Evaluation of analgesic efficacy and associated plasma concentration of tramadol and O-desmethyltramadol following oral administration post ovariohysterectomy

Tramadol is used in both human and veterinary medicine to treat postoperative pain. In human subjects, variation in metabolism of tramadol has led to inconsistent analgesia in some individuals. The purpose of the current study was to evaluate tramadol as a sole postoperative analgesic and to compare drug plasma concentration to clinical pain score. A high or low dose of tramadol was randomly assigned and administered to 14 mixed breed female dogs after ovariohysterectomy. The Short-Form of the Glasgow Composite Measure Pain Scale was used for pain evaluation post-operatively. Plasma was collected for evaluation of tramadol and O-desmethyltramadol concentrations. The effect of weight and dose on pain scores as well as how pain score correlated with plasma concentration of tramadol and O-desmethyltramadol was evaluated. A significant difference in pain score was noted between doses when weight classes were pooled. The plasma concentrations did not correlate with pain score. Based on our results, the use of tramadol as a sole analgesic agent provides inadequate postoperative pain control but further evaluation of tramadol as a postoperative analgesic agent is needed.