Sarah G. Jamison, MD
Diagnosis of Acute MI in the Setting of New Left Bundle Branch Block
A 56 y/o woman with past medical history of hypertension, diabetes, and hypothyroidism presents with complaints of chest pain x 3 hours. Her pain is described as pressure-like, sub-sternal, non-radiating, 8/10, and associated with shortness of breath and diaphoresis.
Vitals: BP 103/62, HR 120, RR 18, T 98.1, Sat 98% RA
General app: patient was noted to be clutching chest, mildly dyspneic, diaphoretic, and appears uncomfortable.
CV: sinus tachycardia, no mrg
Resp: mild bibasilar crackles.
Ext: 1+ pitting edema to lower extremities bilaterally.
Initial EKG showed sinus tachycardia with LBBB. V3 was found with 3mm ST elevation following a 14mm S-wave, V4 noted with a 5mm S-wave followed by 2mm discordant ST elevation, and Lead II has 3 mm discordant ST depressions. Previous EKGs showed no LBBB. EKG below is a representative and is not patient’s ekg.
Image courtesy of google images: hqmeded-ecg.blogspot.com
Labs were significant for Troponin and CK of 3.04 and 2005, respectively. Chest x-ray confirmed mild/moderate pulmonary congestion.
The catheterization laboratory was activated where angiography revealed 100% mid-LAD occlusion. Patient received a drug-eluding stent to the mid-LAD and was transferred to the CCU. Patient remained hemodynamically stable with serial EKGs showing normalization of previous ischemic changes. Care was optimized with beta-blocker, aspirin, Plavix, and statin therapy. Patient was transferred to floors on POD 4 with no further complications.
Left bundle branch block (LBBB) is a cardiac conduction abnormality causing delayed activation and subsequent contraction of the left ventricle. Structural cardiac abnormalities, myocardial ischemia, electrolyte imbalances, and drug toxicities are common etiologies associated with the conduction delay. The electrocardiographic evidence of LBBB is as follows: QRS duration of at least 0.120 second in the presence of sinus or supra-ventricular rhythm, a QS or rS complex in lead V1, and an R-wave peak time of at least 0.06 second in leads I, V5, or V6 associated with the absence of a Q wave in the same lead. As discordant ST segment and T wave changes are associated with the aforementioned QRS irregularities, new or presumed new LBBB is understandably a confounding factor to the accurate diagnosis of acute myocardial infarction (MI).
Patients who present with LBBB and suspected acute coronary syndrome are more likely to be older females with pre-existing hypertension, congestive heart failure, and cardiovascular disease than those ACS patients with no LBBB.
According to recently revised guidelines by the American College of Cardiology/American Heart Association (ACC/AHA), new or presumed new left bundle branch block (LBBB) should not be considered diagnostic of acute MI in isolation (1). Prior to this drastic update published in early 2013, the ACC/AHA 2004 STEMI Guidelines recommended that patients with new LBBB associated with ischemic symptoms should receive rapid reperfusion via fibrinolytic therapy or angioplasty (2). These recommendations stood in the face of a growing body of evidence demonstrating that amongst the increasing population of patients receiving angiography and percutaneous coronary intervention (PCI), those with LBBB were not much more likely to have acute coronary arterial occlusion than those without (3). Other studies showed that treating LBBB as an MI equivalent has invariably lead to frequent false catheterization laboratory activation. One such study was Larson et al’s prospective analysis of 1335 patients with suspected STEMI who underwent angiography. It was reported that the prevalence of false-positive catheterization laboratory activation was 14% overall, but among patients presenting with LBBB, the rate of false activation was 44% (4).
Long before the ACC/AHA made recommendations against using new LBBB as an MI equivalent in isolation, The Electrocardiographic Diagnosis of Evolving Acute Myocardial Infarction in the Presence of Left Bundle-Branch Block by Sgarbossa et al. outlined three criteria by which ED physicians may diagnose acute MI in the setting of LBBB (5); they are as follows:
1. ST-segment elevation ≥1 mm concordant with the QRS complex in any lead (+ 5 points)
2. ST-segment depression ≥1 mm in lead V1, V2, or V3 (+ 3 points)
3. ST-segment elevation ≥5 mm discordant with the QRS complex in any lead (+ 2 points)
A score of 3 or more points is over 95% specific for the accurate diagnosis of acute MI in the setting of LBBB, however a meta-analysis of 10 studies exploring the clinical use of Sgarbossa’s criteria among 1614 patients demonstrated a sensitivity of only 20% (6). Idealistically, tests used for the diagnosis of life-threatening conditions such as acute MI should be highly sensitive, thus Smith et al. published a new criterion to replace the third component of Sgarbossa’s Criteria wherein the ST/S ratio is used instead of gross ST discordance (7). Smith found that an ST/S ratio (the ratio of the ST elevation measured at the J point to the R or S wave) ≤ -0.25 increased diagnostic sensitivity to 91% while maintaining a specificity of 90%.
Figure 2 (8)
In the wake of the ACC/AHA’s revised recommendations against emergent reperfusion therapy to patients with ischemic symptoms and new or presumed new LBBB, the amount of false catheterization laboratory activations as well as inappropriate fibrinolytic therapy should be markedly reduced; however, these recommendations may now inevitably cause an increase in the inappropriate denial of reperfusion therapy for this high-risk population. Until further research can provide a prospective validated study to confirm the clinical application of the aforementioned diagnostic tools, prompt and accurate identification of MI in the setting of LBBB remains difficult.
(1) O’Gara PT, Kushner FG, Ascheim DD, et al. 2013 ACCF/AHA Guideline for the Management of ST-Elevation Myocardial Infarction: A Report of the American College of Cardiology Foundation/American Heart Association Task Force on Practice Guidelines. J Am Coll Cardiol. 2013;61(4):e78-e140. doi:10.1016/j.jacc.2012.11.019.
(2) E.M. Antman, D.T. Anbe, P.W. Armstrong, E.R. Bates, L.A. Green, M. Hand, J.S. Hochman, H.M. Krumholz, F.G. Kushner, G.A. Lamas, C.J. Mullany, J.P. Ornato, D.L. Pearle, M.A. Sloan, S.C. Smith. “ACC/AHA guidelines for the management of patients with ST-elevation myocardial infarction–executive summary. A report of the American College of Cardiology/American Heart Association Task Force on Practice Guidelines (Writing Committee to revise the 1999 guidelines for the management of patients with acute myocardial infarction).”, Journal of the American College of Cardiology, 2004.
(3) Chang AM, Shofer FS, Tabas JA, Magid DJ, McCusker CM, Hollander JE. Lack of association between left bundle-branch block and acute myocardial infarction in symptomatic ED patients. Am J Emerg Med. 2009;27:916–21.
(4) Larson DM, Menssen KM, Sharkey SW, et al. “False-positive” cardiac catheterization laboratory activation among patients with suspected ST-segment elevation myocardial infarction.JAMA. 2007;298:2754–60.
(5) Sgarbossa EB, Pinski SL, Barbagelata A, et al. Electrocardiographic diagnosis of evolving acute myocardial infarction in the presence of left bundle-branch block. GUSTO-1 (Global Utilization of Streptokinase and Tissue Plasminogen Activator for Occluded Coronary Arteries) Investigators. N Engl J Med. 1996;334:481–7.
(6) Tabas JA, Rodriguez RM, Seligman HK, Goldschlager NF. Electrocardiographic criteria for detecting acute myocardial infarction in patients with left bundle branch block: a meta-analysis. Ann Emerg Med. 2008;52:329–36. e1.
(7) Diagnosis of ST-Elevation Myocardial Infarction in the Presence of Left Bundle Branch Block With the ST-Elevation to S-Wave Ratio in a Modified Sgarbossa Rule. Smith, Stephen W. et al. Annals of Emergency Medicine , Volume 60 , Issue 6 , 766 – 776
(8) Q. Cai, N. Mehta, E.B. Sgarbossa, S.L. Pinski, G.S. Wagner, R.M. Califf, and A. Barbagelata, “The left bundle-branch block puzzle in the 2013 ST-elevation myocardial infarction guideline: from falsely declaring emergency to denying reperfusion in a high-risk population. Are the Sgarbossa Criteria ready for prime time?”, American heart journal, 2013.http://www.ncbi.nlm.nih.gov/pubmed/24016487