Correct Option of Question#04

Question#04

You are called to the nursery to evaluate a newborn who was diagnosed in utero with pulmonary atresia and ventricular septal defect. On physical examination, she is in no distress, alert, and breathing comfortably at 40 breaths/min. Aeration of the lungs is adequate. Cardiac auscultation reveals a normal S1 and a single S2 sound, and there are no murmurs. Her pulses and perfusion are good, although she has cyanosis of the hands, feet, and perioral area. Her oxygen saturation in room air is 75%.

Of the following, the MOST likely cause of her cyanosis is

A. increased levels of fetal hemoglobin

B. left-to-right shunting at the atrial level

C. methemoglobinemia

D. right-to-left shunting at the ductus arteriosus

E. right-to-left shunting at the ventricular level

Correct Option is 'E'

Explanation:

Cyanosis (ie, bluish discoloration of the skin and mucous membranes) in the newborn may arise from a number of different conditions. Among these are pulmonary pathologies, congenital cardiovascular malformations, persistent pulmonary hypertension of the newborn, and disturbances of the hematologic and metabolic systems. Right-to-left shunting can be thought of as diversion of the desaturated blood away from the lungs and to the systemic circulation.

This can occur because blood does not perfuse the ventilated portions of the lung (intrapulmonary right-to-left shunting), does not perfuse the pulmonary artery from the heart (intracardiac right-to-left shunting), or is diverted away from the pulmonary circuit through the ductus arteriosus (extracardiac right-to-left shunting).

Methemoglobinemia is a rare condition in which the heme iron has been oxidized to the ferric state and no longer can bind oxygen reversibly. Enzyme systems reduce this oxidized hemoglobin and prevent accumulation in the normal state.

When abnormalities exist that limit this reduction, the oxidized form of heme increases, and when blood levels of methemoglobin exceed 15 g/L, the blood appears chocolate brown and the skin has a slate gray cyanotic appearance, even though the oxygen saturation may be normal. True hypoxia may occur at higher levels of methemoglobin.

For the neonate described in the vignette, the cyanosis results from right-to-left shunting at the ventricular level because she has pulmonary atresia and cannot deliver blood from her right ventricle into her lungs. All of the desaturated blood entering her right ventricle, therefore, must exit through the ventricular septal defect, thereby entering the left ventricle and aorta during systole.

The infant may, in fact, have increased levels of fetal hemoglobin, but this would serve to improve oxygen delivery, not cause cyanosis. Left-to-right shunting at the atrial level would not necessarily be expected in this infant, and even if it did occur, delivery of oxygenated left atrial blood to the right side would not produce cyanosis. Patients who have pulmonary atresia require an alternate source of pulmonaryblood flow, which may occur via the ductus arteriosus in a left-to-right flow pathway or through multiple aortopulmonary collateral arteries. Flow cannot occur from right to left in these patients because there is an obligate left-to-right shunt of blood from the system into the otherwise empty pulmonary vascular bed. In fact, when the ductus arteriosus is the only source of pulmonary blood flow in such patients, maintenance with prostaglandin E is vital to maintain pulmonary blood flow and, thus, oxygenation prior to surgical repair.