Intraventricular Blocks

Chapter: Intraventricular Blocks
McMaster Section Editor(s): P.J. Devereaux
Section Editor(s) in Interna Szczeklika: Andrzej Budaj, Wiktoria Leśniak
McMaster Author(s): Juan Gabriel Acosta Velez, Guy Amit, Eder Augusto Hernández Ruiz
Author(s) in Interna Szczeklika: Maria Trusz-Gluza, Wiktoria Leśniak
Additional Information

Definition, Etiology, PathogenesisTop

Intraventricular blocks may be of a His bundle branch block pattern, a fascicular block pattern, or both and result from significant slowing or interruption of conduction. Possible patterns of intraventricular block include:

1) Left anterior or posterior fascicular block.

2) Right bundle branch block (RBBB) (Figure 3.2-5) or left bundle branch block (LBBB) (Figure 3.2-6).

3) RBBB with left anterior or posterior fascicular block (bifascicular block; according to the definition by the European Society of Cardiology, isolated LBBB is also a bifascicular block).

Trifascicular block refers to simultaneous or alternate conduction impairment in all fascicles. This term is also sometimes used in the case of bifascicular block with first-degree atrioventricular (AV) block, but the definition is not accurate, as PR prolongation in such patients could be related to the AV node and not to a block in the remaining fascicle. Simultaneous impairment in all 3 fascicles presents as complete heart block.

Causes of RBBB: Congenital heart disease (most commonly atrial septal defect), ischemic heart disease (IHD), or idiopathic fibrosis. This is frequently an isolated pathology. Features of a pseudo-RBBB with ST-segment elevation are observed in Brugada syndrome.

Causes of LBBB: Structural heart disease: IHD, cardiomyopathy (particularly dilated cardiomyopathy), myocarditis, congenital or acquired heart disease, connective tissue disease, myocardial infiltrates in the course of various conditions, idiopathic fibrosis, or calcifications.

Intraventricular blocks may be caused by antiarrhythmic drugs, particularly class I drugs (see Table 3.4-1) and amiodarone. Bundle branch blocks are more frequent in patients with tachycardia and less common in patients with bradycardia.

Clinical Features and Natural HistoryTop

Intraventricular blocks without advanced AV block are usually asymptomatic. In patients with LV dysfunction and heart failure, LBBB worsens the LV dysfunction and mitral regurgitation, and thus also heart failure.

Patients with bifascicular and trifascicular blocks are at risk of slow progression to advanced or complete AV block (this should be suspected in patients with new-onset syncope). Note the risk of ventricular tachycardia.

DiagnosisTop

Diagnosis is based on electrocardiographic (ECG) criteria.

1. Left fascicular block:

1) Left axis deviation >−30 degrees (anterior fascicular block) or right axis deviation >+90 degrees (posterior fascicular block).

2) QRS complex <0.12 seconds.

3) Waves:

a) A dominant S wave in leads II, III, and aVF, a small Q wave and a dominant R wave in leads I and aVL: Anterior fascicular block.

b) A dominant S wave in leads I and aVL, a dominant R wave in leads II, III, and AVF: Posterior fascicular block.

2. Bundle branch block:

1) QRS complex ≥0.12 seconds.

2) ST segments and T waves are usually discordant from the dominant deflection of the QRS complex.

3) QRS pattern:

a) rsR, rSR, or rsr patterns, or rarely a wide-notched R-wave pattern in V1 to V2: RBBB (see Figure 3.2-3). rS in V1: LBBB.

b) Monophasic QRS complexes with a notched or biphasic R wave in V5 to V6: LBBB (see Figure 3.2-3). Rs, RS in V6: RBBB.

TreatmentTop

1. Management of the underlying condition.

2. Indications for pacemaker implantation in patients with bundle branch block:

1) Syncope, bundle branch block, and positive electrophysiologic study findings defined as a His-ventricular (HV) interval ≥70 milliseconds (conduction time below the AV node) or triggering second-degree or third-degree AV block in the His bundle/Purkinje fibers during atrial stimulation with increasing frequency or pharmacologic challenge.

2) Alternate bundle branch block, regardless of symptoms.

3. Indications for biventricular pacing (cardiac resynchronization therapy): see Chronic Heart Failure.

FiguresTop

Figure 3.2-5. Right bundle branch block. Illustration courtesy of Dr Shannon Zhang.

Figure 3.2-6. Left bundle branch block. Illustration courtesy of Dr Shannon Zhang.

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