A 58-year-old man with a history of hypertension and recent myocardial infarction presents to the emergency department with palpitations and dyspnea. ECG shows a regular narrow-complex tachycardia at 180 bpm with visible P waves buried in the T wave of the preceding beat. Blood pressure is 110/70 mmHg, and he is conscious and alert. After failed vagal maneuvers, intravenous adenosine 6 mg is given, which terminates the arrhythmia within seconds. Which of the following best describes the mechanism of action of adenosine in terminating this arrhythmia?