Public Release: 

New device finds silent clots, may help prevent strokes

American Heart Association

DALLAS, Aug. 2 - For the first time, a device has precisely detected tiny blood clots that can enter brain blood vessels during heart surgery, an advance that may help prevent stroke-causing blood clots and memory loss. Researchers report the findings of two preliminary studies in the August issue of Stroke: Journal of the American Heart Association.

The device, called multifrequency transcranial Doppler (MTCD) ultrasound, also allows physicians to distinguish between blood clots and gas bubbles that often enter the bloodstream during heart procedures and surgery.

Transcranial Doppler ultrasound uses sound waves transmitted through the skull to create images that allow physicians to study blood flow through vessels in the brain. The conventional transcranial Doppler generates sound waves at just one frequency. The MTCD device generates sound waves at two frequencies.

"The distinction is important because a solid clot reflects back more ultrasound signal at the higher frequency, but gaseous bubbles reflect back more signal at the lower frequency," says study author David Russell, M.D., professor of neurology at The National Hospital, Oslo, Norway.

The technology, available in Europe but not in the United States, could greatly reduce one of the most common complications of coronary bypass surgery. In 1999, 355,000 Americans underwent bypass surgery, according to the American Heart Association. Russell says as many as 50 percent of coronary bypass patients experience cognitive decline such as memory loss. Tiny clots that cause minor, symptomless strokes during the surgery lead to this impairment. Conventional monitoring doesn't detect those tiny clots, he says.

"In the past, we could detect something entering the arteries of the brain, but we could not tell if it was a very small gas bubble or blood clot. It was like a policeman who could detect a speeding vehicle but didn't know whether it was a small car or a large truck," Russell says. "With multifrequency transcranial Doppler, we can detect these small clots automatically when they come into the brain, and the surgeon can change the technique to make it safer for the patient."

Noting that bypass surgery can last as long as four hours, Russell says the device can provide prolonged monitoring for blood clots.

In the two articles, Russell and Rainer Brucher, Ph.D., professor of neurology at Ulm University of Applied Sciences in Germany, report findings from a laboratory study and a clinical study involving 15 patients with mechanical heart valves and 45 patients with narrowing of the carotid (neck) arteries.

First, they evaluated the effectiveness of multifrequency ultrasound in detecting material entering the brain through the bloodstream.

It proved highly accurate, classifiying 100 percent of solid and gaseous objects and ruling out 99.3 percent (596 of 600) of artificially generated clots in a laboratory study. In patients, the device identified 98.6 percent (546 of 554) of the gas and solid objects, and it correctly ruled out 98.9 percent (791 of 800) of artificially generated clots.

In the second article, they evaluated the technology's ability to distinguish between solid and gaseous clots. Knowing the type of material could help determine the origin of a clot and point to therapies or changes in surgical procedures to prevent clot formation, Russell explains. In the laboratory portion, researchers used MTCD to distinguish between solid plastic balls and gas bubbles flowing through a silicon tube immersed in water. It correctly identified 95.1 percent of the objects overall. Of 159 plastic balls, it classified 152 (95.6 percent) as solid and seven (4.4 percent) as uncertain but apparently solid. Of 105 gas bubbles, the instrument classified 99 (94.3 percent) as gaseous and six (5.7 percent) as uncertain gaseous.

In the clinical portion, researchers used MTCD to monitor blood flow through the patients' middle cerebral artery. The device accurately categorized 98.6 percent of clot-related events as solid or gaseous in heart valve patients and 94 percent of events in patients with narrowing of the neck artery. It detected 514 clots in mechanical heart valve patients. MTCD classified 433 (84.2 percent) as gaseous, 74 (14.4 percent) as solid, and seven (1.4 percent) as uncertain.

Mechanical heart valves often generate gaseous and solid clots, but the significance of the clots is unclear, say Russell and Brucher. Movement of the valve components may create the gas bubbles, and clots may arise on valves from stimulation of platelets (the blood components that form clots) or accumulation of coagulated material on the valve, they say.

"We hope the detection of blood clots in these patients will help us identify those most at risk of stroke," Russell says.

In patients with narrowed carotid arteries, clots can arise at the site of narrowing and break free and travel through the blood stream to the brain, causing a stroke. MTCD detected 32 symptom-free clots in 17 of 45 (38 percent) patients with carotid narrowing. The device classified 30 symptom-free clots (93.7 percent) as solid and two (6.3 percent) as uncertain solid. None were gaseous. Physicians might also use MTCD to monitor patients during a wide range of operations and other patients at an increased risk of stroke due to blood clots, Russell says.


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