Note that the administration of opioids for labor analgesia can also slow gastric emptying. The routine use of antacids to increased gastric pH is advocated by many. Note that metoclopramide may not effectively reverse the decreased motility caused by opioids. The placenta serves as the interface between mother and fetus allowing for the exchange of physiologically important substances including oxygen, carbon dioxide, waste products of metabolism, drugs, etc. Uterine blood flow is one critical determinant of the proper functioning of the placenta and thus the health of the fetus.
Flow is also inversely related to uterine vascular resistance .
Blood flow is reduced by drugs or events that decrease perfusion pressure or increase vascular resistance. Although it is estimated that the flow can decrease by half before fetal distress is seen there may be unmeasurable fetal effects at higher levels of flow and there is little reserve in the fetus for reduced flow. Alpha agonist drugs can produce direct uterine vascular constriction and therefore are relatively contraindicated. Uterine constriction can also occur with the pain of labor and therefore adequate anesthesia may actually improve fetal condition. As far as anesthesia techniques it is likely that it is their effect in decreasing maternal blood pressure that results in deceased uterine flow rather than direct uterine effects that results in difficulties for the fetus.