The U.S. military has begun developing an ultrasonic tourniquet in an effort to stop life-threatening bleeding during combat.
Called the Deep Bleeder Acoustic Coagulation (DBAC) program, it aims to create a cuff-like device that wraps around a wounded limb. Rather than applying pressure to the wound to stem the flow of blood, the device would use focused beams of ultrasound (sound waves above the audible frequencies) to non-invasively clot vessels no matter how deep they are.
If a major blood vessel is hit and a lot of blood lost quickly, a person can die in a few minutes, says Michael Pashley, head of Ultrasound Imaging and Therapy at Philips Research in Briarcliff Manor, NY, one of the groups taking part in the program.
According to the Pentagon’s Defense Advanced Research Projects Agency (DARPA), “these internal bleeding injuries are the leading cause of death for soldiers in the battlefield,” says Pashley. In light of this, DARPA is committing up to $51 million for the project over four years, to be spread among a number of different research organizations.
The ultrasound tourniquet is intended to buy time, so that a medic can get the patient to a better-equipped medical facility, says Lawrence Crum, director of the Center for Industrial and Medical Ultrasound at the University of Washington’s Applied Physics Laboratory in Seattle, who has been working in this field for more than a decade.
Once applied to a wounded limb, the cuff would automatically detect and then seal damaged blood vessels or arteries, by focusing beams of ultrasonic waves at the wound to clot it, in a process known as high-intensity focused ultrasound, or HIFU.
Ultrasonic waves are usually innocuous, bouncing off tissue. This is the principle behind sonograms, says Crum. But when the ultrasonic waves are focused, the effect is radically different. “If you concentrate ultrasound in the same way as light, you can raise the temperature, particularly if the wave is absorbed by the tissue,” he says.
To achieve this effect, the frequency has to be geared to increase its absorption by the tissue, while the intensity must be roughly one million times greater than imaging ultrasound. When applied to a bleeding wound, the effect is similar to cauterization, Crum says.
HIFU is already approved in parts of the world for treating prostate cancer, while clinical trials are underway to use it to treat liver and kidney cancer. For cancer treatment, the tumor tissue is ablated using the HIFU.
Applying it to bleeding seems like a sensible next step, says Gail ter Haar, a physicist at the Institute of Cancer Research’s Therapeutic Ultrasound Team in the Royal Marsden Hospital, near London. “It is ambitious but it’s quite realizable,” she says.
Surrounding tissue may be damaged in the process, since it will be heated close to the boiling point. But the blood vessels remain functional because “the blood flow in the vessel cools the wall and so protects it,” says ter Haar. So the blood around the opening coagulates, while the blood passing through the vessels keeps on flowing.
The biological feasibility of this technology is well established, says Joseph Eichinger, president of Seattle-based AcousTx, which was spun out of another company, Therus, to take part in DARPA’s research program. Therus, also in Seattle, has also been developing ways to use ultrasound to stop bleeding. In particular, its acoustic hemostasis system is being developed to seal punctures in the femoral artery of the groin that are caused as part of cardiac catheter treatments. Normally, these punctures have to have continuous pressure applied to them, and can take from 30 minutes to several hours to seal, says Eichinger. With the HIFU approach, they seal in just a few seconds.
In its final form, the acoustic cuff will consist of a lightweight, flexible device with both ultrasonic imaging transducers and therapeutic transducers lining its insides. The imaging transducers, which function in the same way as sonograms, will be used to first identify the vasculature within a limb and locate any bleeds. The therapeutic transducers are then focused to stem the blood flow.
All these capabilities have been demonstrated as separate parts, says Eichinger – now comes the engineering hurdle of putting them together in a package capable of surviving the rigors of a battlefield. “It is a very challenging environment,” he says. “It’s hard enough to take an iPod into Iraq and make it work.” Indeed, the heat, humidity, dust, and noisy electromagnetic environment of combat couldn’t be further from a safe and clean hospital treatment room.
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