Review Question - QID 217016

This asymptomatic patient who presents with a continuous "machine-like" murmur best heard in the pulmonic location (left sternal border in the 2nd intercostal space) most likely has a patent ductus arteriosus (potentially secondary to congenital rubella infection) caused by failure of this structure to close in response to increased oxygen tension.

During embryonic development, the ductus arteriosus is a structure that allows blood to bypass the pulmonary circulation. This structure is maintained by the production of prostaglandin E2 as well as low oxygen tension of the blood passing this shunt during fetal life. The increased oxygen tension induced by breathing after birth results in the rapid closure of the ductus arteriosus within the first few days of life. Failure of this process results in a patent ductus arteriosus (PDA) that results in continued shunting of blood from the left heart to the right heart. Potential etiologies for PDA include prematurity, maternal Rubella infection, maternal exposures (alcohol consumption, phenytoin use, prostaglandin use), respiratory distress syndrome, trisomies, or concurrent congenital heart disease.

Ovali presents a review of the molecular mechanisms responsible for the closure of the ductus arteriosus. Among these mechanisms, smooth muscle cells of the ductus have been shown to sense oxygen tension through the mitochondrial network by Rho-kinase function.

Incorrect Answers:
Answer 1: Failure of atrial septal formation involving the ostium primum would result in an atrial septal defect that would be heard on cardiac auscultation as wide and fixed splitting of S2. Ostium primum defects are also often associated with other cardiac or developmental defects.

Answer 2: Failure of atrial septal formation involving the ostium secundum would result in an atrial septal defect that would be heard on cardiac auscultation as wide and fixed splitting of S2. Ostium secundum defects are often asymptomatic and isolated defects that are discovered incidentally.

Answer 4: Failure of development of the cardiac cushions into the infundibular septum would result in Tetralogy of Fallot that would be heard on cardiac auscultation as a loud and harsh systolic ejection murmur on the upper sternal border. Patients will present with hypercyanotic episodes where the acute onset of restlessness causes increased cyanosis, dyspnea, and occasionally syncope.

Answer 5: Failure of regulation of tissue production in the aorta would result in coarctation of the aorta that would be heard on cardiac auscultation as a harsh systolic murmur along the left sternal border. Patients will present with relative hypertension in the upper extremities and underdeveloped legs compared with arms.

Bullet Summary:
The increased oxygen tension induced by breathing after birth results in the rapid closure of the ductus arteriosus within the first few days of life.