Fig. 2. Umbilical arterial to maternal arterial (UA/MA) ratios of estimated unionized plasma concentrations of ephedrine. Values were calculated by application of the Henderson-Hasselbalch equation to measured data from the previous study.1 Box plots  display the 25th, 50th, and 75th percentiles as horizontal lines  on a bar, whiskers  above and below the box indicate the 90th and 10th percentiles, and data beyond the 10th and 90th percentiles are displayed as individual points .