Figure 1. Correlation between measured (volunteers) or calculated (patients) cerebrospinal fluid (CSF) concentrations of clonidine and percent residual pain. Residual pain was determined by pain report to noxious thermal stimulation in volunteers receiving a 700-micro gram epidural clonidine bolus (dashed line), [19]by computer-controlled infusion (solid line), [20]or by amount of patient-controlled analgesia (PCA) morphine use in postoperative patients receiving epidural clonidine infusions (dotted line). [21]