You are called to the neonatal intensive care unit (NICU) to intubate a term 2-day-old neonate with a diagnosis of tetralogy of Fallot. The team has initiated her on a prostaglandin E1 (PGE1) infusion and supplemental oxygen, due to persistent hypoxemia. With these measures she had been stable with oxygen saturations in the 90s. Recently the NICU has observed several sudden drops in oxygen saturations that have resolved with bag-mask ventilation. Complete blood count, cerebrospinal fluid, and urinalysis are normal. Of the following, what is the MOST likely etiology of the desaturation?