During a routine laparoscopic cholecystectomy in a 28-year-old woman, the anesthesiologist notices an unexplained rise in end-tidal CO₂ (ETCO₂) from 38 to 50 mmHg over 10 minutes, muscle rigidity on palpation, and a core temperature of 38.8°C. The patient is on sevoflurane and nitrous oxide. Succinylcholine was not used. What is the most appropriate immediate action?