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LBBB in Patients with Suspected MI
Abstract & Commentary
By Andrew J. Boyle, MBBS, PhD, Assistant Professor of Medicine, Interventional Cardiology, University of California, San Francisco. Dr. Boyle reports no financial relationship relevant to this field of study.This article originally appeared in the May 2011 issue of Clinical Cardiology Alert. It was edited by Michael H. Crawford, MD, and peer reviewed by Ethan Weiss, MD. Dr. Crawford is Professor of Medicine, Chief of Clinical Cardiology, University of California, San Francisco, and Dr. Weiss is Assistant Professor of Medicine, Division of Cardiology and CVRI, University of California, San Francisco. Dr. Crawford is a speaker for Astra-Zenica, and Dr. Weiss reports no financial relationships relevant to this field of study.
Source: Jain S, et al. Utility of left bundle branch block as a diagnostic criterion for acute myocardial infarction. Am J Cardiol 2011;107:1111-1116.
In assessment of acute chest pain in the emergency room, a new or "presumably new" left bundle branch block (LBBB) is considered as an electrocardiographic (ECG) equivalent to ST elevation. In clinical practice, it appears that many patients are taken to the cardiac catheterization lab urgently for presumed acute myocardial infarction (MI) because of their LBBB but do not have an occluded coronary artery. Jain and colleagues examined the Mayo clinic database to determine whether the use of new, or presumably new, LBBB as a diagnostic criterion for MI results in overdiagnosis.
Between 2004 and 2009, 892 patients were admitted with ST elevation MI (STEMI) for whom their institution's STEMI pathway was activated. Four percent (n = 36) of these were STEMI equivalents with LBBB. Baseline demographics differed between those presenting with ST elevation and those presenting with LBBB. Patients with LBBB were older (73 ± 14 years vs 65 ± 15 years, P < 0.001), less likely to be male (50% vs 70%, P < 0.01), had higher thrombolysis in myocardial infarction (TIMI) risk scores (31 vs 23, P < 0.01), and were more likely to have heart failure (P < 0.001). Patients with LBBB were less likely to receive primary percutaneous coronary intervention (22% vs 86%, P < 0.001) and their door-to-balloon times were longer (232 vs 67 minutes, P < 0.001). The 3-month mortality of patients presenting with LBBB was almost double that of those presenting with ST elevation, but this did not reach statistical significance.
Of the patients with LBBB, 14 (39%) had a final diagnosis of acute coronary syndromes (ACS; 12 with MI, 2 with unstable angina); 13 (36%) had cardiac diagnoses other than ACS (8 acute heart failure, 2 complete heart block, 1 atrial fibrillation, 1 severe aortic stenosis, and 1 hypertensive emergency); and 9 (25%) were diagnosed with non-cardiac chest pain. In the ACS group, only five patients (36%) had occluded culprit coronary arteries, of which two involved the LAD. For outcome analysis, they separated the cohort into these three groups based on their final diagnosis: ACS, non-ACS cardiac, and non-cardiac. In-hospital mortality occurred in three (21%) of the ACS group, two (15%) of the non-ACS cardiac group, and none of the non-cardiac group. At 3 months, the mortality rate was 21% in the ACS group, 23% in the non-ACS cardiac group, and 0% in the non-cardiac group.
The authors then examined the utility of three Sgarbossa criteria in the patients with LBBB: (1) ST elevation > 1 mm concordant with QRS; (2) ST depression > 1 mm in lead V1, V2, or V3; and (3) ST elevation > 5 mm discordant with QRS. Only 2 patients in the ACS group met any Sgarbossa criteria (they both had occluded left anterior descending coronary arteries); the other 12 did not. In the non-ACS cardiac group, 3 patients met any Sgarbossa criteria; the other 10 did not. No patients in the noncardiac group met any criteria. The authors conclude that new or presumably new LBBB in patients suspected of having acute MI identifies a high-risk subgroup, but only a small number have MI, with two-thirds of patients having an alternative discharge diagnosis. The Sgarbossa criteria have limited utility because of their low sensitivity.
The presence of LBBB on ECG in patients suspected of having MI can present a real diagnostic challenge. In the current era, most patients with LBBB are referred urgently to the cardiac catheterization laboratory based on STEMI treatment algorithms. Thus, we have a definitive diagnosis on most patients in this study. Jain and colleagues show us that the majority of these are not likely to have ACS. Furthermore, even fewer of those who do have ACS are likely to have occluded coronary arteries; only 5 of 36 patients (14%) presenting with LBBB had coronary arteries that were occluded. These are the patients we are trying to identify, as these patients have the most clinical benefit from rapid reperfusion. Thus, the presence of a LBBB has a very poor specificity for occluded coronary arteries. The Sgarbossa criteria were developed to identify those with LBBB who were more likely to have occluded coronary arteries. Based on this paper, it appears that this approach has very poor sensitivity. However, caution should be used in interpreting studies with such small numbers of patients.
Despite the diagnostic difficulties associated with determining the cause of a LBBB, the presence of a LBBB remains a predictor of adverse outcomes. Importantly, the LBBB patients with ACS or non-ACS cardiac diagnoses had equally poor prognosis. Interestingly, the authors were able to avoid urgent cardiac catheterization in several LBBB patients with low clinical suspicion for ACS by using bedside echocardiography. In order to avoid unnecessary cardiac catheterization, future research is needed to determine the most appropriate way to approach the patient with LBBB; perhaps bedside echocardiography will become a useful clinical adjunct. Until then, LBBB in patients with suspected MI should continue to be considered a STEMI equivalent.