Atrial fibrillation or AF is the most common abnormal heart rhythm, and affects over 1 million people in the UK. In paroxysmal AF, the heart changes from a normal regular ‘sinus’ rhythm into AF and then goes back into sinus rhythm within 7 days, and often within 48 hours. Presyncope, or feeling faint (as opposed to fainting, for which the medical term is ‘syncope’) is a common symptom associated with AF, and indeed both a rapid and a slow heart rate can cause presyncope and syncope. There are many reasons for fainting and feeling faint, including abnormal heart rhythms, medication, stress or anxiety, ‘carotid sinus syncope’ (often associated with turning the head or wearing a tight collar and more commonly found in older patients), and even coughing or passing water. There is no clear reason why it should specifically be associated with going back into normal sinus rhythm, and indeed this is not routinely described by patients. However, it is possible that blood pressure drops in some patients as the heart goes back into normal sinus rhythm, leading to presyncope because of lack of blood to the brain.