RSBI = breathing frequency (breaths/minute)/tidal volume (liters) (1) Oxyhemoglobin saturation was measured using a pulse oximeter (SPO2). Data were presented as mean ± standard deviation (SD) or median (interquartile range). For each outcome variable, comparisons were planned a priori. A paired t or unpaired t test was used for within-group or between-group comparisons. For non-normal data, the Mann-Whitney test was used. The chi-square test or Fisher's exact test was used to compare proportions of categorical variables between the two groups. A p value less than 0.05 was considered to be statistically significant. All statistical analyses were performed using SAS software version 9.3 (SAS Institute Inc., Cary, NC) and Microcal Origin version 4.0 (Northampton, MA, USA).