Asymmetric onset of sympathetic blockade in epidural anaesthesia shows no relation to epidural catheter position.

A radiological study was performed of the relation between onset of sympathetic blockade in lumbar epidural anaesthesia and the position of the epidural catheter. In 20 patients scheduled for extracorporeal shock wave lithotripsy (ESWL), the onset of sympathetic blockade after epidural anaesthesia (catheter insertion at the presumed level L2-L3, and injection of 20 ml prilocaine 2% with epinephrine 5 micrograms/ml) was objectively evaluated by photoplethysmography. The onset was asymmetrical in 18 patients, and symmetrical in only two. Just before the start of ESWL, the position of the epidural catheter was checked by radiography after injection of 0.5 ml iohexol 300 mg/ml (Omnipaque 300). The radiopaque contrast medium was found median (n = 2), right (n = 7) and left (n = 11) of the midline. In only 9 patients was the earliest onset of sympathetic blockade correlated with the side of the catheter position, and thus no relation between catheter position and onset of sympathetic blockade was found.