This study aims to compare human subjects receiving epidural bupivacaine local anesthetic at different doses and concentrations and to assess the effect on vasodilatation (sympathectomy), sensory and motor block.

The hypothesis is that the drug concentration is more important than drug dose in the determination of the extent and intensity of differential sympathetic, sensory and motor block following epidural local anesthetic.

There is controversy regarding the factors that determine the extent and intensity of epidural drug action. Most studies have focused on the inter-related triad of dose, concentration and volume.

We compare three groups: group A: 10ml of 0.5% bupivacaine; group B: 10ml of 0.25% bupivacaine and group C: 40ml of 0.0625% bupivacaine. When comparing these groups, groups A & B have the same volume of epidural drug, and groups B & C have the same total dose.

In particular this study compares different methods of assessing the sympathetic block (causing vasodilatation). These methods include skin temperature changes, hemodynamic changes and photoplethysmography.

Inclusion Criteria:

• Patients scheduled for extracorporeal shock wave lithotripsy (ESWL) under epidural anesthesia between March to September 1999

Exclusion Criteria:

• graded as ASA 3 or higher, below 18 or above 70 years of age, if they had peripheral vascular disease, baseline neurological deficit, or had any other contraindication for regional anesthesia.