Boxplot of the effect of remifentanil on heart rate before (A) and after (B) noxious stimulation for nonnociceptive conditions and before (C) and after (D) noxious stimulation for intubation. Boxplots represent the median and 25th and 75th percentiles, and the whiskers extend to the most extreme data points; outliers are plotted individually (black dots). The Spearman correlation is given (r_S), with **P < 0.01 and ***P < 0.001. A quadratic polynomial is fitted to the data to guide the eye.