Patient-Controlled Analgesia and Opioid-Induced Respiratory Depression

A 68-year-old man with COPD (FEV1 42% predicted) and sleep apnea is admitted for open prostatectomy. Postoperatively, he is prescribed intravenous patient-controlled analgesia (PCA) with morphine. The nursing staff notes that despite adequate pain control, the patient is increasingly somnolent and his oxygen saturation drops to 88% on room air. Which of the following modifications to his PCA regimen is most appropriate to mitigate opioid-induced respiratory depression while maintaining analgesia?