Atrial Fibrillation Pathophysiology
Overview
Atrial Fibrillation (AF) is the most common sustained cardiac arrhythmia, characterised by chaotic, disorganised atrial electrical activity (350-600 impulses/min) with irregularly irregular ventricular response. It is a major thromboembolic risk due to blood stasis in the left atrial appendage (LAA).
Pathophysiology
- Trigger: Ectopic electrical foci most commonly arising from the pulmonary vein ostia (80-90% of paroxysmal AF cases) fire rapid impulses into the left atrium.
- Perpetuating substrate: Atrial structural remodelling (fibrosis, dilatation), electrical remodelling (shortened refractory periods), and autonomic dysregulation sustain the chaotic activity.
- AF begets AF: Sustained AF causes further atrial remodelling, increasing the likelihood of persistence.
Cardiac Imaging in AF
- Echocardiography: Assesses LA size (dilatation in chronic AF), LV function, valvular disease (especially mitral stenosis), and detects LAA thrombus (TOE far superior to TTE).
- CT Coronary Angiography / Cardiac CT: Pre-ablation mapping of pulmonary vein anatomy — number, shape, and ostial diameter are planned before catheter ablation.
- Nuclear (MPS): Evaluates coexisting ischaemic heart disease.
WarningLeft Atrial Appendage Thrombus
Stagnant blood in the LAA during AF is the primary source of cardioembolic stroke. TOE (Transoesophageal Echo) is the gold standard for LAA thrombus detection before cardioversion. CT demonstrates LAA filling defects but requires careful technique (early/late phase imaging) to distinguish thrombus from flow artifact.
High Yield Facts
LightbulbFRCR / MD Prep Pearl
AF on ECG: absent P waves, irregularly irregular QRS, fibrillatory baseline. CHA2DS2-VASc score determines stroke risk and anticoagulation indication. Catheter ablation targets pulmonary vein isolation (PVI). Watchman device is a LAA occlusion option for patients contraindicated to anticoagulation.