Computed tomography provides anatomy - we need ischemia!

Aug 31 2008

Cardiac computed tomography has revolutionized cardiac imaging in recent years by providing exquisitely detailed cardiac anatomy, including, but not limited to, coronary anatomy.

Non-invasive coronary angiography by computed tomography (CTA) is performed in ever increasing numbers, over 150,000 per year in the United States. However, the specific role of CTA in the diagnostic pathways of cardiology remains to be defined, and practice patterns of the technique often neglect established insights into coronary artery disease. The following characteristics of CTA need to be understood:

• CTA is very good and reliable at excluding coronary artery disease (CAD), with negative predictive values approaching 100%. It is far less good at assessing the hemodynamic relevance of overt disease. Head to head comparisons with nuclear perfusion data show that about half of coronary lesions graded as > 50% diameter stenosis by CTA do not induce ischemia under stress.
• CTA entails substantial radiation (varying widely between 6 and 11 mSv in a recent report) and contrast media exposure (60-80 ml), both with attending risks of malignancy and renal damage, respectively.
• Current accuracy and radiation exposure data from CTA come from highly specialized, experienced centres. It is very unlikely that this quality is maintained when the technique is applied by less experienced operators.

Next, some fundamental insights into CAD should be remembered:

• Extensive experience with tens of thousands of patients undergoing stress imaging tests (by nuclear, echo, or magnetic resonance imaging) has shown very good prognosis for those without objective evidence of stress-inducible ischemia, and a graded prognosis according to the severity and extent of inducible ischemia
• Even angiographically confirmed significant CAD in the absence of inducible ischemia and heart failure carries a relatively good prognosis (< 1% death or infarction per year). Thus, there are no data to support that patients without inducible ischemia should be revascularized. On the contrary, several studies such as COURAGE have shown that even some subsets of patients with ischemic stress responses may be managed conservatively with a good prognosis

Use of CTA is currently crystallizing in two scenarios. The first is the symptomatic patient in a low or intermediate pre-test-likelihood of significant CAD. Depending on individual preferences and cost issues a CTA (or scoring of coronary calcification, without contrast and with less radiation) may provide useful incremental information for risk-stratification and, importantly, obviate coronary angiography if it is negative. However, it should be borne in mind that in a population with a low pre-test likelihood of CAD (< 30%) the positive predictive value of CTA for diagnosing segmental coronary stenosis will drop into the 60% range or lower, while admittedly still preserving a high negative predictive value. The other scenario with a potential role for CTA is the emergency room, in particular in the context of "chest-pain units". Here, the promise is to very quickly exploit the "clearing" potential of a negative scan, which reliably predicts absence of CAD and also perhaps, with a modified protocol, to exclude other important causes of chest pain, such as major pulmonary embolism or aortic dissection. Unquestionably, this seems attractive. Again, mainly in the patients with low pre-test-likelihood of a true acute coronary syndrome an early CTA could reduce in-hospital time and perhaps even cost. In patients with higher likelihood or established CAD it is questionable whether the procedure will add much to or shorten the management, given the problems in grading stenosis severity and the well-established value of an invasive approach especially in troponin-positive patients.

In summary, CTA may facilitate management of symptomatic patients with low pre-test likelihood of CAD, in particular by conclusively showing absence of CAD. However, testing for inducible ischemia remains central for management decisions in both suspected and proven CAD and cannot be replaced by morphologic information. Furthermore, the use of CTA as a screening test for CAD in asymptomatic patients - discouraged by current recommendations anyway - should be abandoned.