In a multiple linear regression model in the ephedrine group (R2 = 63.6%; P = 0.03), only neonatal genotypes at codons 16 (p.16Arg/Arg, partial correlation coefficient = −0.54, P = 0.008) and 27 (heterozygous p.27Gln/Gln, partial correlation coefficient = −0.50, P = 0.02) and lower neonatal birth weight (partial correlation coefficient = 0.52, P = 0.01) had significant effects on UA lactate (lower UA lactate concentrations). Other factors (maternal genotype at codons 16 and 27 of ADRB2, dose of ephedrine, weight, height, body mass index, baseline systolic or diastolic blood pressure, and heart rate) were not significant predictors of UA pH (R2 = 53.5%; P = 0.15). (See Supplemental Digital Content 1, appendix illustrating the regression models with ADRB2 and NOS3, http://links.lww.com/AA/A257.) In the phenylephrine group (n = 49), the dose of phenylephrine administered to maintain baseline maternal blood pressure was not affected by maternal genotype at codon 16 of ADRB2 (Table 5). Maternal ADRB2 genotype at codon 16 did not predict UA pH (7.32 ± 0.04 in p.16Arg/Arg, 7.33 ± 0.03 in p.16Arg/Gly, 7.32 ± 0.05 in p.16Gly/Gly; P = 0.56). Neonatal ADRB2 genotype at codon 16 did not affect UA pH or UA lactate concentrations.
Overall, UA pH was predicted neither by maternal NOS3 genotype (7.30 ± 0.02 in p.298Glu/Glu, 7.27 ± 0.10 in p.298Glu/Asp, 7.27 ± 0.10 in p.298Asp/Asp; P = 0.78) nor by neonatal NOS3 genotype (7.27 ± 0.09 in p.298Glu/Glu, 7.27 ± 0.11 in p.298Glu/Asp, 7.26 ± 0.05 in p.298Asp/Asp; P = 0.98) (Table 6).
This Chinese cohort of healthy women scheduled for cesarean delivery had a distribution of ADRB2 genotypes and allele (genotype) combinations at codons 16 and 27 that was significantly different from that described in other obstetrical cohorts.5,7,8 In particular, in our previous work assessing hypotension and vasopressor requirement in a North American cohort of women undergoing spinal anesthesia for cesarean delivery,5 20% of women carried at least 1 Glu27 allele (heterozygous p.27Gln/Glu or homozygous p.27Glu/Glu). In this current cohort of Chinese women, only 7% of women were found to be heterozygous at codon 27, and no mother was Glu27 homozygous. A comparison between the American and Chinese cohorts reveals an overall haplotype distribution with a significant difference (P < 0.001). This relatively low occurrence of Glu27 homozygosity among Chinese cohorts has been reported.9,10
Another significant finding, also in contrast with our findings in the North American cohort, is that genotype of ADRB2 did not influence the dose of ephedrine administered to maintain maternal blood pressure during spinal anesthesia for cesarean delivery. Our previous work had described a presumed pharmacogenetic effect of ADRB2, with Glu27 carriers requiring lower doses of ephedrine to treat spinal hypotension.5 There are several possible explanations for these discrepant findings. The difference in genotype distribution according to ethnic background could explain why we could not find a pharmacogenetic effect in this Chinese group, because the 2 combinations (p.16Gly/Gly–p.27Gln/Glu and p.16Gly/Gly–p.27Glu/Glu) that were found to reduce the ephedrine requirement in the North American cohort were “underrepresented” in the current study. Alternatively, the dose response to adrenoceptor-agonists could be attenuated in Asians.11
Second, ephedrine was not given in a similar manner in both studies (bolus in the North American cohort versus continuous infusion in the current study), and the criteria applied for treatment of hypotension, and therefore targeted systolic blood pressure, were different (systolic blood pressure decrease >20% or to <90 mm Hg in the North American study versus near-baseline values in the current study). As a consequence, the total ephedrine dose was significantly higher in the current study than were the doses used in our previous work, and this strategy might overwhelm differences among genotype groups. It is also of course possible that our previous finding was a type 1 error (false positive) and that there is no effect of ARDB2 genotype on ephedrine requirements.
The most clinically relevant and intriguing finding was that UA pH was overall higher and UA lactate was lower in neonates that were Arg16 homozygous in comparison with neonates with the 2 other genotypes of ADRB2. Furthermore, among babies born to mothers receiving ephedrine, ephedrine dose was associated with neonatal acidemia (decreased UA pH) only in neonates carrying a Gly16 allele, but not in neonates who were Arg16 homozygous. Because there was no significant difference in ephedrine concentration as determined by maternal and umbilical cord assays (MA, UA, UV, or ratio of UA:UV and UV:MA) among genetic groups, any difference in metabolic markers are unlikely to have resulted from differential transplacental transfer of drug or a pharmacokinetic effect. Arg16 homozygous neonates seem to be protected from the risk of developing acidemia when exposed to ephedrine, irrespective of the dose given to the mother. Previous studies have demonstrated differential metabolic responses based on ADRB2 genotype,12–14 so it is reasonable to postulate that such genetic variants in the fetus could lead to altered responses to a given dose of a cardiac and vascular stimulant such as ephedrine.
One potential mechanistic explanation for the apparent protective effect of Arg16 homozygosity against neonatal acidemia could be increased desensitization of the β-adrenergic receptor in response to ephedrine. Arg16 homozygous individuals have been shown to undergo rapid desensitization in response to β-agonists15; therefore, it is possible that the continuous infusion of ephedrine could have resulted in a greater degree of desensitization, i.e., tachyphylaxis, in p.16Arg/Arg neonates. It should be noted that there is considerable controversy in the literature regarding the assumption of increased desensitization of Arg16 homozygotes in vivo,12,15 and the time course of desensitization in vivo is unclear. Therefore, conclusions about mechanism must be viewed as preliminary.
Genetic distribution of NOS3 p.298Glu/Asp was similar to that in previous reports in a Chinese population.16 A pharmacogenetic effect of NOS3 with an enhanced response to phenylephrine in subjects carrying the Asp298 allele has been shown in a study in Caucasian patients undergoing cardiopulmonary bypass.17 We did not find a difference in phenylephrine dose according to NOS3 genotype in this study, although our study was underpowered for this clinical outcome because of the low prevalence of the rare variant in this ethnic group. Other considerations include the very different study population and conditions (healthy pregnant women undergoing cesarean delivery rather than a cardiac population undergoing cardiopulmonary bypass), ethnicity (Chinese rather than Caucasian), and different mode of phenylephrine administration (infusion in our study rather than increasing bolus dosing), and the targeted blood pressure. Neonatal acidemia and other markers of fetal metabolism were also not associated with any specific genotype of NOS3 in either the phenylephrine group or ephedrine group. On the basis of our findings, NOS3 genotype may not play an important role in determining the response to phenylephrine given as an infusion to maintain baseline systolic blood pressure in pregnant Chinese women under spinal anesthesia.
Obvious limitations of this study relate to the overall small sample of patients. In addition, the possible effect of p.27Gln/Glu on maternal ephedrine response could not be examined because, contrary to our expectations, no mother was found to be homozygous for Glu at codon 27 (only 5 out of 9 possible combined genotypes were found in this cohort instead of the expected 6 of 9). Furthermore, because of the study design, only half of the neonates were exposed to ephedrine, and a smaller proportion of neonates exposed to ephedrine were found to be Arg16 homozygotes (10 out of 45) in comparison with the proportion of Arg16 homozygous neonates in the phenylephrine group (22 of 49). Thus, although we found that Arg16 homozygous neonates had higher pH values, and none had a pH below 7.28, we must acknowledge that this neonatal cohort (Arg16 neonates receiving ephedrine) consisted of only 9 babies. The proportion of Arg16 homozygous women was similar in the ephedrine group (18 of 45) and phenylephrine group (16 of 49), which was expected because women were randomly assigned to one treatment group or the other, so this uneven distribution of neonatal genotype is almost certainly a chance occurrence.
In conclusion, maternal genotypes of ADRB2 and NOS3 did not impact on the total dose of ephedrine or phenylephrine infusions administered to maintain maternal systolic blood pressure close to baseline during spinal anesthesia for cesarean delivery in a healthy cohort of Chinese women. Neonatal genotype and birth weight and not maternal genotypes or ephedrine dose were found to predict acid-base status and neonatal acidemia. Neonatal homozygosity for Arg16 of ADRB2, which was found to occur in >30% of babies in this Chinese cohort, seemed to protect from the risk of developing neonatal acidemia when mothers were treated with ephedrine. Whether this finding is specific to this Chinese cohort and can be replicated in this or other ethnic groups remains to be determined.
Name: Ruth Landau, MD.
Contribution: Design of genetic portion of the study, data analysis, manuscript preparation.
Name: Shih-Kai Liu, MD.
Contribution: Data analysis, manuscript preparation.
Name: Jean-Louis Blouin, PhD.
Contribution: Conduct of genetic analysis, data analysis, manuscript preparation.
Name: Richard M. Smiley, MD, PhD.
Contribution: Data analysis, manuscript preparation.
Name: Warwick D. Ngan Kee, MBChB, MD.
Contribution: Study design, conduct of study, data analysis, manuscript preparation.
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