Tetralogy of Fallot — formative SAQs

SAQ 1 (10 marks)

A four-month-old girl is brought to the emergency department after an episode of sudden cyanosis and rapid breathing that began while feeding. She is alert but dusky, saturating 74 per cent in air, with a harsh systolic ejection murmur at the left upper sternal border and a single second heart sound. Her mother reports she has been "going bluer with feeds" for several weeks. The emergency nurse notes that the murmur has become quieter over the last ten minutes while the cyanosis has deepened. ^{[1] [2]}

1. Give the immediate emergency management of this infant, explaining the physiological rationale for each step. (4) ^[1]
2. Outline the diagnostic strategy that confirms the underlying lesion and rules out the duct-dependent variants, and state the single most important investigation. (3) ^{[1] [2]}
3. Describe the definitive surgical management and its timing, and explain why a transannular patch has lifelong consequences. (3) ^{[5] [6]}

Model answer — SAQ 1

(1) Emergency management (4). This is a hypercyanotic tet spell and must be treated immediately, not observed. Place her in the knee–chest position, which raises systemic vascular resistance and restores venous return to the right heart, reducing the right-to-left shunt. Give 100 per cent oxygen and administer morphine 0.1 mg/kg intravenously to calm her and blunt infundibular spasm. Give an intravenous fluid bolus of 10 mL/kg isotonic saline to increase preload. If the spell continues, give phenylephrine 5 micrograms/kg intravenously to raise systemic vascular resistance and force blood across the outflow tract, or a short-acting beta-blocker such as esmolol to slow the heart and lengthen diastolic filling of the right ventricle. Correct any acidosis with sodium bicarbonate. A spell that does not respond needs an urgent surgical (modified Blalock–Taussig shunt) or transcatheter (right ventricular outflow tract stent) intervention. ^[1]

(2) Diagnostic strategy (3). Echocardiography is the single most important investigation and is diagnostic — it shows the malalignment ventricular septal defect, the overriding aorta, the site and severity of the right ventricular outflow tract obstruction, and the pulmonary arteries, and it distinguishes classic TOF from pulmonary atresia (which is duct-dependent and needs prostaglandin). Pulse oximetry, a chest X-ray (boot-shaped heart, reduced pulmonary markings, possible right aortic arch) and an ECG (right-axis deviation, right ventricular hypertrophy) support the diagnosis but do not replace the echo. If pulmonary atresia is possible, start prostaglandin before the echo rather than wait. ^{[1] [2]}

(3) Definitive surgery and the transannular patch (3). Definitive management is complete primary repair, typically at three to six months of age — closure of the ventricular septal defect with a patch and relief of the right ventricular outflow tract obstruction by infundibular resection. A transannular patch enlarges the outflow but incises the pulmonary valve annulus, leaving the patient with free pulmonary regurgitation: the lifelong consequence is progressive right ventricular dilation, reduced exercise tolerance, a substrate for atrial and ventricular arrhythmia, and the eventual need for pulmonary valve replacement timed by cardiac MRI. This is why modern practice favours valve-sparing repair where the native valve will accommodate it. ^{[5] [6]}