Complete mechanical expiratory obstruction as a result of expiratory valve malposition in an anesthetized dog

A female, mixed-breed dog, aged 8 years, presenting with left temporomandibular joint ankylosis caused by a soft tissue sarcoma was anesthetized for tracheotomy and caudal segmental mandibulectomy. After intramuscular sedation with methadone and dexmedetomidine, oxygen was delivered by face mask, and anesthesia induced and maintained with propofol. A tracheotomy was performed, and a cuffed endotracheal tube was inserted. A circle breathing system that had been leak-tested was connected to the endotracheal tube, and oxygen (2 L minute^-1) was administered. End-expiratory carbon dioxide partial pressure was undetectable despite manual ventilation. The breathing system was disconnected and capnograph function confirmed by the anesthetist exhaling into the sampling adaptor. Correct endotracheal tube placement within the tracheal lumen and absence of kinking was confirmed by direct visualization. The breathing system was reconnected to the endotracheal tube and the lungs were auscultated during manual ventilation, verifying airflow during inhalation. A rapid increase in breathing system pressure was observed on the airway manometer despite an open adjustable pressure-limiting valve. The problem was resolved by using a second anesthetic machine: capnograph displayed a waveform and breathing system pressure was 0 cmH₂O. Throughout troubleshooting, saturation of arterial hemoglobin (pulse oximetry) remained > 97% and no arrhythmias were detected. A misassembled expiratory valve seat that immobilized the flutter valve was found as the cause of the problem on postanesthetic inspection of the anesthetic machine. Correcting the expiratory valve seat placement restored normal valve function. This case highlights the importance of confirming proper unidirectional valve seat assembly before anesthesia. Performing a leak test alone is insufficient to detect unidirectional valve malfunction. The lack of a safety mechanism to ensure correct valve seat placement reveals a design vulnerability that could lead to fatal anesthetic complications.