Neonatal Nurse Practitioner Practice Exam

In a neonate with patent ductus arteriosus, what is the left-to-right shunt dependent on?

1. Pulmonary artery pressure

2. Pulmonary vascular resistance

3. Systemic vascular resistance

4. Ventricular wall compliance

The correct answer is: Pulmonary vascular resistance

In the context of a neonate with patent ductus arteriosus (PDA), the left-to-right shunt is primarily dependent on pulmonary vascular resistance. When there is a connection between the aorta and pulmonary artery, as seen in PDA, the pressure in the aorta typically exceeds that in the pulmonary artery, resulting in blood flowing from left to right. The degree of this flow largely hinges on the resistance within the pulmonary circulation. As pulmonary vascular resistance decreases, which is often the case after birth as a neonate transitions to extrauterine life, it facilitates a greater volume of blood to flow from the aorta to the pulmonary artery. This is particularly crucial as the maturation of the lungs and their blood vessels continues, leading to reduced resistance and thus potentially increasing the left-to-right shunting effect. In contrast, systemic vascular resistance and pulmonary artery pressure play roles in influencing cardiac output and the overall hemodynamics, but they do not directly dictate the shunting mechanism in the same way that pulmonary vascular resistance does. Ventricular wall compliance affects how the heart can handle blood volumes but does not specifically determine the flow dynamics between the aorta and pulmonary artery in cases of PDA.