Spinal Anesthesia

Spinal (subarachnoid) anesthesia offers many advantages for cesarean delivery. It has a very rapid onset and provides a dense neural block. Because of the small doses used, there is little risk of local anesthetic toxicity and minimal transfer of drug to the fetus. In addition, failures (including incomplete or patchy blocks) are very infrequent with spinal anesthesia. Disadvantages of this technique include the finite duration of anesthesia and a higher incidence of hypotension.

Hyperbaric bupivacaine is the most commonly used agent for spinal anesthesia when performing cesarean sections. Its duration of action of 1.5 to 2 hours is perfectly matched to the duration of surgery in most cases. Although some authors have advocated a sliding scale of doses that depend on patient height, most practitioners now use a set dose of hyperbaric bupivacaine.^[168] As noted in Figure 58-10 , neither patient height, weight, nor body mass index appears to correlate with block height.^[168] Although increasing the dose of spinal anesthetic increases block height,^[169] doses above 15 mg significantly increase the risk of complications, including high block, and are not recommended. Spinal hyperbaric bupivacaine is marketed in concentrations of 0.5%, 0.75%, and 1%; however, availability varies from country to country. Although the extent of the blockade does not appear to be greatly affected by concentration, ^[170] the density of the sensory block may be better with the 0.75% than the 0.5% concentration. However, there seems to be little advantage and potential disadvantage to use of the 1% concentration. A recent comparison of 0.75% with 1% bupivacaine found no difference in onset time or quality of block, but an increased incidence of backache was reported in the 1% group.^[171]

Spinal anesthesia can be initiated with the patient in either the sitting or the lateral position and with either plain or hyperbaric solutions. Each method has its advantages and disadvantages. The sitting position, however, appears to be optimal for placement of a neuraxial block in an obese parturient. ^[172] Hyperbaric solutions have the advantage of greater predictability of block height than plain solutions do, and they allow the anesthesiologist the ability to adjust block height by adjusting table position. ^[173]

Despite achieving an adequate (T4) block, some women under spinal anesthesia will experience some degree of visceral discomfort during cesarean section, particularly in situations in which the obstetrician exteriorizes the uterus. The quality of the spinal anesthesia has been

Figure 58-10 Relationship between patient weight and height and block height during spinal anesthesia with 12 mg hyperbaric bupivacaine in 50 term parturients undergoing cesarean section. Circles represent one patient; squares, two patients; triangles, three patients; and diamonds, four patients. (From Shnider SM, Moya F: Effects of meperidine on the newborn infant. Am J Obstet Gynecol 89:1009–1015, 1964.)