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left bundle branch block (LBBB)

An ECG change characterized by an intraventricular conduction delay affecting the left ventricular wall & septum. Acute occurrences most commonly result from myocardial ischemia. Etiology: 1) myocardial ischemia (most common) 2) hypertension 3) dilated cardiomyopathy 4) valvular heart disease 5) idiopathic* * Incidentally found left bundle branch block (LBBB) with normal left ventricular ejection fraction is associated with increased cardiac morbidity & mortality. [2] Special laboratory: - electrocardiogram* (ECG features): - prolonged QRS - monophasic complexes - secondary ST segment & T wave opposite of QRS (repolarization abnormality) - V1 monophasic QRS with sharp downslope - loss of septal Q-wave in V6 - QRS complex & ST segment & T waves are discordant in patients with ventricular pacing or left bundle branch block (upward QRS complex associated with downward ST segment & T wave & downward QRS complex associated with upward ST segment & T wave) - if ST segment depression is concordant with QRS complex, ischemia should be suspected - if ST segment elevation is excessively discordant with QRS complex, ischemia should be suspected [4] - in patients with ventricular pacing or left bundle branch block, ST segment depression >= 1 mm concordant with the QRS complex defines cardiac ischemia - Sgarbossa criteria for myocardial ischemia in patients with ventricular pacing or left bundle branch block: - >= l lead with >= 1 mm of concordant ST segment depression, or - >= l lead of V1 to V3 with >= 1 mm of concordant ST-segment depression, or - >= 1lead any where with >= 1 mm ST segment elevation, with proportionally discordant ST segment elevation ((>= 25% of depth of previous S-wave) [4] - cardiac stress testing - use pharmacologic stress test or vasodilator - Persantine-thallium (myocardial perfusion study) - increased risk of false positive with exercise stress test [3] * Cannot identify LVH or RVH in the presence of LBBB

General

bundle branch block

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References

  1. Practical ECG Interpretation, T Evans, Ring Mountain Press, 1998
  2. Miller WL, Ballman KV, Hodge DO, Rodeheffer RJ, Hammill SC. Risk factor implications of incidentally discovered uncomplicated bundle branch block. Mayo Clin Proc. 2005 Dec;80(12):1585-90. PMID: 16342651
  3. Medical Knowledge Self Assessment Program (MKSAP) 16. American College of Physicians, Philadelphia 2012
  4. NEJM Knowledge+ - Sgarbossa EB. Recent advances in the electrocardiographic diagnosis of myocardial infarction: left bundle branch block and pacing. Pacing Clin Electrophysiol. 1996 Sep;19(9):1370-9. PMID: 8880802 Review. - Meyers HP, Limkakeng AT Jr, Jaffa EJ et al Validation of the modified Sgarbossa criteria for acute coronary occlusion in the setting of left bundle branch block: A retrospective case-control study. Am Heart J. 2015 Dec;170(6):1255-64. PMID: 26678648