What
precautions should be taken to avoid intrauterine fetal asphyxia?
Intrauterine fetal asphyxia is avoided by
maintaining normal maternal arterial oxygen tension (PaO2), PaCO2,
and uterine blood flow. Maternal hypoxemia may lead to fetal hypoxemia and even
fetal demise. General anesthe-sia is a particular risk to the pregnant woman
because man-agement of the airway can be difficult and the rate of hemoglobin
oxygen desaturation is increased due to the decreased functional residual
capacity and increased oxy-gen consumption. However, care must also be taken
during a regional anesthetic because a high segmental level of anesthesia
during a major conduction block, a toxic local anesthetic reaction, or
oversedation can also lead to a hypoxic event. High inspired oxygen tension
does not adversely affect the fetus even if 100% oxygen is administered.
Both maternal hypercapnia and hypocapnia can be
detrimental to the fetus. Severe hypocapnia produced by excessive positive
pressure ventilation may increase mean intrathoracic pressure, decrease venous
return, and lead to a decrease in uterine blood flow. In addition, maternal
alka-losis, as produced by hyperventilation, will decrease uterine blood flow
by direct vasoconstriction and will decrease oxy-gen delivery by shifting the
maternal oxyhemoglobin disso-ciation curve to the left. Severe hypercapnia is
detrimental because it is associated with fetal acidosis and myocardial
depression.
Both drugs and anesthetic procedures affect
uterine blood flow. Placental blood flow is directly proportional to the net
perfusion pressure across the intervillous space and inversely proportional to
the resistance. Perfusion pressure will be decreased by hypotension, which may
be due to the use of an epidural or spinal anesthetic, from aortocaval
compression in the supine position, or from hemorrhage. Vasoconstriction due to
the use of α-adrenergic drugs, decreased PaCO2, or increased
catecholamines such as occurs during pain, appre-hension, or light anesthesia,
will increase vascular resistance and decrease uteroplacental blood flow.
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