Complete ventilation failure due to a defective heat and moisture exchange filter.

Heat and moisture exchange filters are routinely used in anaesthesia to provide passive humidification and protect the breathing circuit. We report a case of a 3-year-old patient where a complete circuit obstruction due to a defective heat and moisture exchange filter occurred. This resulted in a total inability to ventilate the patient's lungs, initially attributed to bronchospasm, followed by the need for cardiopulmonary resuscitation. Manufacturing defects resulting in such critical incidents are rare but remain an important potential differential when managing difficulty in ventilation. This incident underscores the need to maintain a high index of suspicion for equipment malfunction when difficulty in ventilation arises, even in the presence of plausible clinical causes. Automated anaesthetic machine checks do not assess circuit patency; therefore, manual verification, such as the two-bag test, remains essential following any circuit change. This case also illustrates how human factors and system gaps can align to permit equipment defects to reach the patient, while effective teamwork and crisis management can help to mitigate harm. Routine inclusion of circuit patency testing, simulation-based training and rigorous manufacturing quality control are suggested for preventing recurrence and enhancing patient safety in anaesthetic practice.