Coronary computed tomographic angiography for rule-out of acute coronary syndrome in the emergency department may have moved ahead of SPECT myocardial perfusion imaging – its main noninvasive imaging rival – on the strength of recent evidence of advantages in time-to-discharge and radiation exposure, according to Dr. Christopher M. Kramer.
Myocardial perfusion imaging (MPI) has for roughly a decade been seen as the standard of care for imaging assistance in ED triage of patients presenting with chest pain and a nondiagnostic ECG.
It attained this status based upon the favorable results of the very large randomized ERASE (ER Assessment of Sestamibi for Evaluation of Chest Pain) trial (JAMA 2002;288:2693-700) and other studies in which MPI was compared to usual ED care.
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SPECT scan (left) and the corresponding computed tomography angiography image (right) of the left anterior descending coronary artery (curved multiplanar reformatted image). Note the luminal disease between two areas of dense calcification along the left anterior descending coronary artery. This was read as equivocal for obstructive coronary artery disease.
But in more recent randomized trials comparing computed tomographic angiography (CTA) to standard ED evaluation protocols, which now often include MPI, CTA has carried the day, Dr. Kramer said at the annual cardiovascular conference at Snowmass sponsored by the American College of Cardiology.
For example, the multicenter ACRIN-PA trial randomized 1,370 low- to intermediate-risk patients presenting with possible ACS to EDs to either early CTA or standard care, which in most cases included MPI. The CTA group had a higher rate of discharge from the ED: 50%, compared with 23%. They also had a median 18.0-hour length of stay, significantly shorter than the 24.8 hours in the control group (N. Engl. J. Med. 2012;366:1393-403).
Moreover, the CTA group had a higher rate of detection of CAD – 9.0% compared to 3.5% because CTA can detect nonobstructive as well as obstructive plaque, added Dr. Kramer, professor of cardiology and of radiology and director of the cardiovascular imaging center at the University of Virginia, Charlottesville.
The ROMICAT II (Rule Out Myocardial Ischemia/Infarction by Computer-Assisted Tomography) trial was a similar story. ROMICAT II was a multicenter trial in which 1,000 patients who presented to an ED with symptoms suggestive of ACS but a nondiagnostic ECG were randomized to CTA or a standard ED evaluation, most often including MPI. A total of 47% of patients in the CTA group were discharged directly from the ED, compared with just 12% in the control arm. Most notably, the median length of stay in the ED was 8.6 hours with CTA vs. 26.7 hours with a standard evaluation, and the mean length of stay in the hospital was reduced from 30.8 hours with standard evaluation to 23.2 hours, a 7.6-hour reduction (N. Engl. J. Med. 2012;367:299-308).
Follow-up demonstrated no cases of undetected ACS occurred in either group.
The average radiation dose to patients in the CTA arm was 14.3 mSv, compared with about 10 mSv for those patients in the standard evaluation group who got MPI. However, ROMICAT II didn’t use the latest CTA equipment.
Dr. Christopher M. Kramer
"With the modern CTA units, the radiation doses have come down quite a lot. With our flash 256 detector in the ED, we can do a study in a patient with a controlled heart rate and a reasonable BMI [body mass index] with about 1 mSv of radiation now," Dr. Kramer recalled.
Costs in the ED were significantly lower in the CTA group than in controls in ROMICAT II because of the faster patient throughput. However, this savings was neutralized by higher in-hospital costs because the CTA group had more cardiac catheterizations and revascularization procedures because more cases of CAD were detected, as in ACRIN-PA.
"The real advantage to CTA is the rapid discharge from the ED. That’s what patients appreciate. They get their CT angiogram and they can go home within an hour or 2. It really improves their experience in the ED, rather than spending all night waiting for their SPECT MPI results. I don’t think you’re saving costs by doing CTA, though, because of this catch-up phenomenon," Dr. Kramer said.
Aside from the fact that MPI takes 3-5 hours, its other main limitations are that it has difficulty in differentiating acute ischemia from acute infarction or an old infarct, according to Dr. Kramer.
The ability of CTA to detect more cases of CAD by identifying nonobstructive plaque may prove to be of clinical import. That’s the underlying hypothesis of the ongoing National Heart, Lung, and Blood Institute–funded PROMISE (Prospective Multicenter Imaging Study for Evaluation of Chest Pain) study, in which 10,000 patients with symptoms suggestive of CAD are being randomized to an initial CTA or to usual care with a functional test, either MPI, stress echocardiography, or exercise ECG.