The aims of this study are to provide pharmacokinetic-pharmacodynamic data to quantify the analgesic and side effect profiles of epidural methadone, fentanyl and morphine. The investigators will compare the analgesic effect at three dermatomes to assess the rostral spread of drug, the investigators will assess plasma levels to assess the systemic redistribution of drug and the investigators will assess surrogate markers of central opiate effects (nasal capnography and pupilometry). The investigators hypothesize that due to the long-duration of action of methadone, and its intermediate lipophilicity, that methadone will provide a predominantly segmental analgesia of long duration of action, with low rostral spread and low direct central depressant effects (including respiratory depression).

Inclusion Criteria:

• Patients (men and women) scheduled for extracorporeal shock wave lithotrypsy (for nephrolithiasis) under regional anesthesia.

Exclusion Criteria:

• Inability to understand consent form; poor communication Refusal to sign consent form Contraindication to regional anesthesia Chronic opioid administration ASA classification 3 or greater Age < 18 or > 70