You are assessing a 7-month-old infant who presents with listlessness, pallor, and increased work of breathing. The infant's mother tells you that the child was born 2 months premature and was in the neonatal intensive care unit for 3 weeks. She denies any recent vomiting, diarrhea, or fever. The infant's oxygen saturation is 89% and does not improve with supplemental oxygen. Her heart rate is rapid and weak and does not vary with activity. When you apply the cardiac monitor, you will MOST likely encounter a: