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Dangerous Design Flaw in the Ohmeda Aespire Anesthesia System

Mychaskiw, George II DO; Morris, Steve

doi: 10.1213/01.ANE.0000149050.32419.20
Letters to the Editor: Letters & Announcements

Professor of Anesthesiology (Mychaskiw)

Director; Perioperative Biomedical Engineering, Department of Anesthesiology; University of Mississippi School of Medicine; Jackson, MS; (Morris)

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To the Editor:

We believe there is a dangerous design flaw in the Ohmeda Aespire Anesthesia system (Datex-Ohmeda, Helsinki, Finland.). Although this narrative is being prepared as part of a larger article on anesthesia machine design and safety, we feel that this matter is urgent enough to warrant expedited notification of the readership.

Recently during a routine anesthetic, one of our most experienced CRNA’s called for engineering assistance when the Model 7100 ventilator on a new Aespire anesthesia machine began to intermittently fail. During normal controlled ventilation of an adult patient, the ventilator would periodically shut down and a sustained pressure alarm would flash. After switching to bag ventilation, the ventilator would reset and then function for several minutes until it again repeated the failure cycle. The machine was removed from service and evaluated by biomedical engineering. They found that the machine would indeed abruptly shut off the ventilator and flash a sustained pressure alarm while maintaining a pressure of approximately 9 cm H2O in the circuit. The system would reset after several minutes of hand ventilation by bag. On further examination it was found that the waste gas scavenging system’s flow regulator had been turned nearly to off. On the Aespire system this is located, along with the scavenger bag, in a difficult to access location behind the left rear leg of the machine. It was determined that the low suction flow allowed waste gas to accumulate in the bag and when the pressure reached 9 cm H2O in the bag, the ventilator was shut off and the pressure was maintained throughout the patient circuit. We initially assumed this was simply a manufacturing defect, but were very confused by the fact that, even if a defective pressure release valve was not venting waste gas to the atmosphere, why was the waste gas scavenging system linked to the ventilator? After extensive discussion with Ohmeda’s technical services office, we were informed that this was, in fact, purposely designed to function in the manner we encountered! Apparently, the rationale for this design is that, in the event of low suction flow and a buildup of gas in the waste system, it is preferable to shut off ventilation and hold the patient at 9 cm of CPAP, rather than vent the gas to the room and contaminate the atmosphere. Ohmeda states that the threshold for ventilator shutoff is 7.5 cm H2O, but may vary with fresh gas flow. When we advised Ohmeda that this was unacceptable, they offered a choice of replacement valves that would either convert the system to a full passive unsuctioned or full unregulated active suctioned mode. (Strangely, the full active system still had a flow regulator on the suction hose near the line connector.)

We have subsequently filed an FDA Form 3500 expressing our concern about this design feature. Representatives from Ohmeda advise us that this is supplied as standard because some surgical facilities are unable to generate the suction flow required for proper evacuation of the system and do not want the full active system. Until recently, anesthesia machine design and appearance has largely reflected safety innovations as a result of lessons learned through clinical misadventure. Newer systems, however, are sleeker and more esthetically pleasing, while hiding crucial components, such as flow sensors, unidirectional valves, and waste gas scavenging systems. This is, in essence, a triumph of style over safety. Furthermore, the desire to accommodate hypothetical fears of trace anesthesia gas in the OR atmosphere has trumped the very real hazard to individual patients of interrupted ventilation and sustained airway pressure. Users of the Aespire anesthesia system should be aware of this design issue and take care to be sure waste gas is evacuated appropriately. On their newest model, the ADU, this full suction system is the only available waste gas scavenging system.

George Mychaskiw II, DO

Professor of Anesthesiology

Steve Morris

Director, Perioperative Biomedical Engineering; Department of Anesthesiology; University of Mississippi School of Medicine; Jackson, MS;

© 2005 International Anesthesia Research Society