A 6-month-old infant with tetralogy of Fallot is in the pediatric ICU. The infant suddenly develops severe cyanosis and tachypnea. Which intervention should the nurse perform first?

Place the infant in the knee-chest position. This is the highest priority action. If the infant is too large or uncooperative, bring the knees up to the chest. This position increases systemic vascular resistance (SVR), which reduces the right-to-left shunt and forces more blood through the pulmonary artery for oxygenation.

Minimize crying and agitation, as this can worsen the infundibular spasm and hypoxia. Use a calm, reassuring approach, swaddling, and a pacifier if appropriate. Keep the parent/caregiver present to soothe the infant if their presence is calming.

Apply 100% oxygen via a face mask. While not the primary fix for the shunt, it maximizes the oxygen content of the blood that does reach the lungs. Avoid nasal cannula as it may agitate the infant and delivers a less reliable FiO2.

If the spell persists, administer Morphine sulfate (0.1-0.2 mg/kg IV/IM/SC) as prescribed. Morphine suppresses the respiratory center, which can reduce hyperpnea, and has a sedative effect that may help break the cycle of spasm and hypoxia. It also has a mild venodilatory effect.

Administer a bolus of isotonic crystalloid fluids (e.g., 10-20 mL/kg normal saline) to increase preload and SVR. If severe acidosis is present, administer Sodium bicarbonate (1 mEq/kg IV) as prescribed to correct metabolic acidosis, which is a potent pulmonary vasoconstrictor and can perpetuate the spell.

If the above measures fail, prepare for advanced interventions. This includes Esmolol or Propranolol to relax the infundibulum, or Phenylephrine to dramatically increase SVR. Emergency intubation and general anesthesia are the final steps to control ventilation and reduce oxygen consumption.