In this study, we found that pretreatment with the dose of 8 mg of ondansetron did not reduce the incidence and intensity of propofol injection pain compared to pretreatment with lidocaine with the dose of 40 mg. Compared to the normal saline, it did absolutely not support the analgesic effect of pretreatment with intravenous ondansetron. The result of this negative analgesic effect of ondansetron might be from the many different proposed mechanisms of propofol injection pain.
Propofol causes pain by direct irritation of afferent nerve endings in venous endothelium and the indirect effect from the activation of the Kinin cascade. The activation of the Kinin cascade causes the release of prostaglandin E2 (PGE2), which produces local vasodilatation and increased vascular permeability. In consequence, there is increased contact between propofol and free nerve endings.10 A meta-analysis conducted by Pei and colleagues11 showed that ondansetron which is a distinctive 5-HT3 antagonist, can effectively prevent propofol injection pain when combined with occlusion technique, and the effect is similar to that of magnesium sulfate and lidocaine. Ambesh et al.7 reported that the overall incidence of propofol injection pain in the control group was 55%, compared with 25% in the 4 mg ondansetron combined with venous occlusion technique. Alipour et al.12, also studied the venous occlusion technique combined with pretreatment of lidocaine or ondansetron. Only one person in the group receiving saline solution (1.7%) had no complaint about propofol injection pain. In contrast, the number of patients without pain was 39 patients in the lidocaine group (69.64%) and 22 patients in ondansetron group (39.28%). The efficacy of pain relief in both ondansetron and lidocaine groups from our study was inferior to these previous studies.7, 11, 12 The explanation may be due to the venous occlusion technique, which was not applied in our study. The venous occlusion technique was usually performed with a rubber tourniquet on the forearm. The pressure of the tourniquet was between 50 and 70 mmHg, and it was applied for 30 to 120 seconds. With this combination technique, the number needed to treat (NNT) to prevent propofol pain with 40 mg of lidocaine compared with 40 mg of lidocaine only (without venous occlusion technique) was 4.3 to 1.813. The activation of the nerve fibers responsible for pain transmission resulting from the direct irritation effect of propofol on the inner wall of the blood vessels may be the primary or main mechanism of the injection pain. In addition, the principal mechanism of action of lidocaine as the local anesthetic agent is through the blockade of voltage-gated sodium channels leading to the blockade of action potential propagation that can prevent direct irritation of afferent nerve endings from propofol injection. Therefore, the direct analgesic effect of lidocaine was more effective when there was a long enough duration for the drug to take action during the venous stasis from tourniquet occlusion.14 Previous studies in animal models reported the ability of ondansetron to block sodium channels and serotonin (5-HT3) receptors.4,5 Our hypothesis was that the less analgesic properties of ondansetron by sodium channel block were probably not the principal action of ondansetron. In addition, local anesthetics contain hydrophilic and hydrophobic structures separated by an intermediate amide or ester linkage, a structure that ondansetron does not have. Therefore, ondansetron may have less efficacy even after increasing the dose to 8 mg in our study.
The factors that are associated with propofol injection pain include the size of the intravenous catheter, site of injection, speed of injection, and lipid solvent of propofol.15 These confounders were controlled in this study, all patients received the emulsion of 1% propofol in a mixture of medium-chain and long-chain triglycerides as an available preparation in our institution. This study had a lower incidence of propofol pain, compared to the previous study in our institution (85% vs 99%). The possible assumption was the different doses of propofol the patients received before pain assessment. In our study, the propofol dose was 50 mg in every patient but it was ¼ of the induction dose, which was varied in each patient in the previous study. This 50 mg dose of propofol may have a sub-hypnotic effect that affects the patient’s interpretation.
There were no significant differences between the incidences of postoperative nausea and vomiting among the three groups. This may be due to the wide variation in the surgical duration. Even the 8 mg of ondansetron is an adequate dose to prevent postoperative nausea and vomiting. However, the time of administration is also important. The surgical duration in our study commonly lasted 4 hours, longer than the effective duration of ondansetron which should be within 4 hours. The effective antiemetic property of ondansetron usually occur within 30 minutes after administration, so it is, therefore, better to administer ondansetron before the end of the operation. One of the lethal adverse effects of ondansetron is prolonged QT which can produce significant arrhythmias. Nevertheless, there was no patient with prolonged QT or arrhythmia in our study.
The limitation of this study was the administration of a sub-hypnotic dose of propofol before pain assessment. Therefore, a reliable pain assessment can be confusing.