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NEONATAL RESUSCITATION FETAL CIRCULATION
Category: Child Health
Abstract : Cardiovascular adaptation Fetal circulation To understand the
cardiovascular changes that occur in the neonate at birth, it is essential to
have an understanding of normal fetal circulation. The umbilical vein carries
the oxygenated blood from the placenta to the fetus. Blood flow in the umbilical
vein divides at the porta hepatis, with 50-60% of the blood passing directly to
the i
Cardiovascular adaptation
Fetal circulation
To understand the
cardiovascular changes that occur in the neonate at birth, it is essential to
have an understanding of normal fetal circulation. The umbilical vein carries
the oxygenated blood from the placenta to the fetus.
Blood flow in the umbilical
vein divides at the porta hepatis, with 50-60% of the blood passing directly to
the inferior vena cava and the remainder of the blood passing into the portal
circulation. This portal blood flow perfuses the liver and then passes into the
inferior vena cava.
Flow studies have revealed that relatively little
mixing of the blood occurs in the inferior vena cava from these 2 sites. The
more highly oxygenated blood, which has bypassed the liver, streams into the
inferior vena cava to pass preferentially through the patent foramen ovale into
the left atrium. The desaturated blood returning from the liver and lower body
streams into the inferior vena cava to the right atrium. In the right atrium, it
mixes with blood returning from the coronary sinus and superior vena cava and
flows into the right ventricle. The more highly oxygenated blood that crosses
the foramen ovale mixes with the small amount of pulmonary venous return and
then crosses the mitral valve into the left ventricle.
The output from
the left ventricle passes into the ascending aorta to the heart, brain, head,
and upper torso. The less saturated blood from the right ventricle passes into
the pulmonary arteries. Because the pulmonary vessels are constricted and highly
resistant to flow, only about 12% of the blood enters the pulmonary veins. The
remainder of the blood takes the path of least resistance through the patent
ductus arteriosus into the descending aorta. Approximately one third of this
blood is carried to the trunk, abdomen, and lower extremities, with the
remainder entering the umbilical artery where it is returned to the placenta for
reoxygenation.
Neonatal circulation
The aeration of the lung results
in an increase in arterial oxygenation and pH, with a resulting dilation of the
pulmonary vessels. Decompression of the capillary lung bed further decreases the
pulmonary vascular resistance. There is also a corresponding decrease in right
ventricular and pulmonary artery pressures. The decrease in pulmonary vascular
resistance leads to an increase in blood flow to the lungs and in pulmonary
venous return. Clamping of the umbilical cord removes the low resistance
placental vascular circuit and causes a resultant increase in the total systemic
vascular resistance with a resultant increase in left ventricular and aortic
pressures. The increased systemic vascular resistance, combined with the
decreased pulmonary vascular resistance, reverses the shunt through the ductus
arteriosus (from right-to-left shunting to left-to-right shunting) until the
ductus completely closes.
All of these peripartum events result in
closure of the other fetal shunts. With the decrease in right atrial pressure
and the increase in left atrial pressure, the “flap-valve” foramen ovale is
pushed closed against the atrial septum. This functional closure at birth is
followed by anatomical closure that usually occurs at several months of age. The
ductus venosus closes because of the clamping of the umbilical cord, which
terminates umbilical venous return. Functional mechanical closure of the ductus
venosus is accomplished by the collapse of the thin-walled vessels. Anatomical
closure subsequently occurs at approximately 1-2 weeks. The constriction and
closure of the patent ductus arteriosus is accomplished by contractile tissue
within the walls of the ductus arteriosus. The contraction of this tissue is
dependent on both the increase in arterial oxygen related to the onset of
spontaneous respirations and a fall in circulating prostaglandin E2
(PGE2).
The placenta is a major site of fetal PGE2 production, thus the
removal of the placenta from the circulation causes circulating PGE2
concentration to decrease markedly. Further reduction occurs in the
concentration of PGE2 because of increased blood flow to the lungs (the site of
PGE2 metabolism). Functional closure of the ductus generally occurs within 72
hours of life, with anatomical closure by age 1-2 weeks. In summary, functional
postnatal circulation generally is established within 60 seconds; however,
completion of the transformation can take up to 6 weeks.
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