Introduction: Fentanyl is a short-acting lipophilic opioid. Given the increased volume of distribution in people with a very high body mass index (BMI), fentanyl appears to act more effectively in obese people compared with normal-weight people.
Objectives: This study aimed to evaluate effect of spinal anesthesia with and without addition of fentanyl on the occurrence of urinary retention after knee replacement in patients with class III obesity.
Patient and Methods: This randomized single-blind clinical trial was conducted with the participation of 80 patients with a BMI >40 kg/ m² who were candidates for unilateral knee replacement with spinal anesthesia in 2019–20. The patients were randomly assigned to the intervention group (bupivacaine 15–20 mg + 25-μg fentanyl) and control group (bupivacaine (15–20 mg)) and compared in terms of urinary retention.
Results: There was no statistically significant difference between the groups in the time of first postoperative urination (P=0.706). In fentanyl group, twenty patients needed catheterization after surgery, while this was reported in 13 patients in the opioid-less spinal anesthesia group, which was not statistically significant (P=0.115).
Conclusion: Addition of 25 μg fentanyl to bupivacaine in spinal anesthesia can lead to a brief increase in the probability of urinary retention after surgery.
Trial Registration: The trial protocol was approved by the Iranian Registry of Clinical Trials (identifier: IRCT20160202026328N7; https:// www.irct.ir/trial/61035, ethical code; IR.TBZMED.REC.1400.820)