Question

An $11$-month-old boy is rushed to the emergency department due to a sudden episode of severe cyanosis and unresponsiveness. The parents report that the child became extremely agitated and inconsolable during a blood draw at a local clinic, after which his lips and fingers turned deep blue. He has a history of an uncorrected congenital heart defect for which he is awaiting surgical repair. On arrival, the child is lethargic. Vital signs show a heart rate of $175/\text{min}$, respiratory rate of $55/\text{min}$, and oxygen saturation of $65\%$ on room air. Physical examination reveals a harsh, grade 4/6 systolic ejection murmur at the left mid-to-upper sternal border and a single S2. The emergency physician immediately places the infant in a knee-chest position and applies $100\%$ supplemental oxygen via a non-rebreather mask. Intravenous morphine and a normal saline bolus are administered. Ten minutes later, the infant's oxygen saturation remains at $68\%$. Which of the following is the most appropriate next pharmacologic intervention and its primary physiologic mechanism in this patient?