β₂-adrenoceptor genotype affects vasopressor requirements during spinal anesthesia for cesarean delivery

Background: Maternal hypotension is common after spinal anesthesia for cesarean delivery. There is wide variability in the incidence and severity of hypotension and in the response to treatment. The β₂ adrenoceptor (β₂AR) possesses several polymorphic sites. Codons 16 (Arg16Gly) and 27 (Glu27Gln) have been shown to affect desensitization of the receptor. The goal of this study was to determine whether genetic variants of the β₂AR alter incidence of hypotension or the amount of vasopressor treatment required during spinal anesthesia for cesarean delivery. Methods: One hundred seventy healthy women undergoing elective cesarean delivery were studied. Spinal anesthesia was performed with 12 mg hyperbaric bupivacaine, 25 μg fentanyl, and 200 μg morphine. Hypotension was treated with ephedrine and/or phenylephrine intravenously, and β₂AR genotype at codons 16 and 27 was determined. Analysis of variance was used to compare variables between genotypes, with data expressed as mean ± SD. Results: Ephedrine or phenylephrine was used in more than 90% of patients, with no difference in the incidence of hypotension between β₂AR genotypes. However, there was a significant effect of genotype on the amount of vasopressor required. Gly16 homozygotes received significantly less ephedrine (18 ± 14 mg) than Arg16 homozygotes (28 ± 13 mg) and Arg16Gly heterozygotes (30 ± 20 mg; P = 0.0005). Glu27 homozygotes required significantly less ephedrine than Gln 27 homozygotes (14 ± 13 vs. 30 ± 19 mg; P = 0.002). Gln27Glu heterozygotes received less ephedrine than Gln27 homozygotes (23 ± 16 vs. 30 ± 19 mg; P = 0.03). Conclusions: Glycine at position 16 and/or glutamate at position 27 of the β₂AR leads to lower vasopressor use for treatment of hypotension during spinal anesthesia.