Accuracy of the ECG for STEMI
Abstract & Commentary
By Michael H. Crawford, MD
Source: Prasad SB, et al. Clinical and electrocardiographic correlates of normal coronary angiography in patients referred for primary percutaneous coronary intervention. Am J Cardiol. 2008:102:155-159.
Although the ECG is key in the triage of suspected ST wave elevation myocardial infarction (STEMI), it is imperfect. Prasad et al from Australia sought to determine the rate of false-positive STEMI diagnoses and assess the reason for the misdiagnoses. They performed a prospective, observational study of 690 consecutive patients referred for primary percutaneous intervention (PCI) for suspected STEMI in six hospitals that fed into one regional PCI center. The PCI referral was done by emergency department (ED) physicians directly to the interventional cardiologist who activated the catheterization laboratory. Patients found to have no identifiable culprit lesion and normal, or near normal, coronary arteries were assessed independently in a blinded fashion by two experienced cardiologists and compared to a matched control population with culprit lesions in a 1:2 fashion.
Results: Nine patients (1%) had significant coronary artery disease (CAD) but no culprit lesion; they were excluded. Normal or near normal coronary arteries were found in 87 patients (13%). These patients were younger, had fewer risk factors for CAD, and more often presented to one of the feeder hospitals. They usually presented with chest pain (89%), and it was often described as atypical for angina (58%). Their most common discharge diagnosis was pericarditis (83%). Myocarditis was diagnosed in three patients and stress cardiomyopathy in two. Troponin was elevated in 20%, and attributed to myopericarditis in most. Analysis of the ECGs exhibited that the normal coronary patients more often had left bundle branch block (LBBB) compared to the STEMI controls (13 vs 1%, p < .001).
In the remaining ECGs without LBBB, both observers agreed on the diagnosis of STEMI in 92% from the MI group vs 33% from the normal coronaries group. ST elevation criteria for STEMI were present in 55% of the normal coronary group and 93% of the acute MI group. Emergency department (ED) triage resulted in a high sensitivity (100%) but a false positive rate of 13%. Expert review of ECGs enhanced specificity but not sensitivity. Prasad et al concluded that the 13% false positive ED triage diagnosis of acute STEMI could be reduced by more expert interpretation of the ECG.
Commentary
I recently met with the director of our ED because he wanted a cardiologist to read over some suspected STEMI ECGs before they activated the cath lab in order to decrease false activation because of equivocal ECGs. In this light, this study is of interest. The ED physicians had a 100% sensitivity for STEMI and a 13% false positive rate. This might be considered a good outcome, but data from this study suggest that the number of false positives could be cut in half by using strict criteria and an expert reader. Thus, I agreed to a plan where we would look at faxed ECGs that were equivocal to reduce the number of unnecessary cath lab mobilizations.
Although there are many causes of a false-positive ECG for STEMI, in this study the most common were normal variants and early repolarization; the most common discharge diagnosis was pericarditis. They did have a few patients with myocarditis and stress cardiomyopathy (takotsubo). Interestingly, in their study, LBBB was most predictive of normal coronary arteries. Other studies have also called into question the wisdom of considering new LBBB a STEMI equivalent. Some believe LBBB should be dropped as a criterion for reperfusion therapy.
The major weakness of this study is that false-negative STEMI diagnoses were not assessed. We know they are rare, but they occur especially with circumflex CAD. It would have been useful to have this data as well. Until such a study is done, this one suggests an approach with a 100% sensitivity, in which the false-positive rate is only 6%-7% but requires a reading over of the ECG by an experienced individual using strict criteria. At my institution, we only read over equivocal ECGs, not those with obvious STEMI or those that are clearly normal.
Although the ECG is key in the triage of suspected ST wave elevation myocardial infarction (STEMI), it is imperfect.You have reached your article limit for the month. Subscribe now to access this article plus other member-only content.
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