These medications slow the heart rate by retarding conduction of the electrical discharges through the AV node. These medications, however, do not usually convert atrial fibrillation back into a normal rhythm. Other drugs or treatments are necessary to achieve a normal heart rhythm.

Benefits of controlling the rate. In patients with rapid ventricle contractions as a result of atrial fibrillation, slowing the rate of ventricular contractions improves the heart's efficiency in delivering blood (by allowing more time between contractions for the ventricles to fill with blood) and relieves the symptoms of inadequate flow of blood -- dizziness, weakness, and shortness of breath.

With chronic, sustained atrial fibrillation, doctors may decide to leave some patients in atrial fibrillation provided that their heart rates are under control, the output of blood from the ventricles is adequate, and their blood is adequately thinned to prevent strokes. This form of treatment is called rate control therapy (see below).

Limitations of medications for controlling the heart rate. In patients with diseased AV nodes, ventricular contractions may be slower than in patients who have normal AV nodes. Moreover, some elderly patients with atrial fibrillation are extremely sensitive to medications that slow the rate of ventricular contractions, usually because of a diseased AV node. In these patients, the heart rate can become dangerously slow with small doses of medications to slow the heart. This condition is referred to as tachycardia-bradycardia syndrome, or "sick sinus syndrome." Patients with tachycardia-bradycardia syndrome need medications to control the fast heart rate and a pacemaker to provide a minimum safe heart rate.

Medications used in slowing atrial fibrillation generally cannot convert atrial fibrillation to a normal rhythm. Therefore these patients are at risk for the formation of blood clots in the heart and strokes and will need prolonged blood thinning with anticoagulants like warfarin (Coumadin).