A newborn is transferred to the neonatal intensive care unit 4 hours after delivery due to cyanosis. The patient was born via a spontaneous vaginal delivery at 41 weeks gestation to a 22-year-old gravida 2 para 2 woman. The pregnancy and delivery were uncomplicated. The father was changing the patient's diaper and noticed that the patient's legs appeared dusky. Pulse oximetry shows 98% in the right hand and 83% in the left foot. On physical examination, the patient exhibits subcostal retractions and grunting. Cardiac examination reveals a 2/6 holosystolic murmur at the left lower sternal border. Femoral pulses are strong bilaterally. Which of the following is the most likely cause of this newborn's cyanosis?
Show Explanatory Sources
This newborn has differential cyanosis (or differential hypoxemia), which is cyanosis involving the lower extremities but not the upper extremities. This pattern suggests right-to-left shunting of deoxygenated blood at a location distal to the branch point of the arteries supplying the head and upper extremities (ie, distal to the left subclavian artery), as can occur with shunting through the ductus arteriosus.
Postnatal right-to-left shunting through the ductus arteriosus occurs only with an abnormality that causes pulmonary arterial pressure to be greater than systemic arterial pressure. One potential cause is persistent pulmonary hypertension of the newborn (PPHN), in which pulmonary vascular resistance fails to decline immediately after birth. Flow of deoxygenated blood from the pulmonary artery to the aorta through the ductus arteriosus results in decreased postductal oxygen saturation (eg, in the lower extremities) relative to preductal oxygen saturation (eg, in the upper extremities).
With most right-to-left shunts, the pressure gradient across the shunt is low; therefore, the shunt itself rarely creates a murmur (in contrast to the continuous murmur heard with left-to-right shunting through a PDA). The classic murmur of PPHN is a murmur of tricuspid regurgitation resulting from the elevated pulmonary pressure causing tricuspid valve insufficiency (ie, this patient's holosystolic murmur at the left lower sternal border). This patient's strong femoral pulses are also consistent with PPHN (other causes of right-to-left shunting through the ductus arteriosus [eg, critical aortic coarctation] are associated with diminished femoral pulses).
PPHN is often triggered by a pulmonary disorder (eg, lung hypoplasia, meconium aspiration, pneumonia), which can cause respiratory distress (eg, subcostal retractions, grunting) and can also contribute to hypoxemia and cyanosis.
(Choices A, B, and C) A left-to-right shunt involves passage of oxygenated blood from the systemic circulation into the pulmonary circulation. It does not result in deoxygenated blood reaching the systemic circulation; therefore, hypoxemia and cyanosis do not occur.
(Choice D) Patients with PPHN may also experience right-to-left shunting through an atrial septal defect (if present) or the foramen ovale. However, right-to-left shunting through these intracardiac pathways would not cause differential cyanosis (ie, deoxygenated blood would reach both the upper body and the lower body).
Educational objective:
Differential cyanosis (cyanosis affecting only the lower body) suggests right-to-left shunting across the ductus arteriosus. In newborns, a potential cause of this pattern is persistent pulmonary hypertension of the newborn.