Fig. 4. Plasma concentration of total (  A ) and unbound (  B ) ropivacaine in patients receiving 0.2% ropivacaine (  open bars ) or saline (  closed bars ) through the iliac crest catheter. Measurements were made 24 and 48 h postoperatively. Values are presented as mean ± SD. * Significant differences between the two groups (  P < 0.05).