A 72-year-old woman with chronic atrial fibrillation and heart failure (EF 35%) is admitted with rapid ventricular response (HR 140 bpm). Intravenous digoxin is initiated for rate control. Over the next 6 hours, her heart rate slows to 90 bpm. However, she develops nausea, visual disturbances (yellow-green halos around lights), and her serum potassium is 3.2 mEq/L. ECG shows a shortened QT interval and a sagging ST segment. Which of the following best explains the mechanism by which digoxin controls the ventricular rate in atrial fibrillation?