Electrocardiographic Diagnosis of Evolving Acute Myocardial Infarction in the Presence of Left Bundle Branch Block

Clinical Question

Can acute MI be diagnosed electrocardiographically in the setting of LBBB?

Clinical Bottom Line

  1. Three electrocardiographic criteria, present singly or in combination, can predict with high accuracy acute MI in the setting of LBBB if present.
  2. The absence of these electrocardiographic findings does NOT rule out MI.

The Evidence

Derivation Group

UNIVARIATE Analysis Likelihood Ratio (positive) [95% CI] Likelihood Ratio (negative) [95% CI] Multivariate score
ST segment elevation > 1 mm concordant with the QRS direction 9.54 [3.1-17.3] 0.3 [0.22-0.39] 5
ST segment depression in leads V1 - V3 6.58 [2.6-16.1] 0.78 [0.7-0.87] 3
ST segment elevation > 5 mm discordant with the QRS direction 3.63 [2.0-6.8] 0.75 [0.67-0.86] 2

By ROC Curve (0.874) to achieve a > 90% sensitivity and re-applied with a MV score of 3 or >:

Derivation Group Validation Group
Likelihood Ratio (positive) 7.8 9.0
Likelihood Ratio (negative) 0.2 0.7

Comments

  1. There was an independent, blind comparison with a reference standard. The study group was comprised of patients derived from the GUSTO trial (the North American Subgroup), and the control group consisted of patients selected from the Duke Cardiovascular Databank who had LBBB and NO MI or chest pain (but angiographically documented CAD.)
  2. Methods were described in sufficient detail to allow replication.
  3. This was a retrospective analysis of a large cohort of patients entered prospectively.
  4. The overall prevalence of LBBB in the cohort was quite low -- 145/26,003 patients, or 0.6%. Of the 145 patients, 131 had MI diagnosed by serial cardiac enzyme determinations or other means.
  5. Of note is that the results of the derivation group were analyzed univariately and multivariately, and given scores from 1 to 5 based on their intrinsic ability to accurately predict the presence of MI. This was then tested in a validation group (results presented in the second table above.)
  6. The validation group's LR's are fairly close to the derivation groups, at least statistically speaking. Because the validation group was smaller and showed a greater specificity, this was reflected in the higher +LR (farther from 1) and -LR (closer to 1).
  7. The authors noted that there was no distinction made between new or old LBBB. That information was not available in the GUSTO data. However, this is likely to reflect clinical practice as patients usually do not come with previous EKG's, and as was mentioned above, the overall prevalence of LBBB in a large cohort was low, therefore making it likely that this is not a significant issue,
  8. The use of matched controls who had documented CAD and no CP or MI could have increased the specificity of the diagnostic criteria somewhat.
  9. In all, these electrocardiographic criteria are simple enough to learn that ED physicians and PCP's could diagnose MI's earlier in patients with MI and LBBB. Earlier treatment with thrombolytics may reduce the post-MI myocardial damage compared with later treatment. In addition, more EKG's are likely to be correctly interpreted using these criteria.

APPRAISED BY: Katy Wong, MD

DATE: November 5, 1996

REVIEWED BY: Patrick J. Sousa, MD

Sgarbossa EB, et al. Electrocardiographic Diagnosis of Evolving Acute Myocardial Infarction in the Presence of Left Bundle Branch Block. NEJM, 1996;334:481-487.