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.