Atrial Fibrillation

The most common sustained dysrhythmia hides its biggest threat not in the heart rate itself, but in the atria — where stagnant blood forms clots that travel straight to the brain.

Atrial fibrillation (AFib) occurs when multiple ectopic foci in the atria fire chaotically at 350–600 impulses per minute, producing a quivering instead of a coordinated contraction. The AV node filters these impulses irregularly, so the hallmark ECG finding is an irregularly irregular rhythm with no identifiable P waves — just a wavy, fibrillatory baseline. The ventricular rate is unpredictable and often rapid (100–180 bpm uncontrolled). Without effective atrial contraction, the atria lose roughly 25–30% of their contribution to cardiac output (the "atrial kick"). Blood pools in the atria — especially the left atrial appendage — creating a high risk for thrombus formation and embolic stroke. This is why stroke prevention with anticoagulation is the cornerstone of AFib management, often more critical than rate control alone. CHA₂DS₂-VASc scoring guides anticoagulation decisions. Rate control typically targets a resting ventricular rate below 100 bpm, though a lenient target below 110 bpm may be acceptable in select patients. Rhythm control with cardioversion may be pursued, but if AFib has lasted 48 hours or longer, anticoagulation for at least 3 weeks (or a transesophageal echocardiogram ruling out atrial thrombus) is required before cardioversion to prevent dislodging a clot.

Key Distinctions

Don't confuse AFib (irregularly irregular, no P waves) with atrial flutter (regular sawtooth pattern, organized atrial activity). Students often prioritize heart rate over stroke risk — but a patient in chronic AFib at a controlled rate still needs anticoagulation. New-onset AFib with hemodynamic instability (hypotension, chest pain, altered mental status) warrants synchronized cardioversion, not medication trials.

Clinical Pearl

No P waves + irregularly irregular = AFib. But the real danger isn't the rhythm — it's the clot. Think: chaotic atria make clots; clots make strokes.