Data analysis
stroke, Uncertain
STROKE
The distribution of continuous variables was tested by the Shapiro–Wilk test. The variables were expressed as mean ± standard deviation, median (interquartile range) or number (percentage). The comparison of variables between different study times was performed with the paired Student's t-test or the Mann–Whitney test, depending on data distribution. Variables in volume responders and non-responders were compared using the Fisher’s exact test or the Wilcoxon test, depending on data distribution.Receiver operating characteristic (ROC) curves (with 95% confidence interval) were generated to describe the ability to detect volume responsiveness of the following variables: the PEEP-test-induced percent changes of CI (∆CI), PPV (∆PPV), arterial pulse pressure (∆PP) and stroke volume. For each variable, an optimal threshold value was determined to maximize the Youden index (sensitivity + specificity – 1).Gray zones were calculated using the method defining three levels of response: positive, uncertain, and negative. Uncertain responses were defined using a two-step procedure. We first calculated the 95% CI of the Youden’s index resulting from a 1000 population bootstrap [Based on a previous study evaluating the hemodynamic effects of PEEP [