of any concurrent underlying heart disease and to the possible presence of other conduction disturbances: Effect of age Among asymptomatic patients, LBBB appears to have minimal effect on outcomes in younger, apparently healthy subjects, while LBBB in older individuals has been associated with an increase in mortality. (See 'Asymptomatic patients' above.) With coronary artery disease LBBB is an independent predictor of all-cause mortality in patients with known or suspected coronary heart disease. The presence of LBBB is associated with higher short-term and long-term mortality following a myocardial infarction. (See 'Patients with coronary heart disease' above and 'Patients with acute myocardial infarction' above and "Overview of the acute management of ST-elevation myocardial infarction" and "Conduction abnormalities after myocardial infarction", section on 'Bundle branch block'.) With heart failure LBBB is an independent risk factor for mortality in patients with heart failure and is associated with increased all-cause mortality and sudden death at one year. (See 'Patients with heart failure or cardiomyopathy' above.) Evaluation for concurrent conditions When LBBB is present, patients should be evaluated for hypertension, coronary disease, and other disorders that have been associated with LBBB (eg, myocarditis, valvular heart disease, cardiomyopathies). In most patients, this can be accomplished with a careful history and physical examination. (See 'Evaluation' above.) Management If asymptomatic For asymptomatic patients with an isolated LBBB and no other evidence of cardiac disease, no specific therapy is required. If develop symptomatic AV block Permanent pacemaker insertion is indicated for patients with LBBB who develop symptomatic conduction system disturbances, such as third degree or type II second degree AV block, which is not associated with a reversible https://www.uptodate.com/contents/left-bundle-branch-block/print 13/26 7/5/23, 10:35 AM Left bundle branch block - UpToDate or transient condition. (See 'Treatment' above and "Permanent cardiac pacing: Overview of devices and indications".) ACKNOWLEDGMENT The UpToDate editorial staff acknowledges Leonard Ganz, MD, FHRS, FACC, who contributed to an earlier version of this topic review. Use of UpToDate is subject to the Terms of Use. REFERENCES 1. Tawara S. Das Reizleitungssystem des S uegetierherzens. Gustav Fischer, Jena 1906. 2. Rosenbaum M, Elizari MV, Lazzari JO. The Hemiblocks. Tampa Tracings, Tampa 1970. 3. Uhley HN. Some controversy regarding the peripheral distribution of the conduction system. Am J Cardiol 1972; 30:919. 4. 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Kusumoto FM, Schoenfeld MH, Barrett C, et al. 2018 ACC/AHA/HRS Guideline on the Evaluation and Management of Patients With Bradycardia and Cardiac Conduction Delay: A Report of the American College of Cardiology/American Heart Association Task Force on Clinical Practice Guidelines and the Heart Rhythm Society. J Am Coll Cardiol 2019; 74:e51. 55. Tang AS, Wells GA, Talajic M, et al. Cardiac-resynchronization therapy for mild-to-moderate heart failure. N Engl J Med 2010; 363:2385. Topic 914 Version 36.0 https://www.uptodate.com/contents/left-bundle-branch-block/print 18/26 7/5/23, 10:35 AM Left bundle branch block - UpToDate GRAPHICS Normal conduction system Schematic representation of the normal intraventricular conduction system (His- Purkinje system). The Bundle of His divides into the left bundle branch and right bundle branch. The left bundle branch divides into anterior, posterior, and, in some cases, median fascicles. AV: atrioventricular; RA: right atrium; LA: left atrium; RV: right ventricle; LV: left ventricle. Graphic 63340 Version 6.0 https://www.uptodate.com/contents/left-bundle-branch-block/print 19/26 7/5/23, 10:35 AM Left bundle branch block - UpToDate 12-lead electrocardiogram (ECG) showing typical left bundle branch block Electrocardiogram in typical complete left bundle branch block. The asynchronous activation of the 2 ventricles increases the QRS duration (0.16 seconds in this example). The abnormal initial vector results in loss of "normal" septal forces as manifested by absence of q waves in leads I, aVL, and V6. The late activation of the left ventricle prolongs the dominant leftward progression of the middle and terminal forces, leading to a positive and widened R wave in the lateral leads. Both the ST segment and T wave vectors are opposite in direction from the QRS, a "secondary" repolarization abnormality. Courtesy of Ary Goldberger, MD. Graphic 61594 Version 9.0 Normal ECG https://www.uptodate.com/contents/left-bundle-branch-block/print 20/26 7/5/23, 10:35 AM Left bundle branch block - UpToDate Normal electrocardiogram showing normal sinus rhythm at a rate of 75 beats/minute, a PR interval of 0.14 seconds, a QRS interval of 0.10 seconds, and a QRS axis of approximately 75 . Courtesy of Ary Goldberger, MD. Graphic 76183 Version 4.0 https://www.uptodate.com/contents/left-bundle-branch-block/print 21/26 7/5/23, 10:35 AM Left bundle branch block - UpToDate Coronary artery anatomy Graphic 70956 Version 2.0 https://www.uptodate.com/contents/left-bundle-branch-block/print 22/26 7/5/23, 10:35 AM Left bundle branch block - UpToDate Selected causes of prominent T wave inversions Primary T wave inversions Normal variant Juvenile T wave pattern "Early repolarization" variant (sometimes associated with T wave inversions) Myocardial ischemia or infarction (including takotsubo syndrome) Left or right ventricular overload syndromes (formerly called "strain" patterns) Cerebrovascular injury (T waves may be massively inverted) "Memory T waves" (eg, with intermittent left bundle branch block, intermittent ventricular pacing, or intermittent Wolff-Parkinson-White pre-excitation) Post-tachycardia T wave pattern Apical hypertrophic cardiomyopathy (Yamaguchi's syndrome) Other myocardial/pericardial diseases - dilated and restrictive cardiomyopathies, arrhythmogenic right ventricular cardiomyopathy (dysplasia), pericarditis, myocardial tumor, etc. Idiopathic global T wave inversions Secondary T wave inversions Left bundle branch block Right bundle branch block Wolff-Parkinson-White patterns and variants Ventricular beats (paced, premature or escape) Graphic 63807 Version 1.0 https://www.uptodate.com/contents/left-bundle-branch-block/print 23/26 7/5/23, 10:35 AM Left bundle branch block - UpToDate Single lead electrocardiogram (ECG) showing monomorphic ventricular tachycardia Three or more successive ventricular beats are defined as ventricular tachycardia (VT). This VT is monomorphic since all of the QRS complexes have an identical appearance. Although the P waves are not distinct, they can be seen altering the QRS complex and ST-T waves in an irregular fashion, indicating the absence of a relationship between the P waves and the QRS complexes (ie, AV dissociation is present). AV: atrioventricular. Graphic 63176 Version 7.0 Normal rhythm strip Normal rhythm strip in lead II. The PR interval is 0.15 sec and the QRS duration is 0.08 sec. Both the P and T waves are upright. Courtesy of Morton F Arnsdorf, MD. Graphic 59022 Version 3.0 https://www.uptodate.com/contents/left-bundle-branch-block/print 24/26 7/5/23, 10:35 AM Left bundle branch block - UpToDate ECG 12-lead accelerated idioventricular rhythm 12-lead ECG showing idioventricular rhythm with AV dissociation and wide QRS complexes occurring at a rate faster than the sinus rate but slower than 100 bpm (hence not meeting the criteria for ventricular tachycardia). ECG: electrocardiogram. Graphic 118943 Version 2.0 https://www.uptodate.com/contents/left-bundle-branch-block/print 25/26 7/5/23, 10:35 AM Left bundle branch block - UpToDate Contributor Disclosures William H Sauer, MD Grant/Research/Clinical Trial Support: Biosense-Webster [Catheter ablation]; Boston Scientific [Catheter ablation]. All of the relevant financial relationships listed have been mitigated. Mark S Link, MD No relevant financial relationship(s) with ineligible companies to disclose. Susan B Yeon, MD, JD, FACC No relevant financial relationship(s) with ineligible companies to disclose. Contributor disclosures are reviewed for conflicts of interest by the editorial group. When found, these are addressed by vetting through a multi-level review process, and through requirements for references to be provided to support the content. Appropriately referenced content is required of all authors and must conform to UpToDate standards of evidence. 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