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EKG Discussions

LBBB – new considerations

The implication of a LBBB in ACS is evolving. While a new, or presumed new, LBBB with chest pain has long been considered an indication for acute intervention, it has recently been removed as an indication for PCI or thrombolysis from several guidelines including those of the American Heart Association. Several studies demonstrate that LBBB is rarely associated with an acute MI, and the much-maligned Sgarbossa criteria have gained new legitimacy in the evaluation of ACS.

This EKG is a perfect example of LBBB and the striking repolarization changes that are expected. The criteria for diagnosis of LBBB include QRS >120msec and a monophasic R wave in lead V6 (that is, absence of septal Q waves in the lateral leads). The widespread and expected repolarization changes include ST-segment depression and T-wave inversion in the lateral leads with ST-segment elevation in the anterior leads. Notice that expected repolarization changes (ST-segment and T-wave) are discordant (opposite) to the direction of the QRS complexes. Memorize this pattern, or understand its origin, because deviation from the expected may be a cause for concern.

Interpretation of acute ST-segment and T-wave changes due to ischemia and infarction is difficult, if not impossible, with a LBBB and the associated repolarization changes. Historically a new, or presumed new, LBBB with chest pain concerning for ischemia was a criteria for acute intervention (thrombolysis or acute PCI). This recommendation was based on early fibrinolytic trials with the final diagnosis of MI based on cardiac enzymes. Once these patients began to be referred for interventional catheterization it became clear that the majority of LBBB with chest pain presentation were not predictive of acute MI, but were instead a pre-existing marker of structural heart disease (hypertensive cardiomyopathy, prior MI, or valvular disease). It is also clear that a new LBBB associated with acute infarction implies a very large infarct territory, and is typically associated with cardiogenic shock or acute pulmonary edema, and high mortality.

Sgarbossa published an analysis from the GUSTO-1 trial evaluating EKG criteria that indicated acute infarction presenting with LBBB. She found 3 criteria that were useful:

  1. ST-segment elevation concordant with QRS deflection in any lead
  2. ST-segment depression in V1-3 (concordant changes)
  3. Discordant ST-segment elevation ≥ 5mm in any lead (typically V1-3)

These criteria were reported to have low sensitivity but high specificity, and essentially recommended as superfluous to using clinical suspicion with a new, or presumed new, LBBB.

This EKG is a LBBB, though barely meeting QRS widening criteria – note the concordant ST-depression in the anterior leads.

However, as previously claimed, it actually seems that the occurrence of a new LBBB associated with acute MI is actually relatively low. Studies that have looked at the predictive value of a new, or presumed new, LBBB with concerning chest pain, find that an angiographic culprit lesion associated with Sgarbossa’s criteria is sensitive and specific, at least for the first two criteria. The third criteria of discordant ST-segment elevation ≥ 5mm continued to have disappointingly low predictive value.

Modified Sgarbossa criteria have been recommended by Dr. Stephen Smith of Hennepin County Medical Center and subsequently validated by Myers. Smith’s modification involves criteria #3, and he finds that it is not an absolute height of discordant ST-segment elevation, but a proportional amount compared to the amplitude of the associated QRS. The discordant ST-segment elevation (typically seen in V1-3) should never be more than 25% of the associated S wave.

Management algorithm for patient with suspected MI and LBBB

Cai, Sgarbossa, et al. Am Heart J 2013

References

Cai Q, Mehta N, Sgarbossa E, et al. The left bundle-branch block puzzle in the 2013 ST-elevation MI guideline: From falsely declaring emergency to denying reperfusion in a high-risk population. Are the Sgarbossa Criteria ready for prime time? Am Heart J 166:409-13, 2013.

Meyers HP, et al. Validation of the modified Sgarbossa criteria for acute coronary occlusion in the setting of LBBB: a retrospective case-control study. Am Heart J 170:1255-64, 2015.

Neeland I, Kontos, M, de Lemos, J. Evolving considerations in the management of patients with LBBB and suspected MI. J Am Coll Cardiol 60:96-105, 2012.

Kumar V, et al. Implications of LBBB in patient treatment. Am J Cardiol 111:291-300, 2013.

Smith SW, et al. Diagnosis of ST-elevation <I in the presence of LBBB with the ST-elevation to S wave ratio in a modified Sgarbossa rule. Ann Emerg Med 60:766-76, 2012.