Wednesday, October 10, 2007

Screening for coronary artery disease with CT calcium scoring and carotid intima media thickness: ready for prime time?

Prevention guidelines for cardiovascular disease have advocated treatment guided by the assessment of traditional risk factors. The Framingham risk score is often used for this purpose. Recently, interest has grown in the use of noninvasive imaging to assess risk in asymptomatic patients, particularly CT calcium scoring and ultrasound measurement of carotid intima media thickness (IMT), due to accumulating evidence of superior predictive power of these modalities. Risk assessment strategies have not been studied head to head for clinical outcomes. And, unfortunately, they’re not likely to be anytime soon:

That appears unlikely, at least for now, said Dr. Diane Bild, a medical officer at the National Heart, Lung, and Blood Institute, the logical agency to conduct such research. She said specialists there have already rejected the idea of a head-to-head study looking at how patients who received the high-tech screening fared long term, compared with those screened using more traditional methods.

The institute, Bild said, ``has a lot of competing priorities, and this type of study would be very expensive to conduct, and it just hasn't reached that level where we've gone forward with it."
(Competing priorities indeed. Like the multimillion dollar promotion of quackery by the NCCAM which, like NHLBI, is also a subsidiary of the NIH. But I digress). Needless to say we won’t have the benefit of outcome based trials to guide in the assessment of patients. For the foreseeable future we must settle for lower level evidence. Does that mean we can’t make evidence based decisions? No. Evidence based medicine would have us apply clinical judgment and expertise to come up with the best synthesis of evidence we can, even when this evidence is “low level”. But when a group of experts with special interest in cardiac imaging attempted to do just that and promulgated their own guidelines a firestorm of controversy erupted.

This controversy bears careful examination for several reasons, not the least of which is the nagging question of how best to stem the epidemic of cardiovascular disease. It also impacts broader areas of guideline development relating to potential conflicts of interest and disagreement with other guidelines.

The new guidelines, known as the SHAPE guidelines, expand the recommendations for imaging modalities to screen patients, calling for the use of CT calcium scoring or ultrasound IMT measurement for all asymptomatic men ages 45-75 and women ages 55-75 except for those defined as very low risk. This represents a radical departure from the American College of Cardiology Foundation/American Heart Association expert consensus document, just updated this year to recommend screening only for those patients deemed to be at intermediate risk.

Publication of the guidelines in a supplement to the American Journal of Cardiology was supported by Pfizer pharmaceuticals, eliciting the usual knee-jerk cries of “conflict of interest”. (Joining the chorus were medical thought leaders Arnold Relman and Jerome Kassirer. Do those guys ever miss a chance to pounce on the drug companies?).

Dr. Steven Nissen, no stranger to controversy and hyperbole, also chimed in with:

This issue is not about the conservatism of the ACC and AHA, it’s about the practice of evidence-based medicine. The AEHA is a group of shameless self-promoters who have no scientific basis for their assertions.
The controversy was nicely covered in a point counterpoint in the Cleveland Clinic Journal of Medicine. While I believe Nissen was wrong to say the guideline authors had no scientific basis for their assertions I have chosen not to take sides in this controversy. A reasonable clinician could take either view. All physicians involved in preventive medicine should familiarize themselves with both sides of this debate. These tests are being promoted to patients, who will come to their appointments with questions.

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