Cardiac Conduction Disorders

Cardiac conduction disorders encompass a spectrum of abnormalities in the generation or propagation of electrical impulses through the heart's specialized conduction system. They range from benign incidental findings to life-threatening emergencies requiring urgent intervention.

Anatomy of the Conduction System

The normal cardiac conduction pathway:

SA Node → AV Node → Bundle of His → Right & Left Bundle Branches → Purkinje Fibers → Ventricular myocardium

Disruption at any level produces a characteristic conduction disorder.

Classification

I. Sinoatrial (SA) Node Disorders

Disorder	Description
Sinus bradycardia	Rate < 60 bpm; can be physiologic (athletes) or pathologic
Sick Sinus Syndrome (SSS)	Dysfunction of SA node: bradycardia, sinus arrest, sinoatrial block, tachy-brady syndrome
Sinus arrest / SA block	Failure of SA node to fire or conduct to atria

II. Atrioventricular (AV) Block

Degree	ECG Finding	Clinical Significance
1st Degree	PR interval > 200 ms; all P waves conduct	Usually benign; can be drug-induced
2nd Degree – Mobitz I (Wenckebach)	Progressive PR prolongation until a QRS is dropped	Often at AV node level; usually benign; may be vagally mediated
2nd Degree – Mobitz II	Constant PR interval with sudden dropped QRS	Infranodal (His-Purkinje); unpredictable — high risk of progression to complete block
3rd Degree (Complete)	No relationship between P waves and QRS; AV dissociation	Life-threatening; requires pacemaker

III. Bundle Branch & Fascicular Blocks

Block	Key ECG Features	Common Associations
RBBB	rsR' in V1; wide slurred S in I, V6; QRS ≥ 120 ms	ASD, PE, RV strain; also seen in normal hearts
LBBB	Broad monophasic R in I, aVL, V6; no septal Q waves; QRS ≥ 120 ms	Coronary artery disease, hypertensive heart disease, aortic valve disease, dilated cardiomyopathy
Left Anterior Fascicular Block (LAFB)	Left axis deviation (−45° to −90°); qR in I, aVL; rS in II, III, aVF	Most common fascicular block; often isolated
Left Posterior Fascicular Block (LPFB)	Right axis deviation; rS in I, aVL; qR in III, aVF	Less common; diagnosis of exclusion
Bifascicular block	RBBB + LAFB (most common) or RBBB + LPFB	Carries risk of progression to complete block
Trifascicular block	Bifascicular block + 1st/2nd degree AV block	High risk; pacemaker often indicated

Per Harrison's Principles (p. 6801), LBBB specifically is a marker of one of four high-risk underlying conditions: coronary heart disease (often with impaired LV function), hypertensive heart disease, aortic valve disease, and dilated cardiomyopathy.

ECG Example: Multilevel Conduction Disease

Alternating Bundle Branch Block with Second-Degree AV Block (Wenckebach)

This 12-lead ECG shows alternating RBBB and LBBB on a beat-to-beat basis with concomitant Mobitz I (Wenckebach) AV block — representing extensive multilevel His-Purkinje disease with high risk of progression to complete heart block.

Etiology

Ischemic heart disease (most common cause of acquired block)
Degenerative/fibrotic disease (Lev's disease, Lenègre's disease — progressive fibrosis of the conduction system)
Infectious: Lyme disease, Chagas disease, myocarditis, infective endocarditis with abscess
Infiltrative: Sarcoidosis, amyloidosis, hemochromatosis
Metabolic: Hyperkalemia, hypothyroidism
Autoimmune: SLE (neonatal lupus → congenital complete heart block)
Iatrogenic: Drug toxicity (digoxin, beta-blockers, calcium channel blockers, antiarrhythmics), post-cardiac surgery, catheter ablation
Congenital: Congenitally corrected TGA, ASD, Ebstein's anomaly
Neuromuscular disorders: Myotonic dystrophy, Kearns-Sayre syndrome

Clinical Presentation

Symptom	Underlying Mechanism
Fatigue, exercise intolerance	Reduced cardiac output
Presyncope / syncope (Stokes-Adams attacks)	Sudden complete heart block or pause
Palpitations	Escape rhythms, compensatory changes
Dyspnea	Low output, especially with LBBB + LV dysfunction
Asymptomatic	Many BBBs and 1st-degree blocks

Diagnosis

12-lead ECG — cornerstone of diagnosis
Ambulatory monitoring (Holter, event recorder, implantable loop recorder) — for intermittent/paroxysmal blocks
Exercise stress testing — to unmask rate-dependent blocks or exercise-induced AV block
Electrophysiology (EP) study — measures HV interval; HV > 70 ms suggests infranodal disease; HV > 100 ms strongly predicts progression to complete block
Echocardiography — assess structural heart disease, LV dysfunction (especially with LBBB)
Cardiac MRI — evaluate myocardial scar, infiltrative disease (sarcoid, amyloid)
Lab work — electrolytes, thyroid function, Lyme serology, iron studies, ANA/anti-Ro in appropriate context

Management

Acute / Reversible Causes

Correct underlying cause first: treat hyperkalemia, withhold offending drugs, antibiotics for Lyme disease
Atropine (0.5–1 mg IV) for symptomatic vagally mediated bradycardia/AV block
Transcutaneous pacing — for hemodynamically unstable patients
Transvenous temporary pacing — bridge for high-degree blocks pending definitive therapy

Permanent Pacemaker Indications (ACC/AHA Guidelines)

Indication	Class
Symptomatic complete (3rd-degree) AV block	I (mandatory)
Symptomatic Mobitz II 2nd-degree AV block	I
Asymptomatic complete AV block with documented pauses > 3 s or escape rate < 40 bpm	I
Asymptomatic Mobitz II with wide QRS	IIa
Bifascicular or trifascicular block with syncope and suspected advanced AV block	I
HV interval ≥ 100 ms on EP study (non-physiologic)	IIa
SSS with symptomatic bradycardia	I

Device Therapy Considerations

CRT (Cardiac Resynchronization Therapy): Indicated in LBBB with QRS ≥ 150 ms, EF ≤ 35%, NYHA Class II–IV despite optimal medical therapy — restores ventricular synchrony and improves outcomes
His-bundle pacing / Left bundle branch area pacing (LBBAP): Emerging alternatives to conventional RV pacing that preserve physiologic conduction and avoid pacing-induced cardiomyopathy

Complications

Progression to complete heart block (especially Mobitz II, trifascicular block)
Sudden cardiac death
Pacing-induced cardiomyopathy from chronic RV apical pacing (dyssynchrony → LV dysfunction)
Syncope and trauma
Reduced quality of life from fatigue and exercise intolerance

Key Clinical Pearls

RBBB alone in an otherwise healthy person is generally benign and does not require workup beyond baseline ECG.
New LBBB warrants urgent evaluation to exclude acute MI (STEMI equivalent if new with chest pain — Sgarbossa criteria apply).
Alternating bundle branch block (beat-to-beat RBBB/LBBB alternation) signals extensive His-Purkinje disease and is a near-absolute indication for permanent pacemaker.
Lyme carditis is a reversible cause of high-degree AV block — always check Lyme serology in endemic areas with unexplained AV block, especially in young patients.
Mobitz I in inferior MI reflects ischemia to the AV node (RCA territory) — usually transient. Mobitz II in anterior MI indicates massive infranodal ischemia — carries poor prognosis and requires pacing.

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