Testing blood pulled from vessels near the heart can provide 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 delayed to be useful in treating an unstable patient. Soller's monitor, in contrast, noninvasively provides continuous, real-time results.

While other devices, such as Hutchinson Technology's InSpectra StO₂ Tissue Oxygenation Monitor, can noninvasively measure tissue oxygenation, Soller's is 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 her colleagues have been working on their monitor for more than a decade. In October, at the 2008 Center for Integration of Medicine and Innovative Technology Congress, in Boston, they exhibited a new, fully portable version of the device that's under a pound, less than one-eighth its previous weight.

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 operating room for surgeries associated with high blood loss.

Because the device is noninvasive, it has been through extensive preliminary testing. 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 promising sign that it could serve as an effective early-warning system.

Before the device can be used in clinical settings it will need to receive U.S. Food and Drug Administration approval. A company called Reflectance Medical has been founded to help guide the monitor to market. Meanwhile, Hutchinson's InSpectra monitor, which has been commercially available since 2007, is already in clinical use.