An 8-hour-old boy is being assessed for a murmur in the newborn nursery. The patient was born at 37 weeks via cesarean delivery. Apgar scores were 7 and 8 at 1 and 5 minutes, respectively. He was noted to have a harsh ejection systolic murmur of 4/6 intensity during initial newborn examination. Brachial and femoral pulses are 2+ with no delay. While the patient is breastfeeding for the first time, his face, lips, hands, and feet become cyanotic. The murmur cannot be heard during the cyanotic episode. Which of the following oxygen saturation percentages are most likely present during the cyanotic episode?

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11ec160b_3aa1_b4c4_9249_73b6130b082a_MD0001_00 This infant has a harsh systolic ejection murmur that disappears during an episode of cyanosis, an expected finding with tetralogy of Fallot (ToF). The major defects in ToF are right ventricular outflow tract (RVOT) obstruction and a ventricular septal defect (VSD); the murmur is created by blood flow through the narrowed RVOT, not the VSD. The degree of RVOT obstruction is the major driver of symptoms in ToF because it determines the direction and magnitude of shunting through the VSD. A certain degree of RVOT obstruction is fixed, but some degree is dynamic and can change based on activity (eg, feeding, squatting) that alters the underlying hemodynamics. Many patients have left-to-right or neutral shunting at rest (and are acyanotic), but they develop cyanosis ("tet" episodes) when a dynamic increase in RVOT obstruction (eg, precipitated by crying or feeding) leads to increased right-to-left shunting through the VSD. During such an episode, decreased blood flow through the RVOT causes the systolic murmur to disappear. In addition, oxygen saturation (SaO₂) in the left ventricle is reduced (eg, <90%) due to mixing with deoxygenated blood from the RV. The SaO₂ in the left atrium (LA) remains normal as the LA continues to receive oxygenated blood from the pulmonary veins. (Choice B) These values show a step-down in SaO₂ at the LA (pulmonary vein SaO₂ > LA SaO₂), consistent with a right-to-left shunt through an atrial septal defect (ASD). Right-to-left shunting through an ASD (ie, Eisenmenger syndrome) takes years to develop and is not an expected cause of episodic cyanosis in infants. (Choice C) These values show a step-up in SaO₂ at the RV (RV SaO₂ > right atrium SaO₂), consistent with left-to-right shunting through a VSD, as occurs during acyanotic periods in patients with ToF. (Choice D) These values reflect the SaO₂ distribution in a normal heart. (Choice E) These values show a step-up in SaO₂ at the pulmonary artery (pulmonary artery SaO₂ > RV SaO₂), consistent with left-to-right shunting through a patent ductus arteriosus. Left-to-right shunts do not cause cyanosis. Educational objective: Tetralogy of Fallot involves right ventricular outflow tract (RVOT) obstruction and a ventricular septal defect (VSD). Cyanotic ("tet") episodes are commonly precipitated by a dynamic increase in RVOT obstruction that increases right-to-left shunting through the VSD. These episodes involve decreased blood flow through the RVOT, causing disappearance of the associated systolic murmur, and reduced oxygen saturation of the left ventricular blood being pumped into the systemic circulation. __________ References: Tetralogy of Fallot. (http://www.ncbi.nlm.nih.gov/pubmed/19144126)

An 8-Hour-Old Boy Is Being Assessed for a Murmur in the Newborn