July 26, 2023 -- Researchers in Texas are developing a “green light” technology they hope will solve a crucial problem highlighted by the pandemic: the limits of pulse oximeters in patients with darker skin.
A recent study adds weight to earlier findings that their device works.
“It is a new, first-in-class technology,” said Sanjay Gokhale, MD, the bioengineer who is leading this research at the University of Texas at Arlington. “The team conducted extensive preclinical work and carried out phase I studies in human volunteers, demonstrating sensitivity and accuracy.”
It’s one of several projects underway to update pulse oximetry, a technology based on research in lighter-skinned people that has not changed much in 50 years.
The pulse oximeter, or “pulse ox,” measures the saturation of oxygen in your hemoglobin (a protein in red blood cells). But it tends to overestimate the oxygen saturation in patients with darker skin by about 2% to 3%. That may not sound like a lot, but it’s enough to delay major treatment for respiratory issues like COVID-19.
“Falsely elevated readings from commercial oximeters have delayed treatment of Black COVID-19 patients for hours in some cases,” said Divya Chander, MD, PhD, an anesthesiologist in Oakland, CA, and chair of neuroscience at The Singularity Group. (Chander was not involved in the UT Arlington research.)
Early research happening separately at Brown University and Tufts University aims to redesign the pulse oximeter to get accurate readings in patients of all skin tones. University of California San Diego researchers are looking into a method that measures blood oxygen using sound in combination with light. Other solutions try to correct for skin tone with algorithms.
The device from UT Arlington uses an algorithm too, but its main innovation is that it replaces red light with green light.
Red Light, Green Light
Traditional oximetry devices, which typically clip on to the patient’s fingertip, use LEDs to beam light through the skin at two wavelengths: one in the red part of the spectrum, the other in the infrared. The light transmits from one side of the clip to the other, passing through arterial blood as it pulses.
The device calculates a patient’s oxygenation based on how much light of each wavelength is absorbed by hemoglobin in the blood. Oxygenated hemoglobin absorbs the light differently than deoxygenated hemoglobin, so oxygenation can be represented as a percentage; 100% means all hemoglobin is completely oxygenated. But the melanin in skin can interfere with the absorption of light and affect the results.
The green light strategy measures not absorption but reflectance -- how much of the light bounces back. As with traditional oximetry, the green-light method uses two wavelengths. Each is a different shade of green, and the two forms of hemoglobin reflect them differently.
Using an algorithm developed by the researchers, the device can capture readings in patients of all skin tones, the researchers say. And because it works on the wrist rather than a finger, the device also eliminates issues with cold fingers and dark nail polish – both known to reduce accuracy in traditional oximetry.
In the latest experiments, the researchers tested the technology on synthetic skin samples with varying amounts of melanin, Gokhale said. The device picked up changes in blood oxygen saturation even in samples with high melanin levels.
In a study published last year, the technology was tested in 16 people against an invasive handheld blood analyzer and a noninvasive commercial pulse oximeter, and found to be comparable to the invasive method.
A Drawback
The green light approach could be “game changing,” Chander said. But there is a drawback.
Since green light doesn’t penetrate as deeply, this approach measures blood oxygen saturation in capillary beds (small blood vessels very close to the skin surface). By contrast, traditional oximetry measures oxygen saturation in an artery as it pulses -- thus the name pulse oximetry.
Valuable information can be obtained from an arterial pulse.
Changes in arterial pulse, known as the waveforms, “can tell us about a patient's hydration status [for instance],” Chander said. “In a mechanically ventilated patient, this variation with a patient's respiratory cycle can give us feedback about how responsive the patient will be to fluid resuscitation if their blood pressure is too low.”
Given such considerations, the green light method may be useful as an adjunct, not a full replacement, to a standard pulse ox, Chander noted.