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

Upper abdominal surgery causes respiratory muscle dysfunction. Multiple factors have been implicated in the occurrence of such dysfunction; however, the role of pain remains unclear. To elucidate the role of pain, we studied 50 patients undergoing elective upper abdominal surgery in a randomized, controlled investigation. Inspiratory and expiratory muscle function were assessed 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 administration of pethidine (analgesia group) or placebo (placebo group) (Session 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 abdominal surgery decreased Psniff in both the analgesia and placebo groups (from 70 +/- 15 to 42 +/- 11 cm H(2)O [p < 0.05] in the analgesia group, and from 69 +/- 15 to 42 +/- 10 cm H(2)O [p < 0.05] in the placebo group). Intramuscular pethidine caused an increase in Psniff to 56 +/- 14 cm H(2)O (p < 0.05), whereas placebo had no effect. Pain increased comparably after upper abdominal 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 score 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 changes did not reach statistical significance. We conclude that pain contributes to inspiratory muscle dysfunction after upper abdominal surgery.