Monday, November 17, 2008

Noninvasive Trauma Monitors

A portable sensor spots circulatory shock to save lives.

By Jocelyn Rice

Warning light: A portable, noninvasive monitor developed by researchers at the University of Massachusetts Medical School bounces infrared light off a patient's muscle to gather information about oxygen, pH, and red-blood-cell levels. This data, continuously displayed on a handheld screen, can warn doctors of impending circulatory-system failure long before traditional measures would. Credit: Gwenn Ellerby, University of Massachusetts Medical School

Patients with severe injuries or serious infections run the risk of circulatory shock--a life-threatening condition in which the blood can't supply tissues with enough oxygen and nutrients. If shock is recognized in time, the patient can be resuscitated with oxygen, intravenous fluids, and medications. But catching shock early is no simple matter. A small infrared sensor currently under development at the University of Massachusetts Medical School promises to detect impending shock earlier than any other noninvasive test.

Traditionally, patients in critical condition are continuously monitored for changes in blood pressure, heart rate, and pulse oxygen saturation. But the body has mechanisms to compensate for massive blood loss and systemic infection, keeping those parameters steady even while the patient's status deteriorates. "When the blood pressure starts to drop, it's too late," says spectroscopist Babs Soller, who developed the new device along with colleagues at the UMass Medical School. "The patient is already going into shock." The new device instead measures the levels of oxygen, pH, and hematocrit--the proportion of red blood cells in the blood--in a patient's muscle tissue.

"Until now, we've either had noninvasive methods which are very insensitive, like blood pressure, or we've had sensitive methods that are invasive and cumbersome," says George Velmahos, chief of trauma, emergency surgery, and surgical critical care at Massachusetts General Hospital, who was not involved in developing the device. "So the noninvasive and continuous nature of this method is key."

Soller's device beams near-infrared light through the skin over an arm or leg muscle, where it travels through fat and reflects off muscle tissue and back to the monitor. Based on the spectrum of the reflected light, computer algorithms determine the oxygen, pH, and hematocrit levels. Unlike similar infrared biomeasurement devices, the new monitor automatically compensates for differences in skin color and fat thickness between patients to optimize the results.

One of the ways that the body compensates for blood loss or impaired circulation is by prioritizing which tissues most need oxygen. Blood is shunted away from skeletal muscles and internal organs and delivered instead to the heart and brain. A pulse oximeter, which analyzes the blood before it has delivered oxygen to tissues, can't tell whether this kind of compensation is occurring. Because the new device measures oxygen within the muscle, it can give a more complete picture of how well the blood is feeding peripheral tissues. A substantial drop in muscle oxygen while pulse oxygen saturation remains steady could indicate that the patient is compensating for internal bleeding and will soon "crash."