Baltimore, MD - A new study comparing exercise radionuclide scintigraphy with multidetector computed tomography (CT) as a screening tool for asymptomatic CAD has found only a modest agreement between an abnormal exercise single-photon-emission computed tomography (SPECT) result and high coronary-artery-calcium (CAC) scores [1]. Although moderate or severe CAC is often associated with inducible ischemia, investigators conclude that the absence of CAC or the presence of only mild CAC "by no means precludes inducible myocardial ischemia."

Writing in the February 1, 2006 issue of the American Journal of Cardiology, lead author Dr Roger Blumenthal (Johns Hopkins University School of Medicine, Baltimore, MD) and colleagues note that despite the spread of inexpensive coronary-calcium screening modalities in the general population, its role in accurately identifying apparently healthy asymptomatic patients at high risk for CAD is unclear.

"There is a school of thought that says the best way to screen for occult CAD is to do CT for coronary calcium and then to do exercise SPECT in those with high calcium scores," senior author Dr Lewis Becker (Johns Hopkins University School of Medicine) told heartwire. "This might have some advantages over doing exercise SPECT in everyone as the first test, since the CT test is much quicker and somewhat cheaper. However, it was not known how the two approaches compare in this type of population. It is likely that not all coronary lesions are calcified and can be seen on CT scan. We thought that some noncalcified lesions might be able to produce ischemia on an exercise test."

To examine the relationship between calcium score and exercise-tolerance testing (ETT) accompanied by SPECT, the Johns Hopkins investigators screened 260 consecutive siblings, aged 30 to 59 years, of individuals with premature CAD. Overall, 49 subjects had inducible ischemia as demonstrated by ETT/SPECT. The investigators report that exercise-induced ischemia was more frequent in subjects with higher CAC scores but was not exclusive. Although more than 50% of subjects with a CAC score >100 (the commonly suggested CAC cutoff point) had inducible ischemia, it was also present in 12% of patients with a CAC score of zero and in 20% of patients with CAC scores from 11 to 100. In fact, among the 49 subjects with inducible ischemia, 29% of subjects had no coronary calcification and only 40% of subjects had a CAC score >100.

"It is likely that many lesions associated with inducible ischemia are not calcified," said Becker when asked why the two imaging techniques were so discordant. "Further, inducible ischemia is reflective of both coronary artery luminal obstruction and the function of the vascular endothelium in that artery. The presence of calcium does not necessarily mean that there is an obstructive lesion present or that the endothelium is dysfunctional."

"The main point that we want to get across is that some people will have moderate disease and exercise-induced epicardial artery vasoconstriction or inadequate dilatation of the microcirculation causing ischemia in the setting of a family history of premature CHD and the presence of a couple of traditional risk factors," added Blumenthal. "The latter may lead to endothelial dysfunction, resulting in vasoconstriction."

Blumenthal told heartwire that the lack of agreement between CAC scores and SPECT was not too surprising; his group had previously shown that most people with myocardial ischemia and a family history of CAD generally had only 20% to 40% diameter stenosis. Previous studies comparing ETT/thallium and cardiac CT were done in older patients, who are more likely to have at least some coronary calcification.