Why Question Established Practice?
During hypotension resulting from conduction anesthesia in gravid ewes, uterine blood flow (UBF) decreased roughly in proportion to the decrease in maternal blood pressure. Ephedrine or mephentermine significantly increased UBF over that accomplished by metaraminol. Presumably, the preferential effects of these agents were the result of increased cardiac output owing to inotropic and chronotropic actions. However, UBF never exceeded 90% of prespinal levels with any vasoactive agent, and, for a given maternal system, the UBF response was variable, generally increasing but frequently remaining constant or decreasing. For these reasons, all other methods of combating hypotension should be used initially. If vasopressors are still required, agents of choice are those whose principal mode of action lies in cardiac stimulation rather than peripheral vasoconstriction.