A 7-month-old boy is brought to the emergency department by his parents because of a sudde… — USMLE Step 2 CK

A 7-month-old boy is brought to the emergency department by his parents because of a sudden episode of deep blue discoloration of his lips and face. The parents report that he was crying vigorously when he suddenly became intensely cyanotic, tachypneic, and irritable. He has a known history of a congenital cardiac defect for which he is awaiting surgical repair. On arrival, the infant is agitated and visibly cyanotic. Vital signs show a heart rate of $165 \text{ /min}$ , respiratory rate of $55 \text{ /min}$ , and oxygen saturation of $68\%$ on room air. Auscultation reveals a harsh, crescendo-decrescendo systolic murmur over the left mid-to-upper sternal border. The physician immediately places the infant in a knee-chest position and administers $100\%$ oxygen via a non-rebreather mask, but the infant's agitation and cyanosis persist. Which of the following is the most appropriate next step in pharmacologic management?

Intravenous morphineAnswer
B
Inhaled albuterol
C
Intravenous furosemide
D
Intravenous ceftriaxone
E
Immediate endotracheal intubation

Answer

Intravenous morphine

The correct answer is intravenous morphine. This patient is experiencing a hypercyanotic ('tet') spell, a life-threatening complication of Tetralogy of Fallot characterized by severe dynamic right ventricular outflow tract (RVOT) obstruction. The resulting increase in right-sided pressures forces deoxygenated blood across the ventricular septal defect (VSD) into the systemic circulation (creating a massive right-to-left shunt). Initial management includes placing the child in a knee-chest position (which mechanically kinks the femoral arteries to increase systemic vascular resistance) and providing supplemental oxygen. When these physical measures fail, intravenous morphine is the most appropriate next step. Morphine acts to sedate the infant, decreasing agitation and dampening the sympathetic nervous system drive. This relaxes the infundibular spasm, reduces the right-to-left shunt, and improves pulmonary blood flow.

Step-by-Step Solution

Identify the clinical presentation

The patient is experiencing a hypercyanotic ('tet') spell due to Tetralogy of Fallot.

Sudden cyanosis during crying in an infant with a known cardiac defect and a harsh systolic murmur indicates dynamic right ventricular outflow tract obstruction.

Evaluate the initial interventions

Knee-chest positioning and oxygen were administered but were insufficient to break the spell.

Initial mechanical steps aim to increase systemic vascular resistance and improve oxygenation, but pharmacologic intervention is now required.

Select the appropriate pharmacologic therapy

Intravenous morphine is indicated.

Morphine calms the infant, reduces sympathetic tone, and relaxes the infundibulum, thereby decreasing the right-to-left shunt.

Key Concept

The management of hypercyanotic (tet) spells in Tetralogy of Fallot involves measures to increase systemic vascular resistance (knee-chest position) and decrease pulmonary vascular resistance and infundibular spasm (oxygen, morphine, intravenous fluids, beta-blockers).

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