A newborn is evaluated for cyanosis immediately after birth. On examination, oxygen saturation is 70% in all 4 extremities and does not change despite 100% oxygen administration. The patient's lungs are clear to auscultation, and there are no heart murmurs or gallops. Peripheral pulses are normal. A chest radiograph reveals clear lungs and a normal cardiac silhouette but a narrowed mediastinal shadow. Which of the following is the most likely cause of cyanosis in this patient?

Question

A newborn is evaluated for cyanosis immediately after birth.  On examination, oxygen saturation is 70% in all 4 extremities and does not change despite 100% oxygen administration.  The patient's lungs are clear to auscultation, and there are no heart murmurs or gallops.  Peripheral pulses are normal.  A chest radiograph reveals clear lungs and a normal cardiac silhouette but a narrowed mediastinal shadow.  Which of the following is the most likely cause of cyanosis in this patient?

Quizplus · Accepted Answer

11ec160b_3aa3_8989_9249_6d2969891e42_MD0001_00 This cyanotic newborn has severe hypoxemia that does not improve with oxygen administration. These findings suggest a congenital heart defect in which deoxygenated blood bypasses the pulmonary circulation and is delivered directly to the systemic circulation (ie, right-to-left shunt). The most likely etiology is transposition of the great arteries (TGA), which is characterized by the abnormal origination of the aorta from the right ventricle and the pulmonary artery from the left ventricle. The abnormal position of the aorta directly on top of the pulmonary artery results in a narrow-appearing mediastinum. The anatomy in TGA results in disconnected and parallel systemic and pulmonary circuits with delivery of deoxygenated blood from the right ventricle to the systemic circulation via the aorta (effectively a right-to-left shunt). Cyanosis occurs immediately after birth, and cardiac examination is often normal. Oxygen administration (ie, hyperoxia test) does not improve cyanosis or hypoxemia in TGA because the oxygenated blood cannot effectively reach the systemic circulation; instead, it is cycled from the lungs to the left side of the heart and back to the lungs. (Choices A and B) Pulmonary edema secondary to sepsis or heart failure (ie, impaired left ventricular contractility) can lead to hypoxemia and cyanosis due to impaired alveolar-capillary diffusion leading to intrapulmonary shunting. However, crackles would be expected on examination, and abnormal cardiopulmonary chest x-ray findings (eg, cardiomegaly, alveolar infiltrates, pleural effusion) are typical. (Choice C) Congenital methemoglobinemia, which is characterized by the inability to reduce methemoglobin back to hemoglobulin, is a rare cause of neonatal cyanosis. Oxygen saturation fails to improve with oxygen administration; however, methemoglobinemia would not explain a narrowed mediastinum on chest x-ray. Moreover, pulse oximetry is inaccurate in methemoglobinemia and reads no lower than 85% even if true oxygenation is lower. (Choice E) Primary pulmonary pathologies (eg, respiratory distress syndrome [RDS], pulmonary hypoplasia) are typical causes of ventilation-perfusion mismatch and often present with cyanosis at birth. However, chest x-ray findings are usually abnormal (eg, ground-glass opacification in RDS), and oxygen administration would be expected to increase oxygen saturation. Educational objective: Cyanotic congenital heart disease, characterized by right-to-left shunting, should be considered in neonates with severe cyanosis that does not improve with oxygen administration. In transposition of the great arteries, deoxygenated blood from the right ventricle is delivered to the systemic circulation while oxygenated blood cycles through the pulmonary circulation. __________ References: Transposition of the great arteries. (http://www.ncbi.nlm.nih.gov/pubmed/30860704) Cyanotic heart disease. (http://www.ncbi.nlm.nih.gov/pubmed/29763177)

A Newborn Is Evaluated for Cyanosis Immediately After Birth