IEEE Journal of Translational Engineering in Health and Medicine, vol. 6, pp. 1-10, 2018.
Y. Zhang, C. Xu, X. Xu, X. Li, "A comparative study of heart rate monitoring using photoplethysmography and electrocardiography," Journal of Medical Systems, vol. 41, no. 11, pp. 1-8, Nov. 2017
Bone structure:
S. M. Kim, J. Y. Lee, S. Y. Kim, S. S. Lee, S. H. Kim, S. Y. Park, "Comparison of peripheral perfusion index measurements in different fingers of the hands," Korean Journal of Anesthesiology, vol. 71, no. 1, pp. 23-28, Feb. 2018.
A. Nitzan, Y. Koppel, Y. Aviram, "Photoplethysmography in the fingers and toes," Journal of Biomedical Optics, vol. 17, no. 11, pp. 1-13, Nov. 2012.
Skin colour:
M. D. Mehta, S. S. Niranjan, N. Gupta, R. C. Bhatia, "Photoplethysmography: a noninvasive technique for blood pressure estimation," North American Journal of Medical Sciences, vol. 7, no. 3, pp. 77-85, Mar. 2015.
T. Inan, S. Etemadi, K. Paloma, J. Giovangrandi, "On the challenges and opportunities in the wearable devices for blood pressure monitoring," IEEE Journal of Biomedical and Health Informatics, vol. 19, no. 6, pp. 1335-1346, Nov. 2015.
Finger better than Wrist for PPG:
Allen, J. (2007). Photoplethysmography and its application in clinical physiological measurement. Physiological Measurement, 28(3), R1–R39. https://doi.org/10.1088/0967-3334/28/3/R01
Cozzolino, D., Pascale, V., Chiaradia, G., Mazzeo, A., Massaroni, C., & Schena, E. (2020). Wearable sensors for remote photoplethysmography monitoring: A review of technologies and applications. Sensors, 20(20), 5963. https://doi.org/10.3390/s20205963
Gao, Y., Li, X., & Liu, H. (2014). Acquiring a stable and reliable pulse rate from photoplethysmography signals by using a new form of complementary ensemble empirical mode decomposition. Journal of Biomedical Optics, 19(8), 087005. https://doi.org/10.1117/1.JBO.19.8.087005
Nakamura, M., Iwamoto, H., Togawa, T., & Nakamura, T. (2002). Development of a finger-type photoplethysmographic sensor applicable to wearable devices. Journal of Biomedical Optics, 7(1), 98–105. https://doi.org/10.1117/1.1427043
Rhee, J., Kim, J., Kim, H., & Lee, K. (2014). Differences in photoplethysmography signals from the fingers and the wrist. Journal of Clinical Monitoring and Computing, 28(2), 165–170. https://doi.org/10.1007/s10877-013-9491-7