What are the physiologic changes that occur at birth?
When the neonate is delivered, the first breaths expand the lungs with air and alveolar pO2 increases. These changes lead to a dramatic decrease in PVR. At the same time, the umbilical cord is clamped and the low-resistance placenta is removed from the circulation. This results in an abrupt increase in SVR. These changes lead to:
· functional closure of the patent foramen ovale because the pressure on the left side of the heart (SVR) is greater than on the right side (PVR);
· functional closure of the ductus arteriosus because of an increase in arterial pO2. The ductus arteriosus will become the ligamentum arteriosum;
· functional closure of the ductus venosus because of removal of the placenta.
This pattern of circulation closely resembles the adult circulation. However, it is referred to as the transitional circulation because of the reversibility of the above-mentioned changes during adverse events such as hypoxia or acidosis. Any insult that will increase pulmonary vascu-lar resistance will result in reopening of the functionally closed fetal shunts. Factors that will adversely affect PVR are hypoxia, hypercarbia, acidosis, hypothermia, and sympathetic stimulation. Therefore, it is imperative that the initial management of the neonate in the delivery room is to pay meticulous attention to ensuring adequate oxygenation, ventilation, and maintenance of normothermia. Reversion to fetal circulation is referred to as persistent pulmonary hypertension of the newborn (PPHN).