Critical Adverse Effect of Morphine in Pain Management

A 65-year-old female patient with a history of osteoarthritis and chronic pain presents to the hospital for a scheduled knee arthroplasty. The nurse is preparing to administer preoperative medications. The physician has ordered morphine sulfate 4 mg IV for pain management. The patient has a known allergy to codeine. Considering the patient's history and current medication order, which of the following should the nurse monitor for the most critical adverse effect associated with morphine administration?