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).
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