During birth, interrelated changes occur in the respiratory and circulatory systems. When the neonate takes a breath, the lungs expand with air, thereby increasing alveolar pO2, decreasing alveolar partial pressure of carbon dioxide (pCO2), and increasing functional residual capacity. These three changes result in a significant decrease in PVR4 that allows for right ventricular output to now enter the pulmonary circulation easily, thereby now increasing the blood volume returning to the left atrium. Concurrently, the umbilical cord is clamped, removing the low resistance placenta from the circulation. Both these actions, clamping the cord and removing the placenta, result in an increase in neonatal SVR. The increase in blood volume and pressure in the left atrium results in functional closure of the foramen ovale. The DA will functionally close as the arterial pO2 increases. The ductus venosus closes secondary to removal of the placenta from the circulation (Table 2-2).
Table 2-2.Circulatory Changes After Birth |Favorite Table|Download (.pdf) Table 2-2.Circulatory Changes After Birth
|Decrease in pulmonary vascular resistance |
| Expansion of lungs |
| Increased pO2 |
|Increase in systemic vascular resistance |
| Clamping umbilical cord |
| Removal of placenta from circulation |
|Closure of ductus arteriosus |
| Increased pO2 |
| Prostaglandins |
|Closure of foramen ovale |
| Increased pulmonary blood flow |
| Increased blood return to left atrium |
|Closure of ductus venosus |
| Removal of placenta |
The key event that must occur at birth is the decrease in PVR. If PVR does not decrease to levels below SVR, the changes described above will not occur and right-to-left shunting at the atrial level (FO) and extracardiac level (DA) will continue; the neonate will be hypoxic (Figure 2-4). This circulation was once referred to as persistent fetal circulation but is now known as persistent pulmonary hypertension, which reflects the underlying pathophysiology that exists.
Effects of increased pulmonary vascular resistance in the newborn.
The circulation at birth is referred to as transitional because the changes that occur at birth are reversible; the two major shunts are functionally and not anatomically closed. Adverse events such as hypoxia, hypercarbia, acidosis, or hypothermia will result in reopening of the two shunts and a conversion back to a fetal pattern.5 The DA may take 10 to 15 hours to fully functionally close, with complete anatomic closure occurring in 2 to 3 weeks6 of postnatal life; the vestigial organ remaining is the ligamentum arteriosum. The DA will “reopen” if hypoxia is present. The relationship between PVR and SVR will determine the direction of the shunt across the DA. If PVR is greater than SVR, then right-to-left shunting will occur and worsen hypoxia in the neonate.
The FO closes by a flaplike valve located in the atrial septum on the left side secondary to increased left atrial pressure as compared to right-sided pressures. In 25% to 30% of adults, the FO remains anatomically probe patent.7 If PVR remains high or increases, this will create a higher pressure on the right side of the heart, opening the flaplike valve; this results in a patent FO and right-to-left shunting will occur.
Initial management of the neonate in the delivery room should be aimed at ensuring a smooth transition from fetal to transitional circulation. Factors that may adversely affect the decrease in PVR must be avoided. These include hypoxia, hypercarbia, acidosis, and hypothermia. Delivery room management strategies and neonatal resuscitation are discussed in detail in Chapter 11.