Obstetric anesthesia controversies: vasopressor choice for postspinal hypotension during cesarean delivery.

The use of spinal anesthesia for cesarean delivery has increased with the introduction of small-gauge, noncutting spinal needles. Most women in Canada, the United States, Australia, and Europe requiring elective cesarean delivery receive some variation of a spinal anesthetic technique owing to the perceived advantages of a clear end point in locating the subarachnoid space, fast onset of local anesthetic effect, and reduced risk of local anesthetic toxicity. To successfully administer spinal anesthesia for cesarean delivery, however, requires consistent attention to details to minimize side effects, the predominant one being maternal hypotension. Maternal hypotension is an unwanted consequence of the physiologic onset of spinal blockade, and causes both maternal and fetal effects. Maternal symptoms include nausea, vomiting, and a sense of ‘‘impending doom’’ from inadequate cerebral perfusion. Inadequate treatment of hypotension can ultimately end with the loss of consciousness and cardiovascular collapse. The fetus is indirectly affected by the development of hypotension, because of its dependency on maternal uterine artery pressure for adequate uterine blood flow. With a persistent reduction in uterine blood flow, fetal acidosis will occur; uterine blood flow reductions of 65% led to acidosis in 10 minutes in the fetal lamb model. Although anesthesiologists do not allow hypotension to persist, successful treatment can occasionally take longer than desired. The incidence of maternal hypotension is reported to be 50% to 80% depending upon the definition of hypotension used, the position of