Optical pulse detection ‘photoplethysmography’ (PPG) provides a means of low cost, unobtrusive and continuous cardiovascular monitoring that is used ubiquitously for decades in medical settings for blood oxygen monitoring and more recently popular in many wearable devices to monitor various aspects of the cardiovascular system such as heart rate, blood pressure, blood oxygen saturation, respiration rate and more experimentally blood sugar levels.
PPG sensing consists of a light source and photodetector. Light is emitted into the skin and the intensity of light transmitted into the photodetector will vary depending on the volume of blood in the vascular bed of the measurement site, taking advantage of blood’s absorbent qualities to visible and infrared (IR) light.
One of the main factors affecting PPG sensing performance is its susceptibility to interference, from movement of the body. Other significant factors affecting the performance include the amount of blood flowing into the peripheral vascular bed, the varying optical properties of skin and blood, ambient light, and the wavelength used to illuminate the skin.
Race and PPG Sensing
Melanin protects the skin against the harmful ultraviolet (UV) radiation from the sun, its absorbing qualities increase as the wavelength of light decreases, being highly absorbent to shorter wavelengths ranging from UV to yellow light (200-600nm). Smart-watch PPG sensors typically use green light (492-577nm) whilst pulse oximetry sensors typically use red light (622-780nm) and IR light (780-2500nm). Both sensing modalities, however, have had reports on inaccuracies for people with darker skin tone.
A large-scale pulse oximetry study (1609 subjects: 1333 white patients and 276 black patients) found black patients to have three times the frequency of occult hypoxemia (an arterial oxygen saturation estimates of <88% despite an oxygen saturation of 92% – 96%) as white patients. Additionally, a smartwatch PPG sensing study with 256 subjects found darker skin toned subjects had Heart Rate measurements 10 times less accurate than lighter skin toned subjects. However, other research suggests that melanin is not a significant factor.
With anecdotal evidence and conflicting findings, understanding of the magnitude and scope of the potential inaccuracies of current PPG sensing due to skin melanin content is unclear. This is a concerning problem given that 80% of the world population are individuals with pigmented skin and it has been projected that by 2035 half of the black population in USA will be affected by cardiovascular diseases.
To address this problem this research project will increase the diversity of subjects to have proportional representation. Current studies exploring PPG sensing and skin melanin content tend to have smaller numbers of participants with darker skin tones raising concerns of misleading conclusions.
Gender and PPG Sensing
The cardiovascular system of biological males and females differ causing noticeable changes to the collected PPG signals. Biological males have higher average blood pressure and larger diameters of arteries causing the amount and velocity of blood in the measurement site to be greater in biological males allowing for better PPG measurements. Additionally, a biological female heart tends to be smaller in mass than a biological male heart causing biological females to have higher heart rates.
These biological differences compounded with other interferences such as skin tone, movement and ambient light could cause significant differences in signal quality based on an individual’s biological characteristics.
How Is This Study Going to Fix These Issues?
Multi-wavelength PPG shows promising signs of becoming a viable method for remote physiological monitoring. The selection of wavelengths used in PPG sensing is a compromise. Green light (492–577nm) produces the best generalized modulation, however, multi-wavelength approaches for HR (Heart Rate), blood pressure and blood glucose estimations have been shown to out-perform single-wavelength approaches.
Multi-wavelength approaches in clinical applications are not unique to PPG sensing. Narrow-band imaging for gastrointestinal endoscopy has seen improvements from white light endoscopy using blue and green light due to their varying interactions with blood and tissue. Blue light (400–430nm) penetrates to the depth of the capillaries in the superficial mucosa, while the green light (525–555nm) penetrates deeper into the mucosa. With the use of multiple wavelengths, the accuracy, robustness and generalisability of PPG sensing could be dramatically increased.
How Can I Get Involved
If you would like to help tackle gender and racial bias in medical and fitness devices, please consider getting involved in this study. You can register your interest here, evolveppg.co.uk/register-interest as well as follow us on Instagram @evolveppg for updates.
The study will take an hour to complete, and you will be gifted with a £20 Amazon gift card (thanks to the Research Support Award!). During the hour you will be asked to walk/run on a treadmill and cycle on an indoor bike for 10–15-minute periods.
Daniel was awarded a Research Support Award to help fund his research data collection activities. To find out more about Research Support Awards, visit the Funding section in Moodle.
