Statistical analysis
Categorical variables are described as percentages, and continuous variables are expressed as the means or medians with dispersion measures. The Kruskal‒Wallis, and Pearson chi-square tests were used to perform uni and multivariate comparisons of quantitative and qualitative variables, respectively. A longitudinal analysis with repeated measures of SI was performed using a linear mixed-effects model, where we tested the main association of NLA on the change in SI during labour and postpartum, comparing the association of interventions on SI across labour and postpartum. The analyses were fitted using an unstructured random slope and intercept, using the R package ‘lme4’. We chose this structure following the recommendation of using it on outcomes with a ratio scale, such as SI.
We tested whether the changing trend was different between the NLA and non-NLA groups. The primary outcome was the SI values for a patient over time during labour and postpartum periods. Therefore, the primary hypothesis of the study can be formally stated as a test of whether there is a time × intervention interaction. We tested the interaction by fitting a multilevel model with SI as the dependent variable at each time and with the time period, analgesia status (NLA or non-NLA), and the NLA-time period interaction as fixed effects. We assumed that the time between each measure (antepartum and postpartum) was equal for statistical analysis.
The association between NLA status and SI was adjusted in the linear mixed-effects model by maternal age (older than 35 years), haemoglobin level, cervical dilation, and nulliparity. We categorised continuous variables and entered them as terciles in the multilevel model to account for the nonlinearity of the relationship with SI. We also fitted the models for each component of the SI (HR and SBP). We excluded confounders with p values >0.50 in the fitted model with all covariates and excluded these nonsignificant covariates from further analysis. The associations for all other assessments were deemed significant if the p-value was <0.05. We reported the β-coefficients of each fitted model and estimated the 95% confidence interval (95% CI). Finally, we performed sensitivity analyses to measure changes in the β-coefficients and significance by type of NLA, including only the most frequently used type of analgesia worldwide (i.e., epidural analgesia).