Friday, November 06, 2009

Cardiac CT

Here I will try and put together the essentials of this sometimes confusing and ever changing topic. Most of this information is from a lecture by Stephen Frohwein, M.D., delivered at the 2009 Tutorials in the Tetons Update in Cardiovascular Diseases.

The use and indications for cardiac CT are more complex than many people appreciate. Although appropriate use of the various types of cardiac CT is becoming better understood, confusion abounds. Dr. Frohwein did such a good job of reducing it down to the nuts and bolts while maintaining the important distinctions that I thought it useful to summarize a few key points here. I will amplify this post with reference to the ACC Appropriateness Criteria.

The notion of “cardiac CT” is meaningless unless distinctions are made. Three types of scans, with completely different indications, could be referred to: calcium scoring, CT coronary angiography or CT angiography of related structures (pulmonary arteries or aorta).

Calcium scoring

Coronary calcifications detected by CT begin with subintimal plaque rupture. Since these ruptured plaques do not encroach on the lumen, calcium scoring is not a way to evaluate chest pain. What calcification does mean in an asymptomatic patient is that the subclinical atherosclerotic process is underway.

Possible appropriate use: always do a Framingham risk score first. Calcium scoring may be reasonable in asymptomatic patients with intermediate Framingham risk (defined as 2 or more risk factors but with a less than 10% score). It not appropriate for any other category, or in any symptomatic patient. It’s a risk screening tool, but is always ancillary to ordinary clinical risk screening tools. So, think of its role as similar to that of hsCRP or KIF6 testing. It involves no contrast and, while not covered by insurance, is cheap at around $150.

Note: in the ACC appropriateness criteria calcium scoring is NOT rated as appropriate for ANY indication! The highest rating it is given is “undetermined!”

A high score does not indicate hemodynamically significant CAD but does increase the probability of such. So what do you do with a high score? You target the patient for a full court press of preventive efforts.

CT coronary angiography

This test involves iodinated contrast and much more radiation than calcium scoring. It is used to evaluate patients with chest pain (never for screening) with intermediate probability ACS. It is not reliable in evaluating stent patency. The ACC appropriateness criteria deal with nuanced indications and problems evaluating patients who have had CABG.

CTA of aorta and pulmonary arteries

I have dealt with this topic in other posts, and it is beyond the scope of this post, except to say that CTA of the aorta and pulmonary arteries can be combined with CT coronary angiography as the “triple rule out.” The ACC has, in its appropriateness criteria, given “triple rule out” an “undetermined” rating.

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