Meperidine infusion for postoperative analgesia in grossly obese patients

Postoperative analgesia was provided to nine grossly obese patients with a continuous intravenous meperidine infusion. The narcotic was infused at an initial rate of 1.3 mg . min-1 for 45 minutes, then 0.7 mg . min-1 for 30 minutes followed by 0.5 mg . min-1 for the next 24 hours. Pain was assessed at hourly intervals, serial blood samples were taken for measurement of plasma meperidine concentrations, and respiratory function was assessed by serial measurement of vital capacity and arterial blood gas analysis. Analgesia was gradual in onset and from six hours after commencement of the infusion, seven of the patients suffered little or no pain. Plasma meperidine concentrations reached a peak of 0.33 +/- 0.05 microgram . ml-1 at one hour decreased gradually after three hours. Surprisingly, there was poor correlation between analgesia and plasma meperidine concentration. All patients breathed spontaneously and maintained satisfactory blood gas tensions. However, there was a marked reduction in postoperative vital capacity. Thus meperidine administered by continuous intravenous infusion can provide good postoperative analgesia in the obese patient without producing respiratory depression. However, the lack of correlation between analgesia and plasma narcotic concentration suggests that further study is required before intravenous regimes can be prescribed by application of pharmacokinetic principles.