Progressive Hypercapnia and Rigidity during Isoflurane Anesthesia in a Dog

An 8-year-old neutered male Toy Poodle weighing 7.2 kg underwent isoflurane anesthesia for computed tomography (CT) followed by planned excision of a left cranial cervical cutaneous-subcutaneous mass.During CT, the dog breathed spontaneously with intermittent manual ventilation as needed, and end-tidal carbon dioxide (ETCO 2 ) ranged from 35 to 55 mmHg.Following hospitalization and fluid therapy, body temperature had decreased to 38.4°C before anesthesia.At the start of surgery, body temperature was 38.8°C and ETCO 2 was 59 mmHg.Within approximately 5 min, ETCO 2 increased to 75 mmHg, body temperature increased to 39.2°C, and mild limb rigidity was observed.Isoflurane was discontinued, the anesthetic system was flushed with 100% O 2 , intermittent positive-pressure ventilation was initiated, and active cooling was intensified.Despite these interventions, ETCO 2 increased to 110 mmHg, body temperature reached 41.0°C, generalized rigidity and ventricular tachycardia developed, and cardiopulmonary arrest followed.Return of spontaneous circulation was not achieved, and the dog died.The temporal pattern was compatible with a hypermetabolic anesthetic crisis, with malignant hyperthermia considered a major differential diagnosis.This case highlights the importance of recognizing progressive unexplained ETCO 2 elevation and muscle rigidity as actionable early warning signs during volatile anesthesia.