Saving Soldiers

Scientists look at ways to quickly diagnose blood loss

by Cindy Tumiel

Blood loss—whether brought on by gunshot wound, flying shrapnel or explosive concussion—has been the leading cause of battlefield death. The body’s own survival systems can trick a medic into misunderstanding how much blood a wounded soldier has lost and how close that soldier may be to dying.

Researchers are trying to find a more definitive indicator of blood loss. Then they can develop diagnostic tools for the battlefield that rapidly measure these body cues to give a medic better information about the patient’s true state.

The first step, though, is finding a safe way to simulate hemorrhage in a laboratory setting.

William Cooke, a professor of kinesiology at UTSA, is employing a negative pressure chamber that applies changes to the lower body of volunteers as a means to simulate battlefield blood loss. The experiments, funded by the Department of Defense and carried out in collaboration with PERL Research, a Huntsville, Ala., engineering firm, are a first step in a long road to give combat medics a rapid diagnostic tool that could help save soldiers’ lives.

The current problem is this: The human body has an intricate survival system that kicks in to compensate for hemorrhage. As blood flows out of the body, pressure receptors in the large arteries sense decreasing blood volume and send cues to the heart to speed up the pumping rate. So a wounded soldier with a damaged leg or arm or with severe internal injuries can still have a strong pulse, and in fact may be conscious enough to tell a medic, “I’m OK.”

But that compensatory system is maximized once the body loses about 40 percent of its blood, and a pulse doesn’t tell the medic how close the injured soldier is to this critical point. Death can occur moments after a medic felt a strong pulse and had a conversation with the patient.

“What a medic really needs to know on the battlefield is how much blood the soldier has lost,” Cooke says. “You can see he is bleeding, but three or four others may be bleeding, too. How does the medic decide which person to go to? The heart rate does not give you enough information.”

Cooke’s negative pressure chamber works along the same principle as an iron lung, a device invented in the early 1900s that used negative pressure changes to assist the lungs of polio sufferers who could not breathe on their own. Years later, the space mission Skylab began taking humans on long deployments into orbital space, and researchers developed a lower body negative pressure device to help the astronauts cope with extended periods at zero gravity.

The sleek white cylinder in Cooke’s laboratory follows this model from the space program. It opens wide enough on one edge to slide a human test subject’s legs and lower torso into the tube. Then, a neoprene skirt is used to seal the test subject at the waist before the device is activated.

As negative pressure builds in the tube, it pulls blood away from the upper body and brain and into the lower torso and legs. This displacement tricks the heart and brain into thinking that the body is bleeding, and the brain activates coping mechanisms that defend against dangerous reductions in blood pressure.

“The heart does not know the difference if blood is being displaced or if blood is being lost,” Cooke says. “What it notices is that there is not a lot of blood coming back to it.”

The laboratory tests will simulate blood loss that can occur when a limb is severed or severely damaged. Researchers will pay close attention to stroke volume, or the amount of blood being pumped with each beat during the simulated blood loss. Stroke volume falls as blood returning to the heart decreases. Such information would be helpful in battlefield triage, when medical personnel are determining which soldiers have the most critical injuries.

“Stroke volume is what a medic needs to know on the battlefield but can’t,” Cooke says. “The pulse rate doesn’t tell you that. So what other physiologic variables do we have that we can measure? That is the point of the study.”

Cooke has a hypothesis that skin temperature of the face and head decreases in conjunction with decreases of stroke volume. If experiments with the negative pressure chamber confirm the hypothesis, they will develop and test computer-driven sensors that could be used in the battlefield to make rapid measurements of the skin temperature in the web of arteries below the scalp.

“The amount of blood leaving the heart directly reflects how much blood is entering the heart,” Cooke says. “If we know that a medic can’t measure stroke volume and heart rate gives him no information, then we need some better tools. A better tool might be the thermal image of the arteries of the brain and how these temperatures change and correlate with reductions in pressure.”

The temperature sensor could be deployed with the medic. But the long-range goal of the research project is to mount these new tools onto robotic devices that could go into the danger zone of a battlefield in advance of the medic.

“Medics have incredibly high fatality rates,” he says. “Our idea is to use robotic remote triage.