🩺 Advanced EKG Interpretation: Bundle Branch Blocks & Supraventricular Tachycardia (SVT)

Survival Guide: Advanced EKG Interpretation

Bundle Branch Blocks (BBB)

• The Bottom Line

  • Delayed ventricular depolarization due to a block in one of the main bundle branches, leading to a widened QRS complex (≥ 0.12s) and characteristic morphology.

• Key Mechanisms

  • Conduction block within the Right Bundle Branch (RBBB) or Left Bundle Branch (LBBB).
  • RBBB: Right ventricle depolarizes *after* the left, via muscle-to-muscle conduction from the left ventricle.
  • LBBB: Left ventricle depolarizes *after* the right, via muscle-to-muscle conduction from the right ventricle.
  • Common Causes: Ischemia, infarction, fibrosis, cardiomyopathy, valvular heart disease, congenital heart disease, hyperkalemia (LBBB), pulmonary embolism (RBBB).

• EKG Criteria

  • QRS Duration: ≥ 0.12 seconds (3 small squares).
  • Right Bundle Branch Block (RBBB) Specifics:
    • V1/V2: RSR' ("M" pattern), rsR', or broad, notched R wave.
    • V5/V6/I/aVL: Broad, slurred S wave ("W" pattern).
    • ST-T wave changes: Discordant to QRS (ST depression and T-wave inversion in V1-V3 is common and expected; it is a secondary repolarization abnormality).
  • Left Bundle Branch Block (LBBB) Specifics:
    • V1/V2: Broad, deep S wave or rS pattern.
    • V5/V6/I/aVL: Broad, notched or slurred R wave ("M" pattern). Q waves are absent (unless pre-existing MI).
    • ST-T wave changes: Discordant to QRS (ST elevation and upright T-wave in V1-V3; ST depression and T-wave inversion in V5, V6, I, aVL is common and expected).

• Clinical Significance

  • New LBBB with symptoms of ischemia: Treat as STEMI equivalent until proven otherwise. Requires emergent cardiology evaluation.
  • RBBB: Can be benign, but may indicate underlying heart disease or pulmonary pathology (e.g., pulmonary embolism).
  • Bifascicular Block: RBBB + Left Anterior Fascicular Block (LAFB) or RBBB + Left Posterior Fascicular Block (LPFB). Increases risk of complete heart block, especially if symptomatic.
  • Trifascicular Block: Inferred from bifascicular block with a prolonged PR interval. Represents disease in all three fascicles and carries a high risk of complete heart block.

• Red Flags / Warnings

  • New LBBB with acute chest pain/symptoms: Consider acute myocardial infarction. Use Sgarbossa or modified Sgarbossa criteria for superimposed ischemia.
  • Bifascicular/Trifascicular Block with syncope, presyncope, or dizziness: High risk for complete heart block; requires urgent evaluation for permanent pacing.
  • Any BBB in the setting of acute symptoms: Always rule out acute coronary syndrome, pulmonary embolism, hyperkalemia, or drug toxicities.

Supraventricular Tachycardia (SVT)

• The Bottom Line

  • A rapid heart rhythm originating above the ventricles (atrial or AV nodal), characterized by a narrow QRS complex (<0.12s) unless aberrancy or pre-existing BBB is present. Typically regular and paroxysmal.

• Key Mechanisms

  • Re-entry Circuits (Most Common):
    • AVNRT (AV Nodal Reentrant Tachycardia): Re-entry within the AV node (slow/fast pathways). P waves often buried within QRS, or retrograde (pseudo-r' in V1, pseudo-S in inferior leads).
    • AVRT (AV Reentrant Tachycardia): Re-entry involving an accessory pathway (e.g., Wolff-Parkinson-White Syndrome). Can be orthodromic (narrow QRS, common) or antidromic (wide QRS, less common).
  • Increased Automaticity / Triggered Activity:
    • Atrial Tachycardia (AT): Focal origin in the atria. P waves have an abnormal morphology and axis, followed by a QRS.
    • Sinus Tachycardia: Physiologic response, typically gradual onset/offset, normal P wave morphology. Usually distinguished by identifiable cause and P waves.

• EKG Criteria

  • Rate: Usually 150-250 bpm (can be 100-300 bpm, depending on etiology).
  • QRS Duration: < 0.12 seconds (narrow QRS) in the absence of aberrancy or pre-existing BBB.
  • Rhythm: Usually regular, though can be irregular in some atrial tachycardias (e.g., MAT) or AF/AFlutter with varying block.
  • P Waves:
    • Often absent, obscured, or retrograde (inverted in inferior leads, upright in aVR).
    • May have abnormal morphology/axis if focal atrial tachycardia.
    • P:QRS ratio may be 1:1, or P waves may appear before or after the QRS.
  • Initiation/Termination: Often abrupt ("on/off" phenomenon).

• Differential Diagnosis (for Narrow Complex Tachycardia)

  • Sinus Tachycardia (gradual onset/offset, normal P waves).
  • AVNRT.
  • AVRT (Orthodromic).
  • Atrial Tachycardia.
  • Atrial Flutter with regular conduction (e.g., 2:1 block often presenting at ~150 bpm).
  • Atrial Fibrillation with rapid ventricular response (irregularly irregular, no distinct P waves).

• Red Flags / Warnings

  • Hemodynamic Instability: Hypotension, altered mental status, signs of shock, acute heart failure, ischemic chest pain. Requires immediate synchronized cardioversion.
  • Wide Complex Tachycardia (WCT) in presumed SVT: Always assume Ventricular Tachycardia (VT) until proven otherwise. SVT with aberrancy or pre-existing BBB can mimic VT but is less common. Use Brugada or Verakey criteria for differentiation.
  • WPW with Atrial Fibrillation/Flutter: This is a critical emergency. High risk of degeneration to Ventricular Fibrillation (VF) due to rapid conduction over the accessory pathway. AV nodal blocking agents (Adenosine, Calcium Channel Blockers, Beta Blockers, Digoxin) are absolutely contraindicated as they can increase conduction down the accessory pathway. Treat with Procainamide or synchronized cardioversion if unstable.
  • Recurrent symptomatic SVT: May warrant electrophysiology study and catheter ablation for definitive management.