A 42-year-old woman was in labor. The fetal heart rate monitor showed intermittent variable decelerations with good recovery. The obstetrician ruptured membranes and noted that the amniotic fluid was meconium stained. Because the fetal heart rate monitor showed good beat-to-beat variability, it was decided to allow the mother to continue in labor and deliver vaginally. The infant was delivered vaginally 2 hours later and was noted to be meconium stained.
Describe the fetal circulation.
The fetal circulation ( Figure 82-1 ) is a parallel circuit, in contrast to a series circuit in the adult. In the fetus, gas exchange occurs at the placenta and not the lungs. Blood leaving the placenta enters the fetus via the umbilical vein. This relatively well-oxygenated blood (pO 2 30–35 mm Hg) enters the fetus and predominantly bypasses the liver via the ductus venosus. Most of this blood on entering the right atrium is preferentially shunted across the patent foramen ovale to the left side of the heart and out the ascending aorta to the cerebral and coronary circulation. The brain and heart receive most of the relatively well-oxygenated blood. Blood returning from the cerebral circulation via the superior vena cava, which is considerably less oxygenated (pO 2 12–14 mm Hg), enters the right side of the heart. This blood is preferentially directed to the right ventricle and exits through the pulmonary artery. Because of the high pulmonary vascular resistance (PVR) that exists in utero, only 10% of this blood enters the pulmonary circulation to provide nutrients for lung growth. The remaining blood is shunted across the ductus arteriosus because of the low systemic vascular resistance (SVR). SVR is low in the fetus because of the relatively large ductus arteriosus and the placenta. Blood enters the descending aorta and supplies the lower fetal body, returning to the placenta via the iliac veins to the umbilical arteries.
What physiologic changes occur at birth?
When the neonate is delivered, the first breaths expand the lungs with air, and alveolar pO 2 increases. These changes lead to a dramatic decrease in PVR, although not to the normal adult values. 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 the following:
Functional closure of the patent foramen ovale because the pressure on the left side of the heart (SVR) is greater than the pressure on the right side (PVR)
Functional closure of the ductus arteriosus because of an increase in arterial pO 2 ; the ductus arteriosus becomes 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 increases pulmonary vascular resistance results in reopening of the functionally closed fetal shunts. Factors that adversely affect PVR are hypoxia, hypercarbia, acidosis, hypothermia, and sympathetic stimulation. It is imperative in the initial management of the neonate in the delivery room 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.
How is neonatal resuscitation managed in the delivery room?
As soon as the newborn is delivered, a rapid assessment should be performed to determine the need for resuscitation ( Figure 82-2 ). This rapid assessment answers the following three questions:
Is the newborn full term?
Is the newborn crying or breathing?
Is the muscle tone good?
If the answer to all three questions is yes, resuscitation is not necessary, and the newborn can remain with the mother. If the answer to any of these questions is no, resuscitation should be initiated in the following order:
Warm by radiant heat source
Clear the airway if needed
Dry the newborn
Administration of epinephrine or volume expansion or both
In the first 60 seconds, known as the “golden minute,” the following should be completed: the initial steps; reassessment of the respiratory status and heart rate ( Box 82-1 ); and, if needed, administration of supplemental oxygen or positive pressure ventilation. Positive pressure ventilation should be at a rate of 40–60 breaths per minute, and the minimal inflation pressure to achieve a heart rate >100 beats per minute should be applied. Heart rate is assessed by auscultation of the precordium or palpation of the umbilical artery. When supplemental oxygen or positive pressure ventilation or both are begun, the assessment now consists of evaluation of respirations, heart rate, and oxygenation (see Box 82-1 ). Oxygenation is ideally monitored by pulse oximetry. However, it may take 1–2 minutes before pulse oximetry is functional, and in low-flow states it may not work at all. During every intervention, success is measured by an increase in heart rate.