Atrial fibrillation

Atrial fibrillation is a heart rhythm disorder that begins in the atria, the two upper chambers, the ones that take in blood before passing it down. Every heartbeat starts with an electrical signal that tells the muscle when to squeeze; in atrial fibrillation that signal comes out wrong, so instead of one clear command the muscle gets a scramble of impulses. The result is that the atria contract irregularly, with no steady beat: it's the difference between a room where everyone claps together and a room where each person claps whenever they feel like it.

Atrial fibrillation carries a second risk: it makes it more likely that clots, solid plugs of blood, will form inside the heart. A clot like that can end up causing an ischemic stroke, meaning part of the brain stops getting the blood it needs, the way a blocked pipe leaves everything downstream of it dry.