Introduction & Objective: Fentanyl has been used to improve quality and increase the duration of sensory block and analgesia after surgery in spinal anesthesia, but the effects of different doses are not still correctly specified. The aim of this study was to compare the duration of sensory block and side effects of adding different doses of intrathecal fentanyl to lidocaine 5% in spinal anesthesia.

Materials & Methods: 140 adult patients undergoing elective surgery under spinal anesthesia were enrolled in a double blind placebo-controlled clinical trial. For spinal anesthesia in all patients, 100 mg of 5% intrathecal lidocaine was used and based on the drug added, patients were randomly assigned to one of four groups of control (2 ml distilled water), group two (25 microgram fentanyl), three (50 microgram fentanyl) and four (100 microgram fentanyl) groups. Duration of sensory block and the occurrence of complications were evaluated.

Results: The groups were similar regarding demographic variables. (P>0.05) Mean duration of analgesia in the control group was 100.28±25.05 minutes and in group two was 112.28±18.4 minutes and in the third group was 135.28±10.63 minutes, and in group four, it was 140±12.42 minutes. Except for the differences of the control group with group two and those of group three with group four, the differences between all groups were statistically significant. (P<0.001) The frequency of sedation and respiratory depression in was significantly higher in groups three and four compared to the control group and group two. (P<0.01) The frequency of itching in the control group was lower than those of groups three and four and the patients in group two experienced less itching compared to group four (P<0.01). The frequency of nausea and vomiting and shivering in the control group was higher than those of groups two and three and four(P<0.01).

Conclusion: Adding 50 microgram of fentanyl to 100 mg intrathecal lidocaine significantly increases the duration of analgesia, but it is also associated with the risk of respiratory depression.