About a third of the oxygenated blood in the right atrium is shunted through the foramen ovale to the left atrium, and the remaining blood mixes with deoxygenated blood and flows to the right ventricle. Blood from the right ventricle flows to the pulmonary artery, but because of the high fetal PVR, the majority of the blood shunts through the ductus arteriosus. Soon after birth the increased negative intrathoracic pressure and a number of circulatory changes allow for the pulmonary vasculature to dilate and decrease the PVR, allowing the lungs to transition as the sole organ for gas exchange.

Within the first 24 hours, there is a functional closure of the ductus arteriosus and foramen ovale, with a complete anatomic closure of the ductus arteriosus usually within 2-3 weeks after birth.

Cardiac diseases associated with diminished pulmonary blood flow (and therefore diminished pulmonary markings on the chest X-ray) include tricuspid atresia, pulmonary atresia, pulmonary stenosis, tetralogy of Fallot (TOF), and Ebstein’s anomaly. TOF is one of the most common cyanotic congenital heart diseases presenting in the newborn period, and the degree of cyanosis depends on the severity of pulmonary stenosis. These five cardiac lesions are among the most common cardiac lesions that depend on the presence of a patent ductus arteriosus for communication of the pulmonary and systemic circulations.

Lesions associated with increased pulmonary blood flow and therefore increased pulmonary markings on the chest X-ray include (persistent) truncus arteriosus (fails to divide properly into pulmonary artery and aorta) and total anomalous venous return.

In patients with truncus arteriosus, the cyanosis may not be clinically apparent. A clue to a condition associated with increased pulmonary blood flow is that cyanosis does not respond to prostaglandin administration, which maintains patency of the ductus arteriosus. These two lesions require surgical correction soon after birth.

Cardiac conditions that present with severe heart failure and cyanosis in the neonatal period include hypoplastic left heart syndrome, critical aortic stenosis, interrupted aortic arch, and transposition of the great arteries. These lesions are ductal dependent, so infants may present with cyanosis and shock when the ductus arteriosus closes. Infants with ductal-dependent lesions may be asymptomatic at birth and present to the ED with shock and/or cyanosis in the first 2-3 weeks of life.

Table 2. Chemical Causes Medications

• Amyl nitrite
• Benzocaine
• Dapsone
• Lidocaine
• Nitric oxide
• Nitroglycerin
• Nitroprusside
• Phenzaopyridine
• Prilocaine
• Quinones
• Sulfonamides