7/6/23, 2:48 PM Chronic bifascicular blocks - UpToDate Official reprint from UpToDate www.uptodate.com 2023 UpToDate, Inc. and/or its affiliates. All Rights Reserved. Chronic bifascicular blocks : William H Sauer, MD : N A Mark Estes, III, MD : Susan B Yeon, MD, JD, FACC All topics are updated as new evidence becomes available and our peer review process is complete. Literature review current through: Jun 2023. This topic last updated: Dec 06, 2022. INTRODUCTION Bifascicular block, a pattern seen on the surface electrocardiogram (ECG), results when normal physiologic activation in the His-Purkinje system is interrupted. The normal sequence of activation is altered dramatically in patients with bifascicular block, with a resultant characteristic appearance on the ECG that varies depending upon the exact fascicles which are blocked. Interruptions in conduction may result in right bundle branch block (RBBB), left anterior fascicular block (LAFB), or left posterior fascicular block (LPFB), with bifascicular block resulting when two of these three are identified from the ECG. A 2009 American Heart Association/American College of Cardiology Foundation/Heart Rhythm Society (AHA/ACCF/HRS) scientific statement on the standardization and interpretation of the electrocardiogram recommends against using the term bifascicular block (and also trifascicular block) since these patterns do not have unique anatomic and pathologic substrates [1]. However, these terms are still widely entrenched in clinical practice and scientific literature, meriting their discussion here. The anatomy, clinical manifestations, differential diagnosis, prognostic implications, and treatment of bifascicular block (RBBB with either LAFB or LPFB) will be reviewed here. Though technically a type of bifascicular block, complete LBBB is discussed separately, as are conduction system abnormalities involving only a single fascicle. (See "Left bundle branch block" and "Right bundle branch block" and "Left anterior fascicular block" and "Left posterior fascicular block" and "Left septal fascicular block".) https://www.uptodate.com/contents/chronic-bifascicular-blocks/print 1/19 7/6/23, 2:48 PM Chronic bifascicular blocks - UpToDate DEFINITIONS Bifascicular block The term bifascicular block most commonly refers to conduction disturbances below the atrioventricular (AV) node in which the right bundle branch and one of the two fascicles (anterior or posterior) of the left bundle branch are involved. Although this definition is most commonly used, left bundle branch block (LBBB) is also a type of bifascicular block since LBBB, as noted, implies block in both fascicles [2,3]. Trifascicular block The term trifascicular block is most commonly used to describe bifascicular block associated with prolongation of the PR interval (ie, first degree AV block). However, this description, though commonly used in clinical practice, is inaccurate as the conduction delay resulting in the PR interval prolongation does not usually occur in a fascicle, but in the AV node. True trifascicular block would involve block of the right bundle branch and both fascicles of the left bundle branch; this manifests as third degree (complete) heart block and is referred to as such. Sinus rhythm with alternating left/right bundle branch block or right bundle branch block (RBBB) with alternating fascicular blocks on a beat-to-beat basis is a very rare manifestation of trifascicular block, usually heralding complete AV block. (See "Third-degree (complete) atrioventricular block".) ANATOMY AND BLOOD SUPPLY Anatomy The bundle of His divides at the juncture of the fibrous and muscular boundaries of the intraventricular septum into the left and right bundle branches ( figure 1). The right bundle branch is a long, thin, discrete structure that courses down the right side of interventricular septum near the endocardium in its upper third, deeper in the muscular portion of the septum in the middle third, and then again near the endocardium in its lower third. The right bundle branch does not divide throughout most of its course, but begins to ramify as it approaches the base of the right anterior papillary muscle with fascicles going to the septal and free walls of the right ventricle. The main left bundle branch penetrates the membranous portion of the interventricular septum under the aortic ring and then divides into several fairly discrete branches. The components of the left bundle branch are ( figure 1) [4-8]: A pre-divisional segment An anterior fascicle that crosses the left ventricular outflow tract and terminates in the Purkinje system of the anterolateral wall of the left ventricle https://www.uptodate.com/contents/chronic-bifascicular-blocks/print 2/19 7/6/23, 2:48 PM Chronic bifascicular blocks - UpToDate A posterior fascicle that fans out extensively inferiorly and posteriorly into Purkinje fibers In some hearts, a median fascicle to the interventricular septum Blood supply The blood supply to the fascicles is complex and somewhat variable between patients: The right bundle branch receives most of its blood supply from septal branches of the left anterior descending coronary artery, particularly in its initial course. In most patients, it also receives some collateral supply from either the right or circumflex coronary systems depending upon the dominance of the coronary system ( figure 2). The left anterior fascicle (and median fascicle, when present) is supplied either by septal branches of the left anterior descending artery or by the atrioventricular (AV) nodal artery ( figure 2). The proximal part of the left posterior fascicle is supplied by the artery to the AV node and, at times, by septal branches of the left anterior descending artery. The distal portion has a dual blood supply from both anterior and posterior septal perforating arteries. As is true for the right bundle branch, the left fascicles may receive some collateral flow from the right and circumflex coronary systems. ETIOLOGY The right bundle branch is vulnerable to stretch and trauma for two-thirds of its course when it is near the subendocardial surface ( figure 1). Additionally, conduction in both the right and left bundle branches can be compromised by both structural and functional factors (eg, chronic ventricular pressure or volume overload, myocardial ischemia, myocarditis, etc). A more extensive discussion of the etiologies of conduction disturbances in the right and left bundles is presented elsewhere. (See "Right bundle branch block", section on 'Etiology' and "Left bundle branch block", section on 'Etiology'.) CLINICAL PRESENTATION, DIAGNOSIS, AND EVALUATION ECG findings Bifascicular block may present with one of three potential appearances on the surface electrocardiogram (ECG): Right bundle branch block (RBBB) and left anterior fascicular block (LAFB) ( waveform 1) RBBB and left posterior fascicular block (LPFB) ( waveform 2) LAFB and LPFB (ie, left bundle branch block [LBBB]) ( waveform 3) https://www.uptodate.com/contents/chronic-bifascicular-blocks/print 3/19 7/6/23, 2:48 PM Chronic bifascicular blocks - UpToDate Detailed descriptions of the ECG findings associated with RBBB, LAFB, LPFB, and LBBB are presented separately. (See "Right bundle branch block", section on 'ECG findings and diagnosis' and "Left anterior fascicular block", section on 'Electrocardiographic findings' and "Left posterior fascicular block", section on 'Electrocardiographic findings' and "Left bundle branch block", section on 'ECG findings and diagnosis'.) Asymptomatic patients In nearly all instances, the clinical presentation of bifascicular block is asymptomatic and fairly benign, as bifascicular block in and of itself does not produce symptoms, and there are no specific signs of bifascicular block during physical examination. As such, bifascicular block is identified when patients are undergoing an ECG for another indication. For asymptomatic patients with bifascicular block, no further diagnostic evaluation or therapy is required. However, patients should be carefully screened for symptoms and signs suggesting occult cardiac disease, as concomitant structural heart disease is frequently present. If underlying cardiac disease is suspected, additional diagnostic testing and therapy would proceed accordingly. Symptomatic patients For patients who present with presyncope or syncope and are noted to have bifascicular block on ECG, additional monitoring and evaluation are required, as such patients may have intermittent complete heart block that results in hemodynamic instability leading to their symptoms of presyncope or syncope. In those patients with syncope or presyncope who have suspected advanced conduction disease, we perform continuous ECG monitoring for 24 to 48 hours, usually in an inpatient setting, to monitor for high-grade AV block that would require a permanent pacemaker [3]. Additionally, cardiac imaging with echocardiography is indicated, as this presentation could be the initial manifestation of structural heart disease [3]. In our opinion, patients presenting with unexplained syncope and bifascicular block should be evaluated immediately as possible progression to heart block is unknown at initial presentation. In those patients with a structurally normal heart and unexplained syncope with bifascicular block, an electrophysiologic study (EPS) could identify occult infranodal conduction disease and prompt permanent pacemaker implantation [2,9]. Patients with an abnormal EPS (HV >70 msec or His-Purkinje AV block with pacing or pharmacologic challenge) would generally benefit from permanent pacemaker implantation. In those patients with unexplained syncope and no obvious etiology, long-term monitoring with an insertable cardiac monitor (also sometimes referred to as an implantable cardiac monitor or an implantable loop recorder) is indicated. (See "Ambulatory ECG monitoring".) https://www.uptodate.com/contents/chronic-bifascicular-blocks/print 4/19 7/6/23, 2:48 PM Chronic bifascicular blocks - UpToDate Limited data suggest that tilt table testing is not helpful in patients with bifascicular block with unexplained syncope, and as such we do not recommend tilt table testing in this population. In a study comparing such patients with control subjects with bifascicular block and no syncope, no difference in the incidence of a positive tilt table test (28 versus 32 percent) was observed, suggesting that test specificity in this population is poor [10]. DIFFERENTIAL DIAGNOSIS While bifascicular block has one of two fairly characteristic appearances on electrocardiogram (ECG), there are other conditions in which the ECG may have a similar appearance that need to be excluded prior to confirming the diagnosis of bifascicular block. Ventricular tachycardia and accelerated idioventricular rhythm If the dominant ventricular rhythm originates from a pacemaker in the ventricle, the QRS will be widened and can have the appearance of bifascicular block. However, both ventricular tachycardia (heart rate greater than 100 beats per minute) ( waveform 4) and accelerated idioventricular rhythm (heart rate between 60 and 100 beats per minute) ( waveform 5) are associated with atrioventricular (AV) dissociation, which should distinguish the rhythm from a supraventricular rhythm with bifascicular block. (See "Sustained monomorphic ventricular tachycardia: Clinical manifestations, diagnosis, and evaluation" and "ECG tutorial: Ventricular arrhythmias", section on 'Accelerated idioventricular rhythm'.) Ventricular pacing Ventricular pacing from the right ventricle typically results in a QRS complex resembling that seen with LBBB on the surface ECG. Biventricular pacing, in theory, could also result in the appearance of bifascicular block. In nearly all patients, however, the presence of pacemaker spikes preceding the QRS complex differentiates a paced complex from bifascicular block. Ventricular pre-excitation (Wolff-Parkinson-White syndrome) In some patients with manifest accessory pathways, the pre-excitation pattern can mimic bifascicular block. In Wolff- Parkinson-White (WPW) syndrome, however, the PR interval is typically short, which is generally not the case with bifascicular block. NATURAL HISTORY AND PROGNOSIS Progression of chronic bifascicular block and bifascicular block with a prolonged PR interval to complete heart block appears to be infrequent among asymptomatic patients [11,12]. In one https://www.uptodate.com/contents/chronic-bifascicular-blocks/print 5/19 7/6/23, 2:48 PM Chronic bifascicular blocks - UpToDate study of 554 patients with bifascicular or trifascicular block who were followed for an average of 42 months, only 1 percent per year progressed to complete heart block [11]. Among patients with syncope or other symptoms at baseline, the likelihood of progression to symptomatic high-grade heart block appears high. In a study of 249 patients with bifascicular block (of which 41 percent were left bundle branch block [LBBB]), 57 patients required a permanent pacemaker for "significant atrioventricular (AV) block" over a median follow-up of 4.5 years (5 percent per year) [13]. Otherwise unexplained syncope in the presence of bifascicular block is an indication for a permanent pacemaker [2,14]. However, because unexplained syncope can be due to nonarrhythmic causes, it is possible for those with a pacemaker to continue to experience syncope, and thus an accurate diagnosis is desirable [15]. Despite the expectation for prevention of syncope with pacing, patients who receive a pacemaker with fascicular block do not have a different mortality rate compared with those without pacers [16]. This is likely due to competing causes of cardiac death independent of heart block progression. The significance and treatment of bifascicular or trifascicular block appearing during acute myocardial infarction is considered separately. (See "Conduction abnormalities after myocardial infarction".) TREATMENT Management of patients with chronic bifascicular block begins by looking for and correcting reversible causes of impaired conduction such as myocardial ischemia and drugs that may slow conduction or prolong the refractory period of fascicular tissue. (See "Etiology of atrioventricular block".) If no reversible causes are present, management involves the avoidance of medications that impair atrioventricular (AV) nodal conduction (when possible). Consideration of additional treatment with a permanent pacemaker depends on the presence or absence of symptoms: For patients with bifascicular block and no apparent symptoms, no specific treatment is required. For patients with bifascicular block and symptoms of syncope or presyncope of suspected cardiac etiology (specifically due to suspected intermittent complete heart block with bradyarrhythmia), we suggest permanent pacemaker implantation. Our approach is consistent with the guideline recommendations of various professional societies for patients with unexplained syncope in the setting of chronic bifascicular block if https://www.uptodate.com/contents/chronic-bifascicular-blocks/print 6/19 7/6/23, 2:48 PM Chronic bifascicular blocks - UpToDate other likely causes of syncope have been excluded [9,14]. (See 'Natural history and prognosis' above.) A randomized trial of permanent pacing versus implantable loop recorder monitoring in patients with bifascicular block and syncope demonstrated a significant reduction in a composite endpoint of cardiovascular death, syncope, bradycardia, and device-related complications with empiric pacing [17]. Interestingly, syncope was still observed in 29 percent of patients who received a pacemaker, indicating a vasodepressor etiology in many of these patients. This clinical trial adds to the clinical evidence supporting the pacing recommendation. A number of neuromuscular diseases are associated with conduction abnormalities. These include myotonic muscular dystrophy, Kearns-Sayre syndrome, Erb dystrophy (limb-girdle), and peroneal muscular atrophy. These patients represent a special class and are treated more aggressively with pacemakers due to the potential for unpredictably rapid progression of conduction disease. (See "Inherited syndromes associated with cardiac disease" and "Permanent cardiac pacing: Overview of devices and indications", section on 'Neuromuscular diseases'.) Detailed reviews of the indications for permanent pacemaker placement and of the modes of cardiac pacing are presented separately. (See "Permanent cardiac pacing: Overview of devices and indications", section on 'Acquired AV block' and "Modes of cardiac pacing: Nomenclature and selection".) SUMMARY AND RECOMMENDATIONS Bifascicular block, a pattern seen on the surface electrocardiogram (ECG), results when normal electrical activity in the His-Purkinje system is interrupted. Interruptions in conduction may result in right bundle branch block (RBBB), left anterior fascicular block (LAFB), or left posterior fascicular block (LPFB), with bifascicular block resulting when two of these three are identified on the ECG. Bifascicular block most commonly refers to conduction disturbances involving the right bundle branch and one of the two fascicles (anterior or posterior) of the left bundle branch. (See 'Introduction' above.) In nearly all instances, the clinical presentation of bifascicular block is asymptomatic and fairly benign, as bifascicular block in and of itself does not produce symptoms, and there are no specific signs of bifascicular block during physical examination. As such, bifascicular block is identified when patients are undergoing an ECG for another indication. For asymptomatic patients with bifascicular block, no further diagnostic evaluation or therapy is required, although patients should be screened carefully for symptoms and signs suggesting occult cardiac disease. (See 'Asymptomatic patients' above.) https://www.uptodate.com/contents/chronic-bifascicular-blocks/print 7/19 7/6/23, 2:48 PM Chronic bifascicular blocks - UpToDate For patients who present with presyncope or syncope and are noted to have bifascicular block on ECG, additional monitoring and evaluation are required, as intermittent complete heart block may result in hemodynamic instability leading to their symptoms. In such patients, we perform continuous ECG monitoring for 24 to 48 hours, usually in an inpatient setting, to monitor for high-grade atrioventricular (AV) block. We also perform echocardiography to assess for underlying structural heart disease. (See 'Symptomatic patients' above.) Progression of chronic bifascicular block and bifascicular block with a prolonged PR interval to complete heart block is infrequent, with an annual rate of approximately 1 percent in asymptomatic patients and up to 5 percent in symptomatic patients. (See 'Natural history and prognosis' above.) Management of patients with chronic bifascicular block begins by looking for and correcting reversible causes of impaired conduction such as myocardial ischemia and drugs that may slow conduction or prolong the refractory period of fascicular tissue. If no reversible causes are present, treatment involves the avoidance of medications that impair AV nodal conduction (when possible) and evaluation for permanent pacemaker placement. (See 'Treatment' above and "Permanent cardiac pacing: Overview of devices and indications", section on 'Acquired AV block'.) For patients with bifascicular block and no apparent symptoms, no specific treatment is required. For patients with bifascicular block and symptoms of syncope or presyncope of suspected cardiac etiology (specifically due to suspected intermittent complete heart block with bradyarrhythmia), we suggest permanent pacemaker implantation (Grade 2C). Use of UpToDate is subject to the Terms of Use. REFERENCES 1. Surawicz B, Childers R, Deal BJ, et al. AHA/ACCF/HRS recommendations for the standardization and interpretation of the electrocardiogram: part III: intraventricular conduction disturbances: a scientific statement from the American Heart Association Electrocardiography and Arrhythmias Committee, Council on Clinical Cardiology; the American College of Cardiology Foundation; and the Heart Rhythm Society: endorsed by the International Society for Computerized Electrocardiology. Circulation 2009; 119:e235. https://www.uptodate.com/contents/chronic-bifascicular-blocks/print 8/19 7/6/23, 2:48 PM Chronic bifascicular blocks - UpToDate 2. Brignole M, Moya A, de Lange FJ, et al. 2018 ESC Guidelines for the diagnosis and management of syncope. Eur Heart J 2018; 39:1883. 3. 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. 4. Tawara S. Das Reizleitungssystem des S uegetierherzens. Gustav Fischer, Jena 1906. 5. Rosenbaum M, Elizari MV, Lazzari JO. The Hemiblocks. Tampa Tracings, Tampa 1970. 6. Uhley HN. Some controversy regarding the peripheral distribution of the conduction system. Am J Cardiol 1972; 30:919. 7. Hecht HH, Kossmann CE, Childers RW, et al. Atrioventricular and intraventricular conduction. Revised nomenclature and concepts. Am J Cardiol 1973; 31:232. 8. Demoulin JC, Kulbertus HE. Histopathological examination of concept of left hemiblock. Br Heart J 1972; 34:807. 9. Brignole M, Auricchio A, Baron-Esquivias G, et al. 2013 ESC Guidelines on cardiac pacing and cardiac resynchronization therapy: the Task Force on cardiac pacing and resynchronization therapy of the European Society of Cardiology (ESC). Developed in collaboration with the European Heart Rhythm Association (EHRA). Eur Heart J 2013; 34:2281. 10. Englund A, Fredrikson M, Rosenqvist M. Head-up tilt test. A nonspecific method of evaluating patients with bifascicular block. Circulation 1997; 95:951. 11. McAnulty JH, Rahimtoola SH, Murphy E, et al. Natural history of "high-risk" bundle-branch block: final report of a prospective study. N Engl J Med 1982; 307:137. 12. Schneider JF, Thomas HE, Kreger BE, et al. Newly acquired right bundle-branch block: The Framingham Study. Ann Intern Med 1980; 92:37. 13. Mart -Almor J, Cladellas M, Baz n V, et al. [Novel predictors of progression of atrioventricular block in patients with chronic bifascicular block]. Rev Esp Cardiol 2010; 63:400. 14. Shen WK, Sheldon RS, Benditt DG, et al. 2017 ACC/AHA/HRS Guideline for the Evaluation and Management of Patients With Syncope: 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 2017. 15. Kalscheur MM, Donateo P, Wenzke KE, et al. Long-Term Outcome of Patients with Bifascicular Block and Unexplained Syncope Following Cardiac Pacing. Pacing Clin Electrophysiol 2016; 39:1126. https://www.uptodate.com/contents/chronic-bifascicular-blocks/print 9/19 7/6/23, 2:48 PM Chronic bifascicular blocks - UpToDate 16. Moulki N, Kealhofer JV, Benditt DG, et al. Association of cardiac implantable electronic devices with survival in bifascicular block and prolonged PR interval on electrocardiogram. J Interv Card Electrophysiol 2018; 52:335. 17. Sheldon R, Talajic M, Tang A, et al. Randomized Pragmatic Trial of Pacemaker Versus Implantable Cardiac Monitor in Syncope and Bifascicular Block. JACC Clin Electrophysiol 2022; 8:239. Topic 1063 Version 26.0 https://www.uptodate.com/contents/chronic-bifascicular-blocks/print 10/19 7/6/23, 2:48 PM Chronic bifascicular blocks - 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/chronic-bifascicular-blocks/print 11/19 7/6/23, 2:48 PM Chronic bifascicular blocks - UpToDate Coronary artery anatomy Graphic 70956 Version 2.0 https://www.uptodate.com/contents/chronic-bifascicular-blocks/print 12/19 7/6/23, 2:48 PM Chronic bifascicular blocks - UpToDate 12-lead electrocardiogram (ECG) showing bifascicular block with right bundle branch block and left anterior fascicular block The 12-lead ECG from a patient with a history of an anteroseptal myocardial infarction (Q waves seen in lead V1-V3) shows bifascicular block with RBBB and LAFB. A typical RBBB is seen with a QRS duration of 0.16 seconds and an rSR' configuration in lead V1 and a deep S wave in V6. The QRS complexes in leads II, III, and avF are negative, with a rS morphology, diagnostic of a pathologic left axis deviation, known as LAFB. ECG: electrocardiogram; RBBB: right bundle branch block; LAFB: left anterior fascicular block. Reproduced with permission by Samuel Levy, MD. Graphic 69886 Version 4.0 https://www.uptodate.com/contents/chronic-bifascicular-blocks/print 13/19 7/6/23, 2:48 PM Chronic bifascicular blocks - UpToDate ECG bifascicular block RBBB and LPFB The 12-lead ECG from a patient with bifascicular block with RBBB and LPFB. ECG: electrocardiogram; RBBB: right bundle branch block; LPFB: left posterior fascicular block. Reproduced with permission from: Nathanson LA, McClennen S, Safran C, Goldberger AL. ECG Wave-Maven: Self-Assessment Program f Clinicians. Available at: http://ecg.bidmc.harvard.edu (Accessed on January 11, 2017). Graphic 111524 Version 2.0 https://www.uptodate.com/contents/chronic-bifascicular-blocks/print 14/19 7/6/23, 2:48 PM Chronic bifascicular blocks - 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/chronic-bifascicular-blocks/print 15/19 7/6/23, 2:48 PM Chronic bifascicular blocks - 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/chronic-bifascicular-blocks/print 16/19 7/6/23, 2:48 PM Chronic bifascicular blocks - 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/chronic-bifascicular-blocks/print 17/19 7/6/23, 2:48 PM Chronic bifascicular blocks - 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/chronic-bifascicular-blocks/print 18/19 7/6/23, 2:48 PM Chronic bifascicular blocks - 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. N A Mark Estes, III, MD Consultant/Advisory Boards: Boston Scientific [Arrhythmias]; Medtronic [Arrhythmias]. All of the relevant financial relationships listed have been mitigated. 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. Conflict of interest policy https://www.uptodate.com/contents/chronic-bifascicular-blocks/print 19/19