New method developed for measuring oxygen in blood

In select operating rooms worldwide, surgeons are using a new kind of noninvasive oxygen monitor that makes up for the current technology’s major blind spot—detecting low oxygen levels that arise because of low or obstructed blood flow even though the lungs are working just fine.

According to recently published studies, many conducted at Stanford, the monitor works and has probably saved lives. The inventor, former Stanford associate professor David Benaron, MD, suggests that the device also shows promise for uses outside of surgery and critical care. Among these: tumor detection and drug development, said Benaron.

Like the pulse oximeter—the standard oxygen monitor used in the ICU and surgery— the new device determines oxygen levels by noninvasively reading the blood’s color. But unlike the pulse oximeter, this new monitor can zero in on the amount of oxygen reaching specific tissues. An added benefit is that it works even if the patient has no pulse. The FDA granted clinical approval in November.

The first clinician to give the device serious consideration was Stanford professor of anesthesiology John Brock-Utne, MD, PhD. He learned about Benaron’s vision for the device when it was still in the planning stages. Once it was built he was the first to test it. “John encouraged me to build the monitor and use it to study the ‘blind spots of oximetry’—periods of low oxygen saturation that pulse oximetry doesn’t reveal,” said Benaron.

Brock-Utne, Benaron, Pieter van der Starre, MD, and 16 others produced a study in animals and humans and showed that the device reliably monitors oxygen levels, even in situations in which pulse oximetry fails. The study appeared in the June 2004 Anesthesiology.

Among pulse oximetry’s blind spots is one that occurs during bypass surgery. Because the pulse oximeter relies on the pulsing of a patient’s blood vessels to assess the oxygen level, it’s of little use during such an operation, as this pulse ceases. “Many studies from different institutions have shown that between 5 and 10 percent of cardiac bypass patients experience subtle declines in intellectual ability due to diminished oxygen supply to the brain,” said van der Starre.

The pulse oximeter, invented in 1978 by William “Bill” New, MD, PhD, then a member of Stanford’s anesthesia clinical faculty, uses red and infrared light to measure the blood’s color: the redder the blood, the higher the oxygen. Prior to its invention, surgical teams had no easy way to monitor oxygen. For the most part they kept an eye on the patient’s skin color: A bluish hue meant oxygen was low.

Benaron’s monitor uses shorter light waves, primarily blue and green, to measure the blood’s color. The use of shorter light waves allows the device to monitor only the smallest blood vessels, called capillaries, where oxygen is delivered to the tissues. In contrast, the pulse oximeter’s longer light waves offer information about arteries, which are the larger vessels that merely move the blood to the capillaries.

Benaron, who founded the biomedical optics laboratory at Stanford in 1990, started a company, Spectros, in 2000 to develop the device, the T-Stat Oximeter. Benaron serves as the company’s president. So far, the company has distributed 25 of the light-based oximeters for testing worldwide, with more than half at Stanford. Teams of clinicians here have conducted five trials and three others are under way. Results from two trials have been published and three more are in press.

Among those impressed with the oximeter are Stanford’s vascular surgeons. “Vascular surgeons are a conservative group. If something works we just stick with it,” said Cornelius Olcott, MD, clinical director of Stanford’s vascular surgery division. “But this new oximeter does seem to offer advantages. We’re taking a look at it.”

The vascular surgeons believe the device could alert surgeons to a rare but deadly complication of a surgery to repair an aneurysm in the abdominal section of the aorta. In about 3 percent of those undergoing the procedure the colon becomes starved of oxygen. The problem is, there’s no way to detect the problem until days later, and 57 percent die before leaving the hospital, said Olcott.

Olcott’s hope is that the device will reveal ischemia of the colon while the patient is still in the operating room, giving surgeons a chance to restore flow before damage occurs. Surgeons in his department have begun using the device to monitor the oxygen level of blood flowing to the colon. Vascular surgery resident Eugene Lee, MD, now an assistant professor at UC-Davis, has been gathering clinical data, assessing 40 vascular surgeries using the oximeter. In three cases, the oximeter alerted the surgeons to ischemic conditions they wouldn’t have known about otherwise.

Associate professor Daniel Sze, MD, PhD, in the Division of Interventional Radiology, used the device to monitor oxygen during minimally invasive surgery, and reported results similar to Lee’s.

Benaron said that he wouldn’t be shocked if the device takes off in other areas in addition to vascular surgery. Already several researchers at Stanford are testing the device for its value in targeting cancer treatment, as areas with tumor cells tend to have lower oxygen levels. “Our new device will walk in all sorts of directions,” said Benaron. “It could even enable procedures that couldn’t be done before.”

Benaron founded the biomedical optics laboratory at Stanford in 1990, and holds equity in and receives financial compensation from Spectros Corporation, an NIH- and NCI-supported medical device company that developed the device described in this article.