Conduction Blocks & Intervals

Conduction blocks occur when electrical impulses are delayed or completely blocked as they travel through the cardiac conduction system. They can occur at any level - from the AV node to the bundle branches and fascicles.

The cardiac conduction system has a predictable hierarchy:

• AV node: Connects atrial and ventricular conduction; normally the only electrical bridge between atria and ventricles
• Bundle of His: Single pathway that emerges from the AV node and penetrates the cardiac skeleton
• Bundle branches: Right bundle branch (single fascicle) and left bundle branch (divides into anterior and posterior fascicles)
• Purkinje network: Terminal fibers that deliver impulses to ventricular myocardium

Block at each level produces characteristic ECG patterns and carries different prognostic implications. The key clinical question is always: does this patient need a pacemaker?

First-degree AV block is technically a conduction delay rather than a true block - every atrial impulse still reaches the ventricles, just more slowly than normal.

ECG Characteristics

• PR interval: Prolonged >0.20 seconds (more than 5 small boxes), but constant and consistent
• Every P wave is followed by a QRS: 1:1 AV conduction is maintained
• QRS: Narrow (unless coexisting BBB)
• Rhythm: Regular

Clinical Significance

• Often benign - seen in athletes, young healthy individuals, and with increased vagal tone
• Can be caused by medications (beta-blockers, calcium channel blockers, digoxin, amiodarone)
• May be associated with inferior MI (AV node ischemia) or myocarditis
• No specific treatment required in isolation
• Extreme PR prolongation (>0.30 sec) may cause symptoms due to loss of AV synchrony (atrial contraction against closed AV valves - "pseudo-pacemaker syndrome")

Second-degree Type I (Mobitz I / Wenckebach) block occurs at the level of the AV node. It represents progressive fatigue of AV nodal conduction until a beat is dropped.

ECG Characteristics

• Progressive PR prolongation: Each successive PR interval gets slightly longer until a P wave is not followed by a QRS (dropped beat)
• Grouped beating: Repetitive cycles of progressive PR lengthening followed by a dropped QRS
• RR intervals: Gradually shorten before the dropped beat (the increment of PR prolongation decreases with each cycle)
• The pause: Contains the dropped P wave; the pause is less than twice the shortest R-R interval in the group
• QRS: Narrow (block is at the AV node level)

Clinical Significance

• Usually benign with low risk of progression to complete heart block
• Block is at the AV node level (supra-Hisian) - reliable escape rhythms exist
• Common in: athletes, sleep, young healthy patients, inferior MI
• Drug-related: beta-blockers, calcium channel blockers, digoxin
• Rarely requires pacing unless symptomatic with hemodynamic compromise

Second-degree Type II (Mobitz II) block occurs below the AV node - at the level of the bundle of His or bundle branches. This is a much more dangerous block with high risk of progression to complete heart block.

ECG Characteristics

• Constant PR interval: The PR interval does NOT change before the dropped beat (the defining feature that distinguishes it from Wenckebach)
• Intermittently dropped QRS: P waves march through at a regular rate, but some fail to conduct
• Conduction ratio: May be 2:1, 3:1, 4:1, or variable
• QRS: Often wide (reflecting infra-nodal disease) - especially with coexisting BBB pattern
• No progressive PR prolongation: This is the critical distinguishing feature from Wenckebach

Clinical Significance

• High risk of progression to complete heart block - often unpredictably and suddenly
• Block is infra-Hisian; escape rhythms are slow (20-40 bpm), unreliable, and wide-complex
• Associated with anterior MI (LAD territory affecting the interventricular septum), degenerative conduction disease, and infiltrative cardiomyopathies
• Almost always requires permanent pacemaker implantation
• Transcutaneous pacing pads should be applied immediately while arranging definitive pacing

Third-degree (complete) AV block represents complete failure of conduction between atria and ventricles. The atria and ventricles beat independently with no relationship between P waves and QRS complexes.

ECG Characteristics

• AV dissociation: P waves and QRS complexes occur independently - P waves march through at the sinus rate while QRS occurs at the escape rate
• Regular atrial rate: P-P intervals are constant
• Regular ventricular rate: R-R intervals are constant, but slower than the atrial rate
• No consistent PR relationship: PR intervals vary randomly because P waves and QRS complexes are independent
• Escape rhythm: Junctional (narrow QRS, 40-60 bpm) if block is at AV node; ventricular (wide QRS, 20-40 bpm) if block is infra-nodal

Junctional vs. Ventricular Escape

In RBBB, the right bundle branch is blocked and the right ventricle depolarizes late via cell-to-cell spread from the left ventricle. This produces a characteristic wide QRS with terminal right-directed forces.

ECG Criteria

• QRS duration: ≥0.12 seconds (complete RBBB); 0.10-0.12 seconds (incomplete RBBB)
• V1-V2: rsR' pattern (M-shaped QRS) - "rabbit ears" with R' taller than r
• V5-V6, I: Wide, slurred S wave (late right ventricular activation moving away from left-sided leads)
• Secondary ST-T changes: ST depression and T wave inversion in V1-V3 (expected, not ischemic)

Clinical Significance

• Isolated RBBB: Can be a normal variant, particularly in young healthy individuals
• New RBBB: May indicate acute right ventricular strain (PE, RV infarction), anterior MI, or myocarditis
• Chronic RBBB: Seen with right ventricular hypertrophy, ASD, chronic lung disease
• RBBB does NOT obscure STEMI diagnosis: ST elevation in the setting of RBBB is interpretable (unlike LBBB)

In LBBB, the left bundle branch is blocked and the left ventricle depolarizes late via right-to-left septal activation. This produces a wide QRS with prolonged left-directed forces and profoundly alters the ECG baseline, making ischemia difficult to diagnose.

ECG Criteria

• QRS duration: ≥0.12 seconds
• V1: Broad, deep QS or rS pattern (W-shaped)
• V5-V6, I, aVL: Tall, broad, monophasic R wave (M-shaped) - often notched or slurred
• No septal Q waves: In V5-V6, I (because septal activation is reversed)
• Appropriate discordance: ST-T changes opposite to QRS direction (expected - this is NOT ischemia)

Clinical Significance

• LBBB is almost always pathologic - unlike RBBB, it is rarely a normal variant
• Common causes: Hypertensive heart disease, coronary artery disease, cardiomyopathy, aortic valve disease, degenerative conduction disease
• New LBBB with chest pain: Historically treated as STEMI equivalent; current guidelines emphasize using the Sgarbossa criteria to identify MI in the setting of LBBB

Sgarbossa Criteria (Diagnosing MI with LBBB)

• Concordant ST elevation ≥1 mm in any lead with a positive QRS (5 points) - most specific
• Concordant ST depression ≥1 mm in V1-V3 (3 points) - posterior MI equivalent
• Excessive discordant ST elevation: ST elevation ≥1 mm AND ratio of ST elevation to S wave depth >0.25 (Modified Sgarbossa / Smith criteria) - 2 points
• Score ≥3 suggests acute MI in the setting of LBBB

The left bundle branch divides into two fascicles: the left anterior fascicle (thin, with single blood supply) and the left posterior fascicle (thick, with dual blood supply). Block of one fascicle produces axis deviation without significant QRS widening.

Left Anterior Fascicular Block (LAFB)

• Axis: Left axis deviation (−45° to −90°)
• QRS duration: Normal or slightly prolonged (<0.12 sec)
• Lead pattern: Small q in I, aVL with tall R; small r in II, III, aVF with deep S
• Clinical significance: Common, often benign. The anterior fascicle is thin with a single blood supply (LAD), making it vulnerable
• Most common fascicular block

Left Posterior Fascicular Block (LPFB)

• Axis: Right axis deviation (>+90°)
• QRS duration: Normal or slightly prolonged (<0.12 sec)
• Lead pattern: Small r in I, aVL with deep S; small q in II, III, aVF with tall R
• Clinical significance: Much less common because the posterior fascicle is thick with dual blood supply (LAD + RCA). When present, usually indicates more severe conduction disease
• Diagnosis of exclusion: Must rule out RVH, PE, lateral MI, and other causes of right axis deviation before diagnosing LPFB

Bifascicular and Trifascicular Block

• Bifascicular block: RBBB + LAFB (most common combination) or RBBB + LPFB
• Trifascicular block: Bifascicular block + 1st-degree AV block (suggests disease in the remaining fascicle but does NOT guarantee it)
• Clinical significance: Bifascicular block with syncope or alternating BBB patterns should prompt evaluation for permanent pacing

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