A patient converts from normal sinus rhythm at 80 bpm to atrial fibrillation with a ventricular response at 166 bpm.Blood pressure is 162/74 mm Hg. Respiratory rate is 20 breaths per minute with normal chest expansion and clear lungs bilaterally.Intravenous heparin and Cardizem are given. The nurse caring for the patient understands that the main goal of treatment is what?