CCU Rotation: What is the significance of new LBBB in AMI?

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Left bundle branch block (LBBB) is an abnormal electrocardiogram (ECG)
finding that is often associated with underlying heart disease, abnormal conduction and
increased cardiac risk factors. The criteria for LBBB includes QRS >0.12 seconds, broad
monomorphic R waves in lateral leads (I and V5-V6) with no Q waves, broad
monomorphic S waves in V1 and prolonged R wave >0.06s in V5-V6. The presence of
LBBB can mask or mimic acute myocardial infarction (AMI) making the diagnosis more
difficult. Currently, the American College of Cardiology /American Heart Association
(ACC/AHA) recommends rapid reperfusion therapy, via fibrinolytic therapy or
angioplasty, for new LBBB or presumed new LBBB associated with ischemic symptoms.
These recommendations were based on the Fibrinolytic Therapy Trialists’ (FTT) review
of the major randomized clinical trials of fibrinolytic therapy, which found that
individuals with a bundle-branch block (right or left) had higher baseline mortality and
had the greatest improvement in survival from fibrinolytic therapy. However, majority
of the studies did not distinguish between RBBB and LBBB.

Later studies showed inconsistency between the numbers of new or presumed
new LBBB and acute coronary occlusions, rising questions on current ACC/AHA
recommendations. For example, Chang et al. conducted an observational cohort study
that looked at 7937 patients with cardiac ischemic symptoms and received ECGs. Of
these patients, 55 had new or presumed new LBBB, 136 had old LBBB and 7749 had no
LBBB. There was no difference between the 3 groups. The presence of LBBB did not
predict acute MI. However, revascularization and coronary artery disease were more
common in patients with new or presumed new LBBB.

Sgarbossa et al. identified patients with myocardial infarction and LBBB from
the Global Utilization of Streptokinase and Tissue Plasminogen Activator for Occluded
Coronary Arteries (GUSTO) trial and compared them to the ECGs of patients with
chronic coronary artery disease and LBBB. Sgarbossa identified three criteria useful to
diagnose (AMI) in presence of LBBB:
1. Concordant ST segment elevation >=1mm (score 5)
2. Concordant ST segment depression >= 1 mm in leads V1-V3 (score 3)
3. Discordant ST segment elevation >5mm (score 2)

A total score of 3 or higher has a specificity of at least 90 percent for diagnosing
AMI, but the criteria has a low sensitivity. Furthermore, the third criteria was found to
have little diagnostic value since a high take-off of the ST segment in leads V1 to V3 has
been described with uncomplicated LBBB. Wong et al. demonstrated that patients with
ST segment changes associated with their LBBB, especially those meeting Sgarbossa’s
criteria 1 or 2, have a higher 30-day mortality than LBBB with no segment changes.
A meta-analysis by Tabas et al. also showed a Sgarboss score of at least 3 is useful for
diagnosing acute myocardial infarction in patients who present with LBBB. A score of

2 has an ineffective positive likelihood ratios and a score of 0 is not useful in excluding
acute myocardial infarction.

Dr. Stephen Smith believes that the lower sensitivity to Sgarbossa’s criteria is
due to the low sensitivity of ST elevation in MI (including STEMI and non-STEMI), even
in normal cardiac conduction. It is, however, more sensitive for coronary occlusion
(STEMI only). Also, occlusions in the left anterior descending coronary artery (LAD)
rarely cause concordance changes in leads V1-V3. Therefore, using the current
Sgarbossa’s criteria will miss LAD occlusions. He proposed the Smith modified
Sgarbossa rule:
1. Concordant ST segment elevation >=1mm in at least one lead
2. Concordant ST segment depression >= 1 mm in at least one lead of V1-V4
3. Proportionally excessive discordant ST elevation in V1-V4, as defined by an
ST/S ratio >= 0.20 and at least 2 mm of STE.
In his study of 20 patients with LAD occlusion and 129 controls with ischemic
symptoms and LBBB, ST elevation of at least 2mm in any leads V1-V4 and an ST/S ratio
>=0.02 was highly specific. Discordant ST elevation >5mm without increased ST/S ratio
did not have LAD occlusion.

In conclusion, current literature agrees that only a small portion of new LBBB
requires rapid reperfusion. ECG criteria should be utilized to better identify patients
with coronary artery occlusions. However, ACC/AHA continues to recommend
reperfusion for patients with new or presumed new LBBB and ischemic symptoms.

1. Fibrinolytic Therapy Trialists’ (FTT) Collaborative Group. Indications for fibrinolytic therapy in suspected acute myocardial
collaborative overview of early mortality and major morbidity results from all randomized trials of more than 1000 patients. Lancet

2. Chang et al., Lack of Association with between left bundle-branch block and acute myocardial infarction in symptomatic patients,
American Journal of Emergency Medicine (2009) 27, 916-921.

3. Sgarbossa et al., Electrocardiographic Diagnosis of Evolving Acute Myocardial Infarction in the presence of Left Bundle-Branch
Block, NEJM 1996;334:481-7.

4. Al-Faleh et al., Unraveling the spectrum of left bundle branch block in acute myocardial infarction: insights from the Assessment
of the Safety and Efficacy of a New Thrombolytic (ASSENT 2 and 3) trials. Am Heart J. 2006;151(1):10.

5. Wong et al. Patients with Prolonged Ischemic Chest Pain and Presumed-New Left Bundle Branch Block Have Heterogenous
Outcomes Depending on the Presence of ST-segment Changes, J Am Coll Cardiol 2005;46:29-38.

6. Tabas et al. Electrocardiographic Criteria for Detecting Acute Myocardial Infarction in Patients With Left Bundle Branch Block: A
Meta-analysis. Ann Emerg Med 2008; 52:329

7. Dodd KW. Aramburo L. Broberg E. Smith SW. For Diagnosis of Acute Anterior Myocardial Infarction Due to Left Anterior
Descending Artery Occlusion in Left Bundle Branch Block, High ST/S Ratio Is More Accurate than Convex ST Segment Morphology
(Abstract 583). Academic Emergency Medicine 17(s1):S196; May 2010.

Above Article By: Jenny Luo MD

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