Contribution of pain to inspiratory muscle dysfunction after upper abdominal surgery - A randomized controlled trial

Upper abdominal surgery causes respiratory muscle dysfunction. Multiple fac tors have been implicated in the occurrence of such dysfunction; however, t he role of pain remains unclear. To elucidate the role of pain, we studied 50 patients undergoing elective upper abdominal surgery in a randomized, co ntrolled investigation. Inspiratory and expiratory muscle function were ass essed through sniff mouth pressure (Psniff) and maximal expiratory pressure (MEP), respectively. Pain during the pressure maneuvers was assessed with a visual analog scale (VAS), Measurements were made before surgery (Session 1), 24 h after surgery (Session 2), and 1 h later, after intramuscular adm inistration of pethidine (analgesia group) or placebo (placebo group) (Sess ion 3). To evaluate the effect of pain, we used a mixed-effects model with random intercept, having either Psniff or MEP as the dependent variable and both surgical operation and the level of pain as fixed effects. Upper abdo minal surgery decreased Psniff in both the analgesia and placebo groups (fr om 70 +/- 15 to 42 +/- 11 cm H2O [p < 0.05] in the analgesia group, and fro m 69 +/- 15 to 42 +/- 10 cm H2O [p < 0.05] in the placebo group). Intramusc ular pethidine caused an increase in Psniff to 56 +/- 14 cm H2O (p < 0.05), whereas placebo had no effect. Pain increased comparably after upper abdom inal surgery in both groups (from 0.3 +/- 0.6 to 4.4 +/- 1.5) [p < 0.05] in the analgesia group and from 0.4 +/- 0.5 to 4.3 +/- 1.5 [p < 0.05] in the placebo group). Intramuscular pethidine decreased pain as measured by VAS s core to 2.1 +/- 1.0 (p < 0.05) in the analgesia group, whereas placebo had no effect. Psniff had a statistically significant relationship to pain (p < 0.001). Adjusting for the occurrence of surgical operation did not affect this result. MEP showed the same tendency as Psniff, but the observed chang es did not reach statistical significance. We conclude that pain contribute s to inspiratory muscle dysfunction after upper abdominal surgery.