The American Society of Echocardiography has published this month a consensus statement regarding the use of carotid ultrasound to identify subclinical vascular disease and evaluate cardiovascular disease risk by measuring the carotid intima-media thickness.
The statement, appearing in Journal of the American Society of Echocardiography, is intended to address issues of standardization and help improve the availability of experienced clinical laboratories that can perform high-quality carotid intima-media thickness (CIMT) studies.
The researchers, led by James H. Stein, MD, from the University of Wisconsin School of Medicine and Public Health in Madison, noted that nine published prospective studies, which included at least 1000 asymptomatic participants, have examined CIMT and cardiovascular disease (CVD) risk.
Each study demonstrated that CIMT was significantly associated with risk for myocardial infarction, stroke, death from coronary heart disease, or a combination of these events. In most of these studies, the ability of CIMT to predict future CVD events was independent of traditional risk factors. Furthermore, nine large studies have demonstrated similar or greater predictive power for carotid plaque and CVD.
Measuring CIMT and identifying carotid plaque can be useful for refining CVD risk assessment in patients at intermediate CVD risk. The test can be considered if the level of aggressiveness of therapy is uncertain and additional information about the burden of subclinical vascular disease or future CVD risk is needed, according to the statement.
Researchers recommend that CIMT measurements be limited to the far wall of the common carotid artery and be supplemented by a thorough scan of the extracranial carotid arteries for the presence of carotid plaque, to increase sensitivity for identifying subclinical vascular disease.
Ultrasound imaging of the far wall of the carotid artery produces two echogenic lines: the lumen-intima interface and the media-adventitia interface. The combined thickness of the intimal and medial layers of the arterial wall constitutes the CIMT. Current ultrasound technology is not sufficiently sensitive to measure the thickness of the intima alone, the authors noted.
The presence of carotid plaque or CIMT greater than or equal to the 75th percentile for the patient’s age, sex and race/ethnicity are indicative of increased CVD risk and may signify the need for more aggressive risk-reduction interventions. The authors do not recommend serial studies of CIMT to address progression or regression.
The relationship between increasing CIMT and incident CVD events has been established across a wide age range; however, the strongest data are for individuals between 42 and 74 years of age. For younger adults (18-42 years old), consistent, strong relationships between increasing risk factor burden, emerging risk factors and CIMT have been demonstrated.
While CT can measure coronary calcium as a CVD risk factor, the authors recommend carotid ultrasound because it does not involve ionizing radiation, an important factor to consider particularly when imaging asymptomatic young people.
B-mode imaging is preferred over M-mode imaging, according to the statement. Although M-mode has superior temporal resolution, it provides measurement of only a single point of thickness, rather than a segmental value. Carotid wall thickening is not uniform, so a single value without considering a wider region is difficult to reproduce and may not accurately represent arterial changes. Perpendicular imaging also is challenging using M-mode.
The Centers for Medicare & Medicaid Services has established a CPT code (0126T) for “Common CIMT study for evaluation of atherosclerotic burden or coronary heart disease risk factor assessment.”
For CIMT imaging, the authors recommend 3-5 beat cine-loop and optimized R-wave-gated still frames at each angle -- distal 1 cm of each common carotid artery. Technique includes:
∑ Longitudinal images from three imaging planes: optimal angle of incidence and two complementary angles (anterior, lateral, and posterior);
∑ Use cursor to mark location of bifurcation;
∑ Display clear images of distal common carotid artery perfectly horizontal with double lines on near and far walls, indicating true perpendicular scanning plane; and
∑ Optimize transducer depth (usually 4 cm) to avoid slice thickness artifacts.