The characteristics of postoperative fentanyl effect-site concentration during intravenous fentanyl analgesia after posterior lumbar spine fusion

Objective: To examine the characteristics of postoperative fentanyl effect-site concentrations during intravenous analgesia in patients requiring or not requiring a fentanyl bolus and in patients with or without postoperative nausea and vomiting (PONV). Design: Retrospective observational study. Setting: University-affiliated general hospital. Patients: Sixty patients who underwent posterior lumbar spine fusion. Main outcome measures: The authors simulated the fentanyl effect-site concentration for 48 postoperative hours and compared it between patients who did and did not require a fentanyl bolus and between patients who did and did not experience PONV. Results: At the end of anesthesia, the fentanyl effect-site concentration was similar between 37 (61.7 percent) patients requiring and 23 (38.3 percent) patients not requiring a postoperative fentanyl bolus (p = 0.97). Within the first 12 postoperative hours, the concentration decreased in both groups (p<0.01). The fentanyl effect-site concentration was higher in patients requiring a postoperative fentanyl bolus (within 12 hours, 1.4 ± 0.32 ng/mL vs 0.89 ± 0.35 ng/mL; between 12 and 48 hours, 0.94 ± 0.19 ng/mL vs 0.57 ± 0.09 ng/mL) (p < 0.05). PONV occurred in 22 (36.6 percent) patients, but more so in women (68.2 percent; p < 0.01); PONV was similar between patients requiring and not requiring a fentanyl bolus (p = 0.78). Between the 12th and 48th postoperative hours, fentanyl effect-site concentrations were higher in patients with PONV (0.61 ± 0.10 ng/mL vs 0.57 ± 0.10 ng/mL, p = 0.03). Conclusions: Patients needing a postoperative fentanyl bolus require a higher fentanyl effect-site concentration possibly because of individual variations, and PONV depends on the postoperative fentanyl effect-site concentration.