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An understanding of neonatal circulatory pathology
is intrinsically linked to an understanding of circulatory adaptation
to extrauterine life (see Chapter 43). In
the fetal circulation, better oxygenated blood is returned from
the placenta to the fetus via the umbilical vein. This blood is
streamed by the ductus venosus across the right atrium, through
the foramen ovale, and into the left atrium, facilitating delivery
of the best oxygenated fetal blood to the brain and upper body.
Blood returning via the vena cavae is streamed through the right
side of the heart and into the descending aorta via the ductus arteriosus.
This blood preferentially streams through the ductus because of
arteriolar constriction and the high vascular resistance in the
fetal lungs. The right-to-left ductal blood flow supplies the lower
part of the body and also returns blood to the placenta via the umbilical
arteries, which arise from the iliac arteries.
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At birth, this blood flow pattern changes quickly. The lungs
expand with the first breaths, the pulmonary arterioles dilate,
right heart pressures fall, and blood pours into the pulmonary circulation
to collect oxygen. The removal of the low-resistance placenta from the
systemic circulation increases resistance and pressure on the left
side of the circulation, while the pulmonary blood flow increases
the left heart preload. The result is a dramatic increase in the
workload of the left heart. The muscle in the wall of the ductus
arteriosus constricts in response to rising oxygen levels, closing
functionally within the first 24 hours after birth and structurally
after several days to become a fibrous band.
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During the last trimester, much of the fetal cardiopulmonary
development is in preparation for the changes that have to occur
at birth. Babies born prematurely have exquisite circulatory vulnerability
during this period of the transitional circulation. More mature
babies are also vulnerable if born in a compromised condition or
if they become unwell shortly after birth.
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Poor color, increased heart rate, prolonged capillary refill,
and low urinary output suggest circulatory compromise. Blood measurements such
as low pH and rising lactate can supplement the clinical assessments.
These indicators are useful in identifying the baby with severe
circulatory compromise, but they have limited accuracy for babies
with lesser degrees of compromise.2
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Blood pressure can be accurately measured and continuously monitored
if there is intra-arterial vascular access. Because blood pressure
is relatively easy to monitor, it has traditionally been the primary
indicator of neonatal circulatory status, and much conventional
circulatory support has focused on increasing the low blood pressure.3 Strong
data exists for the importance of a normal blood pressure range4 (Fig. 56-1), but controversy arises over what
constitutes an adequate blood pressure that is, the blood pressure
below which organ injury can result.5 Further,
an emerging body of evidence questions the accuracy of blood pressure
as a gold standard for circulatory well-being.6-8 There ...