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Cardiac Arrhythmias for USMLE Step 1 & COMLEX-USA Level 1

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Mechanisms of Arrhythmias
  • Abnormal Automaticity:
    • Occurs when non-pacemaker cells spontaneously depolarize.
    • Example: Ectopic atrial or ventricular beats.
  • Triggered Activity:
    • Caused by afterdepolarizations (early or delayed) that trigger premature contractions.
    • Example: Torsades de pointes in long QT syndrome.
  • Re-entry Circuits:
    • Electrical impulses re-enter previously excited areas due to altered conduction pathways.
    • Example: Atrioventricular reentrant tachycardia (AVRT) in Wolff-Parkinson-White syndrome.
Types of Arrhythmias
  • Supraventricular Arrhythmias (Above the Ventricles):
    • Atrial Fibrillation (AF): Irregularly irregular rhythm with no distinct P waves.
    • Increases the risk of embolic stroke.
    • Atrial Flutter: “Sawtooth” pattern on ECG due to rapid atrial contractions.
    • Paroxysmal Supraventricular Tachycardia (PSVT): Sudden-onset tachycardia caused by re-entry at the AV node.
  • Ventricular Arrhythmias:
    • Premature Ventricular Contractions (PVCs): Early, wide QRS complex without a preceding P wave.
    • Ventricular Tachycardia (VT): Series of three or more consecutive PVCs. Can lead to ventricular fibrillation.
    • Ventricular Fibrillation (VF): Chaotic and uncoordinated electrical activity, requiring immediate defibrillation.
Bradyarrhythmias
  • Sinus Bradycardia:
    • Heart rate <60 bpm due to increased vagal tone or medications (e.g., beta-blockers).
  • Atrioventricular (AV) Blocks:
    • First-Degree AV Block: PR interval >200 ms with no missed beats.
    • Second-Degree AV Block:
    • Mobitz Type I (Wenckebach): Progressive PR lengthening followed by a dropped QRS.
    • Mobitz Type II: Sudden dropped QRS complex without PR prolongation.
    • Third-Degree AV Block: Complete dissociation between atrial and ventricular activity.
Key ECG Findings
  • Atrial Fibrillation: No P waves, irregular QRS complexes.
  • Atrial Flutter: “Sawtooth” atrial waves.
  • Ventricular Tachycardia: Wide QRS complexes, regular rhythm.
  • Ventricular Fibrillation: Irregular, chaotic pattern with no identifiable waves.
  • Torsades de Pointes: Polymorphic VT with twisting QRS complexes, linked to prolonged QT interval.
Treatment Overview
  • Rate Control vs. Rhythm Control:
    • In atrial fibrillation, initial treatment often focuses on rate control with beta-blockers or calcium channel blockers.
  • Antiarrhythmic Medications:
    • Class I (Sodium Channel Blockers): Quinidine, Lidocaine.
    • Class II (Beta-Blockers): Metoprolol, Esmolol.
    • Class III (Potassium Channel Blockers): Amiodarone, Sotalol.
    • Class IV (Calcium Channel Blockers): Verapamil, Diltiazem.
  • Electrical Therapy:
    • Defibrillation: Used for ventricular fibrillation or pulseless VT.
    • Cardioversion: Performed for hemodynamically unstable atrial fibrillation or flutter.
Key Takeaways
  • Supraventricular arrhythmias originate above the ventricles, while ventricular arrhythmias are more dangerous.
  • Re-entry circuits are the most common mechanism of arrhythmias.
  • Identifying arrhythmias on ECG is crucial for determining treatment strategies.