A 50-year-old man with chronic obstructive pulmonary disease and obstructive sleep apnea presents to the emergen room with the acute onset of shortness of breath and palpitations. On physical examination, the heart rhythm is described as irregularly irregular. What is the most likely cause of this arrhythmia? Atrial flutter with 2:1 conduction. Atrial tachycardia. Ventricular tachycardia. Atrial fibrillation. Sinus tachycardia.