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The three stages, before, during and after pressure
is applied are represented in the figure. The arrows indicate emitted light and reflectance based on the pigment in the finger.
1) Before pressure is applied, the device measures the reflectance of the light when the capillaries are full.
2) At the site of the pressure, there is a distinct color change in the skin causing the emitted light to have a different reflectance pattern.
3) As the blood flows back in the capillaries, the detector identifies a return to the original reflectance pattern, indicating that the capillaries have refilled with blood.
During his daily routine, Vassilios J. Bezzerides, MD, PhD, a clinical fellow in the Cardiovascular Program at Children's Hospital Boston, noticed that it's often difficult to determine whether a patient is properly hydrated. Measuring electrolytes or other markers in blood or urine can provide a good gauge, but they involve cumbersome sample collection and can produce false alarms. There's a quick noninvasive method: placing gentle pressure on the fingernail until it turns white, then timing how long it takes for the finger to regain color, indicating the capillary refill time. But this test is crude and its interpretation highly subjective.
One night, Dr. Bezzerides was checking on a sleeping patient. Not wanting to disturb the patient by turning on the overhead light, he instead observed the color change in the patient's finger using a tricolor LED flashlight. The color in the finger was completely washed out by the white light—the finger seemed to disappear, but slowly reappeared as blood flowed back into the capillaries.
He realized that one of the wavelengths making up the white LED was being absorbed, and that he might be able to image the capillary blood with a bedside tool that took advantage of this phenomenon. So he made a device that put pressure on the patient's finger, then captured the reflection from appropriate wavelengths of light. The device measured capillary refill time quantitatively, with an actual number readout.
His device will have applications not only in the hospital, but also at the bedsides of assisted-living patients and among athletes, who can measure their hydration and optimize fluid intake during rigorous activity. Most important, this test could be used in developing countries with a high incidence of dehydration caused by gastrointestinal infections, especially among children. A hydration device could help triage patients, allowing limited staff to focus on those who need immediate attention.
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