A 56-year-old man undergoes a left upper lobectomy. An epidural catheter is inseed 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, and HCO3 - 28. Which of the following is the most appropriate initial therapy for this patient?