## Diagnosis of Atrial Fibrillation: Investigation of Choice ### Why 12-Lead ECG is the Gold Standard **Key Point:** The 12-lead electrocardiogram is the investigation of choice for confirming atrial fibrillation in any patient with suspected AF, whether symptomatic or asymptomatic. **High-Yield:** A single 12-lead ECG has diagnostic sensitivity and specificity approaching 100% for AF when the rhythm is present during the recording. It is: - Immediately available - Non-invasive - Inexpensive - Diagnostic in acute presentation - Provides additional information (ventricular rate, QRS duration, ischemic changes) ### ECG Findings in Atrial Fibrillation | Feature | Finding | |---------|----------| | **P waves** | Absent; replaced by fine or coarse fibrillatory waves (f waves) | | **Baseline** | Irregular, undulating baseline | | **QRS complex** | Narrow (< 120 ms) unless aberrant conduction or pre-existing BBB | | **Ventricular rate** | Irregular (irregularly irregular) | | **PR interval** | Not measurable (no P waves) | **Clinical Pearl:** The "irregularly irregular" pulse on examination is a clinical clue, but ECG confirmation is mandatory before initiating anticoagulation or rate/rhythm control therapy. ### Role of Other Investigations - **Transthoracic echocardiogram:** Assesses structural causes (LV function, atrial size, valvular disease) and guides prognosis, but does NOT diagnose AF — it is confirmatory and prognostic, not diagnostic. - **Holter monitor:** Used when AF is paroxysmal and not captured on a single ECG; not first-line for symptomatic AF with clear clinical presentation. - **Chest X-ray:** May show cardiomegaly or pulmonary edema but is non-specific and not diagnostic for AF. **Mnemonic:** **ECG-FIRST** — ECG is the gold standard for rhythm diagnosis; F = Fast, I = Irregular, R = Rhythm, S = Screening, T = Test of choice. [cite:Harrison 21e Ch 226]
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