A closed 'pop-off' valve and patient safety incident: A human factors approach to understanding error

A 3-month-old, 1.4 kg, female domestic short hair cat was scheduled for ovariohysterectomy surgery during a final year veterinary student clinical rotation. Shortly after transfer to the operating room (OR) and connection to the anaesthetic machine, signs consistent with high gas pressure build up in the anaesthetic breathing system were observed. Because a clear view of the anaesthetic machine was obscured, the cat was immediately disconnected from the breathing system and transferred to a different anaesthetic machine. Inspection of the OR anaesthetic machine revealed that the adjustable pressure limiting (APL) ('pop-off') valve was closed, despite completion of a machine checkout procedure at the start of the day. The valve was opened, the cat returned to the OR, and surgery completed. Recovery was uneventful. Investigation of the incident revealed several contributing factors leading to the APL valve being left closed.