Statistical analysis
Study population was described according to pH groups. Counts and
proportions were reported for categorial variables, and means +/-
standard deviations (SD) for continuous variables. Normality for
continuous variables was explored both in a graphical way with
histogram. Groups were compared using Chi-squared test for qualitative
variables (unless expected counts were less than 10, in which case
Fisher’s exact test was used) and ANOVA for normal continuous variable
(homogeneity of variance was analyzed using Bartlett’s test) or
non-parametric Kruskal Wallis test for non-normal variables.
Correlation between lactates rate distribution and blood pH rate was
explored with Spearman’s Rho correlation test.
The date of initiation of VA-ECMO cannulation was chosen as the date of
origin for survival outcomes. Survival data were described with
Kaplan-Meier survival curves. Median survival time of the study
population was reported with a 95% confidence interval (95%CI).
Log-rank test was used to assess comparison between pH groups. Survival
rates at 30 days and one year were estimated for each group. Cox
proportional hazard models were used for the assessment of the
association between pH groups and overall survival and a backward
stepwise selection was performed for variable selection. The final model
selected covariates with significance level 0.10. Two covariates
considered as potential confounders were forced into the model: sex and
ECMO indication. Interaction with initial diagnosis was tested. We
checked the collinearity between the covariates in the multivariable
analysis using the variance inflation factor (VIF) for diagnostic for
multicollinearity. A decision curve analysis was performed to
graphically analyze the relationship between pH and the futility of the
procedure.
Statistical analyses were performed with R version 4.0.1.