A 2-month-old boy is brought to the emergency department for evaluation of cyanosis. He became fussy and sweaty while taking his bottle, and his lips "turned blue" for several minutes during the feeding. The infant has had similar episodes during feeding and crying, but these resolved quickly. The patient was born at 39 weeks gestation, and his birth weight was average for gestational age. His weight gain has been slow, currently at the 5th percentile. He is fed pumped breast milk fortified with powdered formula to augment caloric intake. Temperature is 37 C (98.6 F), blood pressure is 80/50 mm Hg, pulse is 150/min, and respirations are 55/min. On examination, the infant is ill-appearing, agitated, cyanotic, and tachypneic. Cardiac auscultation reveals a grade 2/6 crescendo-decrescendo systolic ejection murmur at the left upper sternal border and a single second heart sound. The patient is placed in a knee-chest position immediately. This maneuver improves the patient's condition predominantly by which of the following mechanisms?

Question

A 2-month-old boy is brought to the emergency department for evaluation of cyanosis.  He became fussy and sweaty while taking his bottle, and his lips "turned blue" for several minutes during the feeding.  The infant has had similar episodes during feeding and crying, but these resolved quickly.  The patient was born at 39 weeks gestation, and his birth weight was average for gestational age.  His weight gain has been slow, currently at the 5th percentile.  He is fed pumped breast milk fortified with powdered formula to augment caloric intake.  Temperature is 37 C (98.6 F), blood pressure is 80/50 mm Hg, pulse is 150/min, and respirations are 55/min.  On examination, the infant is ill-appearing, agitated, cyanotic, and tachypneic.  Cardiac auscultation reveals a grade 2/6 crescendo-decrescendo systolic ejection murmur at the left upper sternal border and a single second heart sound.  The patient is placed in a knee-chest position immediately.  This maneuver improves the patient's condition predominantly by which of the following mechanisms?

Quizplus · Accepted Answer

11ec25c8_d63b_4cf0_bc81_ffbffc48bd24_MD0003_00 Tetralogy of Fallot is the most common cyanotic congenital heart defect and has 4 characteristic anomalies: Right ventricular outflow tract (RVOT) obstruction (pulmonary stenosis or atresia)Right ventricular hypertrophyOverriding aortaVentricular septal defect (VSD) Right ventricular outflow tract (RVOT) obstruction (pulmonary stenosis or atresia) Right ventricular hypertrophy Overriding aorta Ventricular septal defect (VSD) The relative resistance between the systemic and pulmonary circulations determines blood flow across the large, unrestrictive VSD. If the resistance of the systemic circulation exceeds the resistance of the pulmonary circulation, the blood will shunt from the ventricles to the pulmonary artery (eg, left-to-right shunt). However, if the pulmonary vascular resistance (PVR) exceeds that of the systemic circulation, the blood will shunt from the ventricles into the aorta (eg, right-to-left shunt), resulting in cyanosis. The degree of RVOT obstruction determines clinical presentation, which can range from minimal symptoms to profound hypoxemia and cyanosis. Patients with even mild stenosis can develop dramatic infundibular spasm with agitation or exertion (eg, feeding, crying, hyperventilation), resulting in near-complete RVOT obstruction. The subsequent increase in PVR (Choice C) shunts deoxygenated blood from the right ventricle across the VSD and into the aorta. This results in sudden cyanosis and is called a hypercyanotic, hypoxic, or "tet" spell. In this patient, the crescendo-decrescendo systolic ejection murmur represents RVOT obstruction. The single second heart sound (S2) comprises the normal aortic and the inaudible pulmonary component of the S2. Treatment of a "tet" spell begins with immediate knee-chest positioning, which kinks the femoral arteries, increases systemic vascular resistance (SVR,) and consequently reduces the degree of right-to-left shunting. In addition, inhaled oxygen (Choice A) stimulates pulmonary vasodilation (decreases PVR) and systemic vasoconstriction. Intravenous fluids improve right ventricle filling and pulmonary flow. knee-chest positioning (Choice B) The knee-chest and squatting positions increase, not decrease, SVR. (Choice E) Although, the knee-chest position can transiently increase venous return to the right heart, this is secondary to increased SVR. Administration of intravenous fluids increases systemic venous return (ie, preload). Educational objective: The degree of right ventricular outflow tract obstruction can fluctuate in tetralogy of Fallot. Patients with mild obstruction can develop a dramatic spasm resulting in hypercyanotic, hypoxic, or "tet" spells. Placement of patients in a knee-chest position during a hypercyanotic spell increases systemic vascular resistance, increases pulmonary blood flow, and improves hypoxemia. References: Squatting. (http://www.ncbi.nlm.nih.gov/pubmed/23791582) Hot topics in tetralogy of Fallot. (http://www.ncbi.nlm.nih.gov/pubmed/24076489)

A 2-Month-Old Boy Is Brought to the Emergency Department for Evaluation