Atrial fibrillation: causes of new onset
Emergency medicine   Wikipedia  
Thyroid
Hypothermia
Embolism (P.E.)
Alcohol
Trauma (cardiac contusion)
Recent surgery (post CABG)
Ischemia
Atrial enlargement
Lone or idiopathic
Fever, anemia, high-output states
Infarct
Bad valves (mitral stenosis)
Stimulants (cocaine, theo, amphet, caffeine)

 Mnemonic:
THE ATRIAL FIBS