During actual and simulated spinal anesthesia in 21 term gravid ewes, induced maternal hypotension was associated with a proportionate decrease in uterine blood flow (UBF). Under these circumstances, vasopressor drugs administered intravenously restored maternal blood pressure (MBP) to normal without improving uterine blood flow. However, rapid infusions of 5% dextrose or dextran solution significantly improved both MBP and UBF proportionately. These results indicate that the prophylactic or therapeutic use of vasopressor agents in conjunction with spinal or caudal anesthesia during pregnancy is contraindicated. Rather, therapy designed to increase effective circulating blood volume should be initiated. This includes a rapid intravenous infusion of dextrose solution, elevation of the legs, and displacement of the uterus to the left to reduce vena caval obstruction and increase venous return to the heart.
Greiss FC, Crandell DL. Therapy for Hypotension Induced by Spinal Anesthesia During Pregnancy: Observations on Gravid Ewes. JAMA. 1965;191(10):793–796. doi:10.1001/jama.1965.03080100011002
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