A 10-year-old boy is brought to the clinic by his parents due to chronic fatigue, shortness of breath, and failure to gain weight. The child was recently adopted from an orphanage in Asia. He weighs 20 kg (44.2 lb). Vital signs are within normal limits. Physical examination reveals a loud systolic murmur. Echocardiogram shows pulmonary stenosis and subaortic ventricular septal defect with deviation of the origin of the aorta to the right. The mother describes occasional episodes of severe dyspnea and cyanosis, during which the child quickly assumes a squatting position. Which of the following mechanisms during squatting relieves this child's symptoms?
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This child has Tetralogy of Fallot (ToF) with ventricular septal defect, overriding aorta, pulmonic stenosis, and right ventricular hypertrophy. The pulmonic stenosis and overriding aorta cause a relatively low ratio of systemic vascular resistance (SVR) to pulmonary vascular resistance (PVR). This low SVR:PVR ratio allows the deoxygenated right ventricular output to take the low-resistance route to the systemic circulation, leading to acute hypoxemia ("Tet spell"). Tet spells are most common in infancy but can occur at any age in children with unrepaired ToF. The mechanism of Tet spells has not been clearly defined but may be caused by acute changes of the infundibular septum leading to more right ventricular outflow tract obstruction.
Children with ToF quickly learn to squat to relieve Tet spells (infants who cannot squat can be placed in the knee-to-chest position by their caregiver). This posture quickly increases SVR without changing PVR, increasing the SVR:PVR ratio. The increased SVR forces a higher proportion of right ventricular output to enter the pulmonary circulation and oxygenate in the pulmonary capillary beds, increasing arterial oxygen concentration and relieving the Tet spell.
(Choices A and D) Squatting restricts chest wall expansion and slightly increases work of breathing. Lung compliance is not changed by this positioning.
(Choices B and C) Squatting does not significantly impact PVR as the compressed vessels are primarily in the abdomen and lower extremities. Pulmonary blood flow increases – not decreases – due to decreased right-to-left shunt with increased SVR.
Educational objective:
In patients with Tetralogy of Fallot, squatting during a Tet spell increases systemic vascular resistance and decreases right-to-left shunting, thereby increasing pulmonary blood flow and improving oxygenation status.