03. Intraventricular Conduction Disturbances and Fascicular Blocks
Disruption of the trifascicular ventricular conduction system produces predictable vector shifts, localized conduction delays, and mechanical dyssynchrony.
1. Anatomy of the Trifascicular Conduction System
- Right Bundle Branch: Long, thin, unbranched cable traveling down the right side of the interventricular septum to the anterior papillary muscle (moderator band).
- Left Main Bundle Branch: Broad sheet dividing rapidly into:
- Left Anterior Fascicle (LAF): Long, slender, single blood supply (LAD septal perforators); highly vulnerable to ischemia.
- Left Posterior Fascicle (LPF): Short, broad, dual blood supply (LAD and posterior descending artery); resistant to isolated injury.
- Left Median / Septal Fascicle: Innervates mid-interventricular septum.
2. Electrocardiographic Criteria for Fascicular Blocks
| Conduction Defect | Frontal QRS Axis | Limb Lead Morphologies | QRS Duration | Diagnostic Caveats |
|---|---|---|---|---|
| Left Anterior Fascicular Block (LAFB) | Left Axis Deviation ($-45^\circ \text{ to } -90^\circ$) | $qR$ pattern in leads I and aVL ($q < 40\text{ ms}, < 2\text{ mm}$); $rS$ pattern in leads II, III, and aVF. | Normal ($<120\text{ ms}$). | Most common intraventricular conduction defect; rule out inferior MI. |
| Left Posterior Fascicular Block (LPFB) | Right Axis Deviation ($>+120^\circ$) | $rS$ pattern in leads I and aVL; $qR$ pattern in leads II, III, and aVF. | Normal ($<120\text{ ms}$). | Diagnosis requires strict exclusion of RVH, lateral MI, and pulmonary embolism. |
| Left Septal Fascicular Block | Normal | Loss of normal septal $q$ in $V_5-V_6$ with prominent anterior $R$ waves in $V_1-V_2$ without bundle branch delay. | Normal ($<120\text{ ms}$). | Mimics posterior myocardial infarction or RVH. |
3. Bifascicular and Trifascicular Blocks
flowchart TD
BIF["Bifascicular Block Patterns"] --> LAFB_RBBB["RBBB + LAFB (Most Common)<br/>• rsR' in V1 (QRS ≥ 120 ms)<br/>• Extreme Left Axis Deviation (-45° to -90°)<br/>• qR in I/aVL, rS in II/III/aVF"]
BIF --> LPFB_RBBB["RBBB + LPFB (High Risk of Progression)<br/>• rsR' in V1 (QRS ≥ 120 ms)<br/>• Right Axis Deviation (> +120°)<br/>• rS in I/aVL, qR in II/III/aVF<br/>• Rule out RVH, lateral MI, PE"]
- Trifascicular Block:
- True Trifascicular Block: Alternating Bundle Branch Block (RBBB on one tracing, LBBB on another) $\to$ Class I Indication for Urgent Permanent Pacemaker.
- Incomplete Trifascicular Block: Bifascicular block (e.g. RBBB + LAFB) combined with first-degree AV block or Mobitz I/II second-degree AV block.
4. Phase-Dependent Aberrancy & The Ashman Phenomenon
- Phase 3 (Acceleration-Dependent) Aberrancy: An early impulse arrives while one bundle branch (usually right bundle due to longer refractory period) is still refractory $\to$ conducts with RBBB morphology.
- The Ashman Phenomenon:
- In Atrial Fibrillation, a short $R-R$ interval following a long $R-R$ interval results in aberrant conduction (usually RBBB) because refractory period duration is proportional to the preceding cycle length ($Long \to Short \to Aberrancy$).
5. Multimodality Investigations & Electrophysiology Study (EPS)
- Invasive His Bundle Electrogram (HBE): Measures the H-V Interval ($35-55\text{ ms}$).
- An $H-V \text{ interval} \ge 100\text{ ms}$ or pacing-induced infra-Hisian block defines critical His-Purkinje disease requiring pacemaker implantation.