Medical Knowledge

A patent ductus arteriosus is a remnant from fetal circulation that provides a connection between the pulmonary circulation and the systemic circulation at the level of the thoracic aorta to bypass the lungs in utero. To understand the impact of a PDA on a newborn, one must first comprehend the transitional pathway from the fetal circulation to the circulation in the postnatal period.

In the fetus, gas and nutrient exchange is provided via the placenta. The oxygenated blood is carried to the fetus via the umbilical vein. Most of the blood flows through the ductus venosus to the inferior vena cava (IVC), and the remainder perfuses the liver. As the blood enters the right atrium, the higher-velocity flow from the IVC is pushed across the foramen ovale and is ejected by the left ventricle into the aorta to perfuse the body. The blood flow entering the right atrium via the superior vena cava enters the pulmonary artery via the right ventricle. The ductus arteriosus allows the pulmonary artery blood flow to join the systemic circulation at the aorta, thus bypassing the lungs with the majority of the right ventricular output. The location of the ductus arteriosus distal to the left subclavian artery allows the head and coronary vessels to receive blood with a relatively higher oxygen saturation compared with the rest of the body.

Figure 80-1 illustrates the fetal circulation. As you trace the path of blood, start with the umbilical vein which sends oxygenated blood from the placenta to the fetal heart.

Figure 80-1 Fetal circulation. Connect the dots using red and blue pencils to trace the fetal circulation. (Note purple lines already drawn to denote the mixing of blood.)