Carotid ultrasound: Familiar tech has new use
Predictive tool for AMI, stroke coming soon
Not too far out on the horizon is a new use for that ultrasound equipment your hospital has been using for years.
"Our new application for ultrasound is not out in clinical practice at the moment, but it could get there very quickly," says Daniel O’Leary, MD, radiologist-in-chief at New England Medical Center and a member of the faculty of Tufts University School of Medicine, both in Boston. He was lead author of a recent study supported by the National Heart, Lung, and Blood Institute that demonstrates a direct relationship between the thickness of the carotid arteries and the risk of future heart attack and stroke.1
O’Leary’s research team showed that ultrasonography can predict the risk of future events in older individuals who have no cardiovascular disease symptoms. "Increases in the thickness of the intima and media of the carotid artery, as measured noninvasively by ultrasonography, are directly associated with an increased risk of myocardial infarction and stroke in older adults without a history of cardiovascular disease," according to the study.
In O’Leary’s words, "Our conclusion was that measuring the thickness of the carotid artery permitted us to identify asymptomatic and presymptomatic individuals’ future risk for heart attack and stroke."
He is describing a screening test that looks for disease. "Does it replace anything? Is it additive? Is this a cost-effective test in terms of population? Answers are unclear at the moment," says O’Leary.
Your staff probably performs carotid ultrasonography on a daily basis, and they are familiar with the technology. But while they may be skilled at looking with doppler for flow abnormalities and high-grade stenosis of the carotid artery, this proposed screen that looks at the artery wall is a different ballgame.
"Only a few technicians — primarily those involved in this study — are skilled in the use of ultrasound to examine the artery wall, measure it, then make predictions based on those measurements," explains O’Leary. "Wall thickness changes with age and sex. There’s no magic number that we can say predicts risk. To get any useful information, a technician performing this new procedure must be able to take the measured value and properly adjust it for age and sex."
While third-party payers reimburse for the routine use of ultrasound now at about $200 to $250 per patient, there’s no guarantee that they will be as comfortable paying for this proposed carotid-wall-screening test. Patients screened for high-grade stenosis to determine stroke risk comprise only about 1% of the general population. When you use ultrasound to measure wall thickness, that test can be applied to a much greater portion of the population — about half.
"In terms of expense," says O’Leary, "testing half the population with ultrasound implies a huge outlay of money. Up until now, insurance companies look at this technology as having a very limited application. Suddenly, we’ll be proposing that it be used in 50% of the population. They won’t like that."
And there are more questions about this technique. Patients with risk factors such as high cholesterol levels and hypertension generally have thickened vessels, so when the ultrasonography finds that thickening, how much information has that added to what is already known? As an across-the-board screening test, does it give us new information? In some individuals it does, but in the majority of people, it doesn’t. It simply confirms what is already known. The questions to ask, advises O’Leary, are "What subset of the population would benefit from having the test?’ and Where might it be cost beneficial?"
You save money not by replacing a cheaper test like a cholesterol or blood pressure screen, but by helping to modify behavior. That ultimately reduces the risk of death, heart attack, or stroke, all of which cost money.
Triaging the borderline subset
People who will benefit from this test are those with borderline elevations of standard risk factors — slightly elevated cholesterol and slightly elevated blood pressure, for example. Physicians are uncertain whether that subset of patients should receive aggressive therapy, and this test may help triage that group. Such therapy includes control of high blood pressure and cholesterol, weight loss counseling, increased physical activity, and aspirin and other drug therapies.
"It won’t help people who are totally healthy, thin, and with no risk factors," says O’Leary, "and on the other hand it will merely confirm what you already know in those with many risk factors."
O’Leary’s study involved nearly 4,500 men and women over 65. Those participants with the most increased carotid intima-media thickness had an almost fivefold greater risk of heart attack or stroke than those with the thinnest measures. Even after accounting for standard cardiovascular disease risk factors such as cigarette smoking, high blood pressure, high blood cholesterol, and diabetes, patients with the thickest artery walls still had more than double the risk of a heart attack or stroke than those with the thinnest walls.
The thickness of the internal carotid artery was as important a predictor of cardiovascular events as the thickness of the common artery, and by combining the measures from both, the investigators gained a more complete picture of the patients’ conditions than either measure alone could yield.
After a median follow-up period of 6.2 years, the research team calculated that the age- and sex-adjusted risk of heart attack or stroke was almost four times higher for subjects with the greatest carotid artery intima-media thickness than for those with the least thickness.
"The strength of the associations between intima-media thickness and outcome was at least as strong as the associations seen with traditional risk factors," the authors wrote. "One of the nice things about [carotid ultrasonography] is that it gives a graphical image," says one. "You can show [it] to a patient and say, Look, here’s what a normal artery looks like and here’s what your artery looks like. Don’t you think you should cut down on all that butter?’ "
O’Leary comments on a part of the study that he terms speculative: "We are now looking at the progression of disease over time as well as at single test measurements."
Over the past 10 years since the data was gathered, the researchers have examined the participants’ vessels twice more. "Let’s say a patient has disease and is being aggressively treated with statins and blood pressure lowering drugs and aspirin," says O’Leary. "Is the treatment doing that patient any good? Are the drugs making a difference? That opens a whole other reason for doing the test."
Tracking change over time helps a physician modify treatment choices. If a patient is spending a lot of money on drugs because of a possible risk, he and his doctor want to know if that medicine is slowing the process down.
Reference
1. O’Leary DH, Polak JF, Kronmal RA, et al, for the Cardiovascular Health Study Collaborative Research Group. Carotid-artery intima and media thickness as a risk factor for myocardial infarction and stroke in older adults. N Engl J Med 1999; 340:14-22.
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