The clinician diagnoses atrial fibrillation through systematic evaluation of rhythm characteristics, underlying etiology, and classification for treatment planning. ECG diagnostic criteria: irregularly irregular rhythm (hallmark), absence of organized P waves (replaced by fibrillatory waves), variable R-R intervals, and narrow QRS complexes (unless rate-related aberrancy or pre-existing bundle branch block). AF classification guides management: paroxysmal (self-terminating within 7 days), persistent (lasting greater than 7 days or requiring cardioversion), long-standing persistent (continuous greater than 12 months when rhythm control strategy is pursued), and permanent (accepted AF where rhythm control is no longer pursued). The clinician evaluates for reversible causes and precipitants: thyrotoxicosis (check TSH in all new AF), alcohol (holiday heart syndrome), pulmonary embolism, acute illness/sepsis, pericarditis, postoperative state, obstructive sleep apnea. Initial workup: 12-lead ECG, echocardiogram (assess LV function, valvular disease, left atrial size -- enlarged LA predicts lower success of rhythm control), TSH, CBC, BMP, liver function tests. The clinician determines rate versus rhythm control strategy: rate control (lenient target resting heart rate less than 110 bpm per RACE II trial, or strict target less than 80 bpm if symptomatic;...