Prevalence of acute myocardial infarction in patients with presumably new left bundle-branch block

Abstract

Objectives

We assessed the prevalence of true acute myocardial infarction and the need for emergent revascularization among patients with new or presumably new left bundle branch block (nLBBB) for whom the primary percutaneous coronary intervention protocol was activated.

Methods and Results

Among 802 patients, 69 (8.6%) had nLBBB. The chief presenting symptom was chest pain or cardiac arrest in 36 patients (52.2%) and shortness of breath in 15 (21.7%). Less than 30% of the patients had elevated cardiac troponin-I, and less than 10% had elevated creatine kinase–MB. Only 11.6% of the patients underwent emergent revascularization; the rate was higher for patients who presented with chest pain or cardiac arrest or shortness of breath than for patients who presented with other symptoms.

Conclusions

Acute myocardial infarction and the need for emergent revascularization are relatively uncommon among patients who present with nLBBB, especially when symptoms are atypical. Current guidelines for primary percutaneous coronary intervention protocol activation for nLBBB should be reconsidered.

Introduction

According to the American College of Cardiology and American Heart Association (ACC/AHA) guidelines, in the absence of contraindications, reperfusion therapy should be administered to patients with symptoms compatible with ST-segment elevation myocardial infarction (STEMI) if these symptoms arose within the prior 12 hours and if an electrocardiogram (ECG) shows new or presumably new left bundle branch block (nLBBB) (level of evidence A).¹ These recommendations stemmed from the Fibrinolytic Therapy Trialists' review of major randomized trials of fibrinolysis versus placebo,² which suggested that patients with a bundle branch block had higher baseline mortality and had the greatest incremental improvement in survival when given fibrinolytics. However, in this meta-analysis, right and left bundle branch block were not analyzed separately, nor were known LBBB and nLBBB. Two studies have found that the prognosis of patients with acute myocardial infarction (AMI) and right bundle branch block (especially those with anterior STEMI) is worse than that of patients with LBBB.3, 4 Subsequent analyses by Shlipak et al⁵ and Gallagher⁶ concluded that treating all patients who have suspected AMI and who present with LBBB (whether it is new or known) with fibrinolytics is preferable to using ECG criteria to diagnose AMI or to determine the age of LBBB.

In 1996, Sgarbossa et al evaluated the ECGs of more than 26, 000 patients in the GUSTO-I (Global Utilization of Streptokinase and Tissue Plasminogen Activator for Occluded Coronary Arteries) trial and determined that an AMI could be diagnosed by using 3 ECG criteria in patients with known or new LBBB: ST-segment elevation (STE) of ≥1 mm that is concordant with the QRS complex; ST-segment depression of ≥1 mm in lead V₁, V₂, or V₃; and STE of ≥5 mm that is discordant with the QRS complex.⁷ Although the use of these criteria is recommended by the ACC/AHA guidelines,¹ several studies have suggested that the Sgarbossa ECG criteria have low sensitivity but high specificity in the detection of AMI.^5,8, 9, 10, 11, 12

Emergency department physicians, as well as cardiologists, are currently under pressure not to miss STEMI and to shorten the first medical contact-to-balloon time. Activating the catheterization laboratory for patients presenting with typical symptoms and LBBB is reasonable because diagnosing ischemic STE in the presence of LBBB is problematic. However, it is well known that up to a third of patients with AMI present with symptoms other than chest discomfort.¹ Thus, in many cases, the primary percutaneous coronary intervention (pPCI) protocol is activated for patients presenting with nLBBB and symptoms other than typical chest pain, including shortness of breath, nausea, and unexplained hypotension.

The current study had 3 aims: (1) to assess the prevalence of AMI and the need for emergent revascularization among consecutive patients with nLBBB in whom the pPCI protocol was activated; (2) to determine whether presenting symptoms may help in predicting AMI and the need for urgent revascularization among patients with nLBBB; and (3) to assess the validity of the Sgarbossa criteria in our nLBBB patient population.

We used both cardiac troponin I and creatine kinase–MB (CK-MB, >X2 upper limit of normal) for the diagnosing AMI. Cardiac troponin I is more sensitive and specific for AMI than CK-MB. However, we felt that positive cardiac troponin I without significant increase in CK-MB probably represent small AMI that could be more compatible with non ST elevation AMI than STEMI.