A 5-mоnth-оld infаnt with а histоry of congenitаl heart problems is brought to the emergency room for worsening breathing difficulty and poor feeding. On assessment, the infant is cyanotic, lethargic, and has an oxygen saturation of 82%. The heart rate, which was initially 155 beats/min, is now 58 beats/min. How should the nurse interpret this change in heart rate?