A 56-year-old man undergoes a left upper lobectomy. An epidural catheter is inserted for postoperative pain relief. Ninety minutes after the first dose of epidural morphine, the patient complains of itching and becomes increasingly somnolent. Blood gas measurement reveals the following: pH 7.24; PaCO2=58;PaO2=100;HCO3-=28. Initial therapy should include