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 difficult. One solution may be a small infrared sensor currently under development at the University of Massachusetts Medical School. It 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 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 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 the development. "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 compensates for differences in skin color and fat thickness between patients for accurate results.

One way the body compensates for blood loss or impaired circulation is by prioritizing which tissues need oxygen most. 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 gives a more complete picture of how well blood is feeding peripheral tissues. For example, 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."

Testing blood pulled from vessels near the heart provides even more information, but that requires a painful, highly invasive, labor-intensive procedure. And since blood samples must be sent to a laboratory for analysis, results are often too late to be useful in treating an unstable patient. Soller's monitor, in contrast, noninvasively provides continuous, real-time results.

While other devices can noninvasively measure tissue oxygenation, Soller's is said to be the only one that measures pH and hematocrit levels as well. Muscle pH is an important indicator of how well treatment is working, says Soller. Cells deprived of oxygen cease to function properly, leading to acid buildup, which lowers pH and causes further damage. When the patient is resuscitated, tissue oxygen levels are restored before tissue pH recovers. Without a good measure of tissue pH, doctors have no way to know whether the patient needs further resuscitation.

Measuring the hematocrit level adds yet another dimension. Administered fluids may rescue a trauma patient's plummeting blood pressure, but they can also dilute the blood, reducing the proportion of red blood cells and impairing oxygen delivery. A falling hematocrit level would alert doctors to this problem early enough to properly address it.

Soller and colleagues have been working on the monitor for more than a decade. In October, at the 2008 Center for Integration of Medicine and Innovative Technology Congress, Boston, they exhibited a more recent portable version of the device that weights less than a pound.

The device, whose development is funded in part by the Army, could potentially be used by combat medics to predict shock in critically wounded patients, and to monitor patients who appear stable for reactions to undiagnosed internal injuries. Soller hopes that, beyond aiding in military applications, the monitor will be useful in civilian ambulances, emergency rooms, and intensive-care units, and in the OR during high-blood-loss surgeries.

In one study, patients had the lower halves of their bodies strapped into a vacuum chamber, which sucked blood toward their feet to mimic massive blood loss. The monitor was able to detect changes before the patients' blood pressure and pulse oximeter readings began to drop, a sign that it could serve as an effective early-warning system.

The device still needs FDA approval. Reflectance Medical, a start-up, has been founded to guide the monitor to market.