Efficacy of a standardized postoperative analgesic protocol in obese patients compared to nonobese and overweight patients: a prospective observational cohort study using visual analogue scale and surgical pleth index

Background: Obesity alters the pharmacokinetics of drugs, including analgesics. The continuous worldwide increase in obesity puts more and more patients at risk for sub therapeutic analgesic treatment or increased toxicity. Objectives: The aim of this study was to determine the effect of obesity on the efficacy of a standard analgesic regimen consisting of paracetamol, ibuprofen, dexamethasone and piritramide PCIA. Design: An observational prospective study in which included patients were cohorted according to BMI in an obese group or a control group. All patients received the same postoperative pain regimen. Setting: Single center, tertiary care hospital, University hospital Ghent Belgium. Methods: Patients (18-70 years) undergoing laparoscopic bariatric surgery (obese patients) or laparoscopic procedures similar in duration and intensity as bariatric surgery (non-obese and BMI<30kg m-2) were included. Patients with preexisting liver disease, pregnancy, alcohol or paracetamol intake were excluded from the study. All patients received a standard analgesic regimen consisting of paracetamol, ibuprofen and piritramide. Data were collected at 9 time points during the first 30 hours postoperatively. Statistical analysis was performed using a linear mixed effects model. Main outcome measures: Pain intensity measured with the visual analogue scale (VAS) both at rest and in motion. Simultaneously the surgical pleth index (SPI) at rest and in motion was recorded. Results: 41 patients were enrolled, of which 13 were non-obese and 28 obese. Mean VAS at rest over all time points was 15.26mm in non-obese patients, compared to 23.94mm in obese patients with a mean difference of 8.68mm (95% CI 0.02 to 17.34). Three hours after first analgesic administration, obese patients scored 21.06mm (95% CI 8.85 to 33.28) higher compared to non-obese participants. We found no statistically significant difference in SPI between non-obese and obese patients. No correlation between VAS and SPI could be demonstrated. Conclusions: We found a significant effect of obesity on VAS, both at rest as in motion, in the first hours after surgery. SPI showed no correlation with patient reported pain intensity. An analgesic regimen of 4g paracetamol q6hrs might be insufficient in patients with obesity. Further studies on safety and efficacy are needed to elucidate this question.