The most common cause of atrial fibrillation/flutter is hypertension. Age is the second most common cause. Obstructive sleep apnea is present in about 40% of patients with atrial fibrillation/flutter, however the exact proportion that it causes is not well defined. There are multiple other causes of atrial fibrillation/flutter that can be remembered with the below pneumonic “PIRATES”:

Pulmonary embolism, pericarditis
Ischemia, intravenous line (central line advanced too far into the right atrium)
Rheumatic valvular heart disease (specifically mitral valve stenosis)
Alcohol, anemia
Thyroid (hyperthyroidism)
Elevated blood pressure (hypertension)
Sleep apnea, sepsis, surgery