Does atrial fibrillation cause fainting?

Atrial fibrillation makes the upper chambers of the heart beat irregularly. The heart may then pump less efficiently, especially when the heartbeat is very fast or changes suddenly. This can cause light-headedness, weakness, breathlessness or a brief loss of consciousness. Some people notice palpitations before they faint, while others do not feel an obvious warning.

Fainting may instead relate to dehydration, a temporary drop in blood pressure, a medication effect, a blood sugar problem or another heart-rhythm issue. The timing of the episode, what you were doing, your recovery and any associated symptoms help a clinician identify the likely cause.

If you feel faint, lie flat with your legs raised if possible, or sit with your head lowered. Avoid driving, swimming alone, climbing or operating machinery until you have been assessed. Arrange a medical appointment promptly, particularly if you have known atrial fibrillation, take heart or blood-pressure medicines, or fainted without warning.

Record what happened: your position, activity, warning symptoms, heartbeat sensations, possible triggers, injury, how long you were unresponsive and how quickly you recovered. A clinician may check your blood pressure and heart rhythm and may arrange an electrocardiogram or other monitoring.

Seek emergency help if fainting happens with chest pain, severe breathlessness, a sustained racing or irregular heartbeat, a severe headache, new weakness, trouble speaking, confusion or a serious injury. Get urgent assessment if you faint during exercise, while lying down, repeatedly, or without any warning. Do not give food or drink to someone who is unconscious; place them on their side if they are breathing and seek emergency help.

Ask whether your fainting could be related to your heart rhythm, blood pressure or medicines, and whether you need an electrocardiogram or a period of rhythm monitoring. Also ask which symptoms should prompt emergency help and whether you should change any activity while the cause is being assessed.