The effects of residual pain on oxygenation and breathing pattern during morphine analgesia

To determine the influence of pain on opioid-induced respiratory depression, we studied oxygenation and breathing patterns in 40 patients scheduled for knee surgery during postoperative patient-controlled analgesia (PCA). After 1 h of morphine PCA, patients were randomized to receive either 20 mL of placebo or bupivacaine 0.25% through a crural nerve catheter and allowed to use PCA for one more hour. Abnormal breathing events were identified and characterized by using the Edentrace II device (Nellcor, Jouy-en-Josas, France). The Spo(2) below which the patient spent 25% and 50% of a studied period was calculated (Spo(225), Spo(250)). Pain relief with regional analgesia increased the incidence of abnormal respiratory events associated with oxygen desaturation: during the second period, the pain score was lower in the bupivacaine group (0.7 +/- 1 vs 4.1 +/- 1.2), morphine consumption was larger in the placebo group (4.2 +/- 1.3 vs 0.7 +/- 1.4 mg), and there were more abnormal obstructive breathing events in the bupivacaine group (11 +/- 16 vs 3.7 +/- 4.3). Spo(225) and Spo(250) were lower in the bupivacaine than in placebo group (91.5% +/- 2.8% vs 93.1% +/- 2.1%, 92.9% +/- 2.4% vs 94.2% +/- 1.8%). Implications: Pain relief with regional analgesia in patients previously treated with opioids increases the incidence of abnormal respiratory events associated with oxygen desaturation.