The atria add about 10-20% to heart function by adding volume to the ventricles right before they contract. Atrial fibrillation, therefore, decreases heart function by about 15-20%. It can decrease heart function much more than that, though. This happens when the ventricular response gets too high. Ventricular responses should be less than 100 bpm (beats per minute), and not cause symptoms such as dizziness. The AV node (atrioventricular node) is the gatekeeper between the atria and the ventricles. If the AV response rate is too rapid, and the patient is having PAROXYSMAL (occasional episodes of) atrial fib, then we can use what's called "the pill in the pocket" technique. We prescribe a beta-blocker. It shows the ventricular response rate to atrial fib. And it's not something the patient has to take every day.