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Eye oximeter traces vital signs

25 Jun 2002

A four-laser device that measures oxygen levels in the retina could become a vital tool in the hospital emergency room.

Scientists in the US have developed a prototype device that directly measures the amount of oxygen reaching the brain. The tool could indicate the health of a patient who has suffered a severe physical trauma before the conventional vital signs show any significant change.

After a heart attack, head injury or internal bleeding, doctors check the supply of oxygen to the brain by measuring the patient's pulse and blood pressure.

However, these "vital signs" can be misleading - a patient with severe internal bleeding will not show any drop in blood pressure until one-third of the body's blood has been lost, for example. In contrast, the oximeter could indicate a life-threatening problem hours earlier.

The four-laser spectroscopic oximeter, developed initially by scientists at the University of Alabama, US, measures the oxygen saturation in retinal blood vessels, which share their blood circulation with the rest of the brain. A single measurement takes only one second to perform.

Sarnoff Corporation, US, has set up a joint venture company with the Alabama scientists called VitalCare Diagnostics to develop the device. It is starting full-scale development in the next few weeks. Chris Gregory, head of biomedical devices at Sarnoff, said: "The eyes are not just a window to the soul, they offer a looking-glass into the brain."

Gregory explained that the four lasers are needed to determine four parameters - oxygen saturation, hemoglobin concentration, blood vessel diameter and a scattering coefficient. Three wavelengths - 635, 670 and 830 nm - are currently provided by diode sources, with the fourth a 488 nm emission supplied by a small argon ion laser.

"The four wavelengths enable us to create a suffciently accurate spectrum to calibrate the oxygen saturation," said Gregory. He added that the argon ion laser will be replaced with a compact diode-pumped source in the eventual product.

Clinical trials of a clinical prototype are expected to begin in about a year, and a marketed device could be available in 2004. Subsequent applications in diabetic retinopathy, stroke screening, glaucoma and hypertension monitoring are envisioned.

Michael Hatcher is technology editor of Opto and Laser Europe magazine.

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