CCTA for Low Risk Chest Pain

Mini JC #4, sorry I'm a week off, but it's been a little busier than usual. This week I figured we would cover something recently in EM RAP, and I am pretty sure it will be coming down the pipeline in the future months to come. 

Today's mini JC focuses on what to do with the low risk-intermediate risk chest pain who you are evaluating for outpatient therapy. Is it your policy to do 2 sets and dc, do you obs and do inpt stress, do you admit and let someone else figure it out?
Chest pain in the low-int risk patient becomes the source of angst and much disagreement between physicians depending on your risk tolerance. Today we will look at the NEJM topic on the use of CCTA for the evaluation of patients with chest pain presenting to the emergency department. 
1. Why is this topic important?
The evaluation and safe discharge of patients with low - intermediate risk chest pain is difficult. Missed myocardial infarctions represent the largest proportion of filed law suits against emergency physician and the lack of sensitivity of any one particular test for excluding coronary artery disease makes safe discharge problematic. Currently the recommendations and gold standard test is a stress test "performed within 72 hours", which has a poor sensitivity and specificity, with an undefined time window for how long a "negative stress test" is protective, if one even exists. CCTA is a non-invasive method of evaluating patients for the presence of coronary artery disease, not dissimilar to a cardiac cath, without the risks of periprocedural MI and catheterization related complications. 
2. What does the study attempt to show?
Low to intermediate probability chest pain patients who the physician deemed required admission or further diagnostic testing to evaluate for coronary artery disease were eligible for inclusion. Patients were assigned to two groups one arm was the coronary CT angiography arm, and the other arm was the usual care "whatever you did before arm". The study attempted to show that a negative CCTA was non-inferior to usual care in terms of safety outcomes at 30 days (death, or MI). 
3. What were the findings?
1370 subjects were in enrolled in a 2:1 fashion CCTA and usual care for a total of 908 CCTA and 462 usual care patients. Only 84% of patients who were assigned to the CCTA arm actually got a CT (767/908) mostly due to elevated resting heart rate. In the subset that received a CCTA 83% (640/767) had maximal stenosis < 50% (defined by investigators arbitrarily as the cutoff for non-significant coronary artery disease.) 7% (52/767) had stenosis between 50-69%, and 4%(28/767) had stenosis > 70%, 6% were non-diagnostic. Of the 640 patients who had a negative CCTA examination 0 had death, or MI at 30 days from the index visit. In addition, in the usual care arm there were also no deaths or MIs at 30 days. Overall 5.1% of patients in the CCTA arm got angiography and 4.2% of patients in the usual care arm got cardiac catheterization. Patients in the CCTA arm were more likely to be diagnosed with coronary artery disease than in the usual care arm when discharged from the hospital. 
4. How is patient care impacted?
This is an interesting study and a lot of data and questions are left unanswered. First off almost all of these "low-int" probability patients did well with no deaths at 30 days in any group. There were a few MI's in each group (none in the negative testing group) but the paper does not describe how these were diagnosed, and does not describe what the delta troponin's were. However, I think what can be interpreted from the study is that a negative CCTA is equally as effective at identifying a patient who is safe for discharge home from the emergency department as patients who would have otherwise been admitted. Therefore, in a low to int probability patient with chest pain CCTA is as effective as stress testing (keep in mind you are still doing the delta troponins) for identifying patients at low risk for death or MI at 30 days. 
5. Is this an area of controversy?
Absolutely. There are many questions left unanswered in this study. Almost all of these patients did well, based on being placed by an ED doc into a low risk category based on HPI and PE, and then even stratified to a lower risk group with negative troponin's. Therefore is a test like CCTA which results in radiation exposure, and increased health care costs a necessary test when the pretest probability of disease is so low? For example, what if instead of CCTA we looked at the WBC and if it was between 4-20K (99% of patients) we identified these patients as low risk to go home. It would have performed equally as well as the CCTA in terms of outcomes at 30 days in terms of death, or MI, simply because no one died or had an MI in this group, despite the fact that you know a WBC between 4-20K has no relationship whatsoever to whether or not a patient will have an MI. 
Therefore what we really need to know is what predicted the patients who were going to have an MI, (the very few that did). Was it a positive troponin, or was it a positive CCTA, and if the CCTA predicted an MI prior to a rise in troponin we have learned something valuable through its use. I think most of us know that a stenosis of > 50% doesn't mean its causing your chest pain, however. A ruptured plaque in a patient with a 30% lesion can become a 100% lesion immediately and result in a STEMI, so is a CCTA really predicting who has the MI or was it the positive biomarker like troponin? I think secondary analysis are coming down the line from this very interested novel study. 
6. Major limitations of the study?
Not all patients enrolled in the CCTA arm got a CCTA, and I think the paper is confusing in what happened to the patients who did not get CCTA. Also in the patients with a positive CCTA how many got cathed and what were the results? If they didn't get cathed what was the point of identifying a patient with a >70% lesion on the CCTA? These questions are not delineated in the paper. Another limitation of the study, is what I eluded to earlier. If you test a population with such a low prevalence of disease how can you accurately determine how effective the diagnostic strategy is compared to the gold standard? In this cohort of 1370 patients there were only 13 MI's in total and NO deaths. 

Litt et al. CT Angiography for Safe Discharge of Patients with Possible Acute Coronary Syndromes. NEJM 2012.