4.6 Intraventricular Conduction Delay, Bundle Branch Blocks & WPW Pattern

Intraventricular Conduction Delay & Bundle Branch Blocks

An intraventricular conduction delay (IVCD) occurs when the electrical impulse is slowed as it travels through the ventricular conduction system (the bundle branches and Purkinje fibers). The hallmark of a conduction delay is a widened QRS complex. Understanding and differentiating these blocks is critical for accurate ECG interpretation, as they can mimic or obscure other severe cardiac conditions.

• Normal QRS Duration: 0.06 to 0.10 seconds.
• Incomplete Block: QRS duration between 0.10 and 0.11 seconds.
• Complete Block: QRS duration ≥ 0.12 seconds.

Comparison Table: RBBB vs LBBB vs WPW

Right Bundle Branch Block (RBBB)

In RBBB, conduction through the right bundle branch is blocked. The left ventricle depolarizes normally first, followed by a delayed depolarization of the right ventricle via cell-to-cell spread. This sequence creates distinct morphological changes, best viewed in the chest leads V1 (right-sided) and V6 (left-sided).

ECG Criteria for RBBB:

1. QRS Duration: ≥ 0.12 seconds (in a complete block).
2. Lead V1: Displays an RSR' pattern (often called "rabbit ears"). The initial R wave reflects normal left ventricular septal depolarization, the S wave reflects left ventricular free wall depolarization, and the tall, wide R' wave reflects the delayed, abnormal right ventricular depolarization.
3. Leads I and V6: Display a wide, slurred S wave. This reflects the delayed right ventricular depolarization moving away from these left-sided leads.
4. ST/T Waves: Secondary ST segment depression and T wave inversion are normally seen in the right precordial leads (V1-V3) opposite to the terminal R' wave. RBBB generally does not interfere with the diagnosis of a myocardial infarction.

Left Bundle Branch Block (LBBB)

In LBBB, conduction through the left main bundle branch is blocked. Depolarization begins in the right ventricle and spreads slowly to the left. Because the left ventricle is much larger, its delayed depolarization dominates the ECG.

ECG Criteria for LBBB:

1. QRS Duration: ≥ 0.12 seconds.
2. Lead V1: Displays a broad, deep, monomorphic S wave (or a QS complex). There is no initial R wave because normal septal depolarization from left-to-right is abolished.
3. Leads I, aVL, and V6: Display a broad, tall, often notched R wave (sometimes described as an "M" pattern). There are no septal Q waves in these leads.
4. ST/T Waves: Secondary ST/T wave changes are present. ST elevation and upright T waves are typically seen in leads with deep S waves (V1-V3), while ST depression and T wave inversion occur in leads with tall R waves (I, V6).

Clinical Note: The presence of a new LBBB in the setting of acute chest pain is often treated as an ST-Elevation Myocardial Infarction (STEMI) equivalent because the abnormal depolarization of LBBB obscures the classic ST segment changes of acute ischemia. Modified Sgarbossa Criteria are required to diagnose MI in the presence of LBBB.

Wolff-Parkinson-White (WPW) Pattern

WPW is a pre-excitation syndrome caused by an accessory electrical pathway (the Bundle of Kent) that bypasses the AV node. This allows the electrical impulse to reach the ventricles prematurely, before the normal AV nodal delay completes, creating a "short-circuit" effect.

ECG Criteria for WPW Pattern:

1. Shortened PR Interval: < 0.12 seconds. Because the AV node is bypassed, there is no physiological delay to allow for ventricular filling.
2. Delta Wave: A slurred, slow upstroke of the initial portion of the QRS complex. This represents the early ventricular depolarization (pre-excitation) via the accessory pathway.
3. Widened QRS Complex: The combination of the Delta wave and the subsequent normal ventricular depolarization widens the total QRS duration to > 0.12 seconds.
4. Secondary ST/T Changes: ST segments and T waves are often directed oppositely to the major delta wave/QRS vector.

Patients with WPW pattern are at risk for life-threatening re-entrant tachycardias, notably Atrioventricular Reentrant Tachycardia (AVRT) and rapid conduction of Atrial Fibrillation. In the setting of Afib with WPW, administering standard AV-nodal blocking drugs (like Amiodarone or Diltiazem) can be fatal, as it forces all impulses down the accessory pathway, leading directly to Ventricular Fibrillation. Proper recognition is imperative.