left bundle branch block (LBBB)
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Introduction
An ECG change characterized by an intraventricular conduction delay affecting the left ventricular wall & septum. Acute occurrences most commonly result from myocardial ischemia.
Etiology
- myocardial ischemia (most common)
- hypertension
- dilated cardiomyopathy
- valvular heart disease
- idiopathic*
* Incidentally found left bundle branch block (LBBB) with normal left ventricular ejection fraction is associated with increased cardiac morbidity & mortality.[2]
Diagnostic procedures
- 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]
- 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
More general terms
References
- ↑ Practical ECG Interpretation, T Evans, Ring Mountain Press, 1998
- ↑ 2.0 2.1 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: https://www.ncbi.nlm.nih.gov/pubmed/16342651
- ↑ 3.0 3.1 Medical Knowledge Self Assessment Program (MKSAP) 16. American College of Physicians, Philadelphia 2012
- ↑ 4.0 4.1 4.2 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: https://www.ncbi.nlm.nih.gov/pubmed/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: https://www.ncbi.nlm.nih.gov/pubmed/26678648