Cardiac Arrhythmias for USMLE Step 3 & COMLEX-USA Level 3
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Mechanisms of Arrhythmias
- Abnormal Automaticity:
- Non-pacemaker cells generate spontaneous action potentials, leading to ectopic impulses.
- Example: Atrial or ventricular premature beats.
- Triggered Activity:
- Early or delayed afterdepolarizations initiate abnormal contractions.
- Example: Torsades de Pointes, often associated with prolonged QT interval.
- Re-Entry Circuits:
- Impulses re-enter previously activated tissues due to altered conduction pathways, causing tachyarrhythmias.
- Example: AV nodal re-entry in Paroxysmal Supraventricular Tachycardia (PSVT).
Supraventricular Arrhythmias
- Atrial Fibrillation (AF):
- Irregularly irregular rhythm with absent P waves.
- Increases thromboembolic risk (managed with anticoagulation, typically DOACs).
- Rate Control: Beta-blockers or calcium channel blockers.
- Rhythm Control: Antiarrhythmics (amiodarone) or electrical cardioversion.
- Atrial Flutter:
- "Sawtooth" pattern on ECG due to rapid atrial contractions.
- Treatment: Similar to AF, with a focus on rate control and possible cardioversion.
- Paroxysmal Supraventricular Tachycardia (PSVT):
- Sudden-onset tachycardia due to AV nodal re-entry.
- Acute management: Vagal maneuvers, adenosine; long-term: beta-blockers or ablation.
Ventricular Arrhythmias
- Premature Ventricular Contractions (PVCs):
- Isolated, wide QRS complexes without preceding P waves.
- Often benign but frequent PVCs may require Holter monitoring or treatment with beta-blockers.
- Ventricular Tachycardia (VT):
- Series of wide QRS complexes (>100 bpm); can be sustained (>30 sec) or non-sustained.
- Management: Antiarrhythmics (amiodarone) and cardioversion for hemodynamically unstable VT.
- Consider implantable cardioverter-defibrillator (ICD) for recurrent VT or structural heart disease.
- Ventricular Fibrillation (VF):
- Disorganized, chaotic ventricular activity causing loss of pulse.
- Immediate defibrillation required for survival.
Bradyarrhythmias
- Sinus Bradycardia:
- HR <60 bpm, typically due to medications or increased vagal tone.
- Symptomatic cases managed with atropine; pacing may be needed if refractory.
- Atrioventricular (AV) Blocks:
- First-Degree Block: PR interval >200 ms, no dropped beats.
- Second-Degree Block:
- Mobitz I (Wenckebach): Progressive PR prolongation with dropped QRS complexes.
- Mobitz II: Sudden dropped QRS without prior PR lengthening, often requiring pacemaker.
- Third-Degree Block: Complete dissociation between atria and ventricles; treated with pacemaker.
Management of Life-Threatening Arrhythmias
- Advanced Cardiac Life Support (ACLS):
- VF and pulseless VT: Immediate defibrillation and epinephrine.
- Bradycardia with hypotension: Atropine, followed by transcutaneous pacing if needed.
- Torsades de Pointes:
- Polymorphic VT linked to prolonged QT.
- Treated with magnesium sulfate and defibrillation if unstable.
- Antiarrhythmic Drug Classes:
- Class I: Sodium channel blockers (e.g., procainamide).
- Class II: Beta-blockers (e.g., metoprolol).
- Class III: Potassium channel blockers (e.g., amiodarone, sotalol).
- Class IV: Calcium channel blockers (e.g., verapamil).
Key Points
- Atrial Fibrillation: Commonly managed with rate control and anticoagulation to reduce stroke risk.
- Ventricular Tachycardia and Fibrillation: Require immediate cardioversion or defibrillation.
- Torsades de Pointes: Linked to prolonged QT and managed with magnesium sulfate.
- Bradyarrhythmias: Advanced AV blocks often need permanent pacemaker placement.
- ACLS Protocols: Critical in managing VF and pulseless VT to improve survival outcomes.