A 6-week-old infant presents with sudden severe cyanosis, hyperpnea, and irritability. Cardiac examination reveals absent pulmonary component of S2, and a right ventricular heave. Echocardiography shows tetralogy of Fallot. What precipitates these 'tet spells' at this age?

• A Infective endocarditis on the pulmonary valve
• B Spasm of the infundibular (subpulmonary) muscle, reducing pulmonary blood flow ✓
• C Paradoxical embolism through the VSD during Valsalva
• D Sudden increase in pulmonary vascular resistance

Explanation

Hypercyanotic (tet) spells are caused by dynamic infundibular (RVOT) spasm that acutely increases right ventricular outflow tract obstruction, diverting deoxygenated blood through the VSD into the aorta. This creates a vicious cycle: hypoxia→acidosis→sympathetic activation→more spasm→more hypoxia. Immediate management includes knee-chest positioning, morphine, and propranolol to break the spasm. Infective endocarditis does not typically present this acutely in early infancy.

Reference: Ghai Essential Pediatrics, 10th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.