Patient-Controlled Analgesia (PCA) optimization in respiratory compromise

A 54-year-old male with a history of chronic pain syndrome is prescribed intravenous patient-controlled analgesia (IV-PCA) with morphine following open spinal fusion surgery. The PCA pump is set with a bolus dose of 2 mg, lockout interval of 8 minutes, and a 4-hour limit of 30 mg. On postoperative day 2, the patient reports inadequate pain control despite frequent demand attempts. Arterial blood gas analysis shows pH 7.32, PaCO₂ 52 mmHg, PaO₂ 78 mmHg, and HCO₃⁻ 24 mEq/L. Which of the following is the MOST appropriate intervention to optimize analgesia while addressing the underlying physiological concern?