Effect of preoperative ondansetron on postoperative patient controlled tramadol consumption

ABSTRACT

Tramadol is an atypical Opioid. It has a dual mechanism of action. The first mechanism is through its weak affinity to opioid receptors with some selectivity for mu-receptors and is produced mainly by its Ml- metabolite. The other mechanism relates to increased central synaptic levels of noradrenaline and serotonin, both are transmitters in the descending pathways that ensure analgesia. Both pharmacokinetic and pharmacodynamic interactions have been hypothsized between tramadol and the antiemetic Ondansetron. Ondansetron competes with tramadol for the Cytochrome P450-2D6 and therefore reduces its M1-metabolite. Ondansetron also blocks 5HT3 receptors, which partially contribute to tramadol analgesia. The aim of this study was to find out the clinical effect of preoperative prophylactic ondansetron administration on postoperative tramadol PCA demand. Sixty patients ASA I or II undergoing orthopedic surgery were studied. Patients were randomly allocated to receive either Ondansetron 4mg slowly IV at time of induction of anaesthesia or placebo. Intravenous Tramadol PCA was used for 24 hours postoperatively in both groups [Loading dose =1 mg/ kg., On-demand bolus = 20 mg. Lock-out interval = 10 minutes and total 4 hours dose limit =400 mg]. Tramadol consumption was recorded at I, 2, 4, 8, 12 and 24 hours following initiation of PCA. The incidence of postoperative nausea and vomiting was registered. The total 24 hours tramadol consumption in the ondansetron group was significantly higher than in the placebo group [612 +/- 72 versus 472 +/- 129.5 mg]. This higher consumption of tramadol occurred mainly during the first 4 hours, then the tramadol consumption was nearly similar in both groups after that. The overall incidence of nausea and vomiting was not significantly different between both groups [50% versus 40%]. A single prophylactic dose of ondansetron given at the time of induction of anaesthesia will increase the postoperative analgesic consumption of tramadol and does not reduce the incidence of PONV