The purpose of this study is to compare the analgesic efficacy and side effects induced by administration of two different infusion rates of remifentanil, 0,05 mcg/ Kg/ min versus 0,1mcg/ Kg/ min, in order to determine which one is the most appropriate analgesic treatment to face extracorporeal shock wave lithotripsy.

Remifentanil is an ultra-short-acting oppioid, which is quickly metabolized by unspecific blood and tissue esterases . This feature vouches for a predictable pharmacokinetics and makes this drug suitable for administration in continuous intravenous infusion in order to achieve patient conscious sedation during non-invasive surgical procedures. Remifentanil could be particularly useful in day surgery setting, thanks also to its faster elimination than other oppioids. This property ensure a swift recovery from conscious sedation, reducing the period of time during which patient remains in recovery room and decreasing also the incidence of side effects, like PONV and respiratory depression. Conscious sedation with remifentanil can provide patient comfort and cooperation during surgical procedure, since the verbal contact with the patient is kept up. Extracorporeal shock wave lithotripsy (ESWL) is a non-invasive procedure, contemplated as the first-line treatment for renal and upper ureteral stones, if stone diameter is ranged from 5 to 20 mm . It can grind calculi into fine particles, that are expelled through micturition. Although many studies have been published in regard to continuous intravenous infusion of remifentanil, a small number of them focuses on which is the most appropriate infusion rate for ESWL procedure.

Inclusion Criteria:

• ASA (American Society of anaesthesiology) physical status between the I and class II
• Body mass index (BMI) between 18 and 30.

Exclusion Criteria:

• Patients who were unable to give informed consent or with diagnosis of depression
• Concurrent treatment with antidepressant
• Anxiolytic or with opioids or with history of abuse and dependence from these substances
• Allergy or intolerance to drugs administered in this study
• Severe cognitive deficits or psychiatric disorders
• Liver or renal impairment (aspartate aminotransferase > 40 UI/L alanine aminotransferase > 40 UI/L, creatinine > 2mg/dL)
• Abnormal values of coagulation (International normalized ratio > 1,2), platelet(< 100.000/µL), arrhythmias and / or defect of atrioventricular conduction.