Medishield AGS system and servo 900 ventilators. Investigation and remedy of hazard caused by defeat of 'low expired minute volume' alarm.

The use of the new Medishield anaesthetic gas scavenging system with ventilators in the Siemens-Elema Servo 900 series was investigated. It was discovered that this combination of equipment creates the hazard that a disconnexion in the patient breathing system may not be detected by the ventilator alarm. This happens because the suction generated by the scavenging system draws a considerable flow of room air through the expiratory system of the ventilator when a disconnexion occurs. Possible solutions to the problem were investigated, and it was discovered that the problem can be overcome by applying a small amount of positive end-expiratory pressure to counteract the suction from the scavenging system.