A 4-week-old boy undergoes cardiac catheterization for closure of a patent ductus arteriosus. The patient was born at 26 weeks gestation due to placental abruption. He has been cared for in the neonatal intensive care unit since delivery. Pulse oximetry shows 99% in the right hand and left foot. Echocardiography shows a moderately sized patent ductus arteriosus. After catheterization but before closure, blood gases are obtained from several locations. Compared with the partial pressure of oxygen (PaO₂) of a normal infant, this patient would be expected to have a higher PaO₂ in which of the following locations?

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11ec160b_3c9e_6ef2_9249_fbb5b40f0d92_MD0001_00 A patent ductus arteriosus (PDA) is a left-to-right shunt that allows continuous flow of oxygenated blood from the aorta into the pulmonary artery. In the pulmonary artery, the oxygenated blood mixes with deoxygenated blood coming from the right ventricle. This leads to a higher partial pressure of oxygen (PaO2) in the pulmonary artery than would normally be seen. (Choices A and B) In patients with a left-to-right shunt (eg, atrial or ventricular septal defect, PDA), the PaO₂ in the aorta remains the same. In contrast, if the shunt becomes right-to-left due to increased right-sided pressures (ie, Eisenmenger physiology) the oxygen concentration in the aorta can be lower than expected. However, patients with Eisenmenger syndrome have cyanosis, which is not seen in this patient. (Choice C) The left atrium receives oxygenated blood from the pulmonary circulation, specifically from the alveolar capillaries via the pulmonary veins. Although the volume of blood received by the left atrium is increased with left-to-right PDA shunts, the PaO₂ in the left atrium is unchanged as all the blood passes through the lungs. (Choice E) The right atrium receives deoxygenated blood from the systemic circulation, primarily the superior and inferior vena cava. The left-to-right shunt of a PDA is "downstream" of the right atrium (ie, in the pulmonary artery), so the PaO₂ of right atrial blood is unchanged. Educational objective: In patients with patent ductus arteriosus, left-to-right shunting of oxygenated blood from the aorta into the pulmonary artery results in a higher-than-expected oxygen concentration in the pulmonary artery. Oxygen saturation in the left and right heart chambers is unchanged.

A 4-Week-Old Boy Undergoes Cardiac Catheterization for Closure of a Patent