How atrial fibrillation is tested on Step 1

Vignettes give an irregularly irregular pulse — often after a binge, with hyperthyroidism, or with mitral stenosis — and ask about stroke prevention, a drug mechanism, or why cardioversion requires anticoagulation.

workspace_premium The danger is the clot

Most AF deaths and disability come from embolic stroke, not the rhythm itself. Anticoagulation decisions follow stroke risk, not whether the rhythm is controlled.

Mechanism and consequences

Ectopic firing from muscle sleeves in the pulmonary veins initiates AF; a dilated, fibrotic atrium maintains it. Over time, AF itself remodels the atrium ("AF begets AF").

PulmonaryveinsEctopictriggersChaoticatria300–600impulses/minNo atrialkick↓ VentricularfillingLAappendageStasis →thrombusEmbolicstrokeOr limb,bowelischemia
AF consequences. The AV node filters impulses irregularly, so the ventricular rate is irregularly irregular and often fast.
AF risk factorsCardiacHTN, HF, mitraldisease, CADMetabolicHyperthyroidism,obesity, diabetesLungSleep apnea,PE, COPDToxicAlcohol binge,stimulantsOtherAge, surgery,sepsis
Risk factors. New AF warrants a TSH and an echocardiogram.

Stroke prevention

The CHA₂DS₂-VASc score estimates annual stroke risk in nonvalvular AF.

LetterRisk factorPoints
CCongestive heart failure1
HHypertension1
A₂Age ≥ 752
DDiabetes1
S₂Prior stroke, TIA, or thromboembolism2
VVascular disease (MI, PAD, aortic plaque)1
AAge 65–741
ScSex category (female)1

Anticoagulate at a score of 2 or more in men or 3 or more in women (2023 ACC/AHA). The 2024 ESC guideline uses CHA₂DS₂-VA, which drops the sex point. Aspirin is not an adequate substitute.

DOAC (apixaban, rivaroxaban)Preferred for nonvalvular AFNo INR monitoringLess intracranial bleedingDose by kidney functionNot for mechanical valvesWarfarinMechanical heart valvesModerate-to-severe mitral stenosisINR target 2–3Many interactionsUsable in end-stage kidney diseasevs
Choice of anticoagulant. Left atrial appendage occlusion is an option when long-term anticoagulation is contraindicated.

Rate vs rhythm control

Rate control slows the ventricles; rhythm control restores sinus rhythm. Both require stroke prevention.

StrategyOptionsNotes
Unstable (shock, ischemia, pulmonary edema)Synchronized cardioversionImmediate
Rate controlMetoprolol, diltiazem, verapamil; digoxin adjunctAvoid non-DHP CCBs in HFrEF; target resting rate < 110
Rhythm control — drugsAmiodarone, flecainide or propafenone (no structural disease), sotalol, dofetilideMonitor QT and toxicity
Rhythm control — proceduresElectrical cardioversion; pulmonary vein isolation (ablation)Ablation increasingly first-line in younger or HF patients

Before elective cardioversion of AF lasting 48 hours or more, anticoagulate for 3 weeks or exclude LA thrombus by TEE, then continue for at least 4 weeks after.

Clinical application

Cardioversion timing is a frequent question.

functions Worked example — cardioversion and clots

A 67-year-old with hypertension has palpitations for an unknown duration and is in AF at 128/min, hemodynamically stable. A trainee proposes immediate electrical cardioversion. What must happen first, and why?

  1. AF of unknown duration is treated as lasting 48 hours or more.
  2. A thrombus may already sit in the left atrial appendage.
  3. Restoring organized atrial contraction can dislodge it, causing stroke.
  4. Meanwhile, control the rate and start anticoagulation.

Answer. Either 3 weeks of therapeutic anticoagulation or a TEE excluding thrombus before cardioversion, then at least 4 weeks of anticoagulation afterward. His CHA₂DS₂-VASc of 2 (age, hypertension) also warrants long-term anticoagulation.