Cause of Device-Related Incident
Device factors; User errors

Clinical Specialty or Hospital Department
Anesthesia; Clinical/Biomedical Engineering

Device Factors
Improper maintenance, testing, repair, or lack or failure of incoming inspection

Document Type
Hazard Reports

External Factors
*Not stated

Mechanism of Injury or Death
Barotrauma; Suffocation

Support System Failures
*Not stated

Tampering and/or Sabotage
*Not stated

User Errors
*Not stated; Device misassembly

Anesthesia Unit Absorbers, Carbon Dioxide [10-140]; Anesthesia Units [10-134]

Disk Misplacement in Anesthesia Exhalation Check Valve

Hazard [Health Devices Oct 1982;11(12):325-6]


An exhalation check valve disk on an anesthesia absorber migrated from its normal position and occluded the hose connection to the anesthesia ventilator. Personnel installed a new disk, perhaps not realizing the hazard associated with a migrated disk or assuming that the original disk had been removed. The resulting disk arrangement caused an occlusion that prevented exhaled gases from refilling the ventilator bellows or reservoir bag. Pressure in the patient breathing circuit helped to hold the misplaced disk in the occluding position. The ventilator pressurized the breathing circuit in a few breaths and the patient became hypoxic and suffered a pneumothorax.


In the reported incident, the disk was probably lost in the valve body during exhalation valve checking or servicing, was not removed immediately (removal can be difficult, especially without forceps or tweezers), then was forgotten. Although this problem occurred with a specific anesthesia absorber, we believe it is possible in any unit with an exhalation valve of similar configuration (i.e., with a downward-projecting outlet beneath the valve). Clearances around the valve seats (with the valve domes removed) and projections on the disks vary widely among brands. Disks most susceptible to migration are flexible ones that lack projections that prevent them from falling or sliding into the valve bodies. (Valves on some units are surrounded by webs or inserts with small holes; these permit gas flow around the valve but prevent migration of the disk.)

While the migration of a disk and installation of a second one may seem unlikely, a similar incident was reported previously.(1)

ECRI tested one model of anesthesia system with an extra valve disk in the same position described in the incident; we found that the system delivered a few normal breaths again. Operation was intermittent at best. With complete occlusion, patient circuit pressure can rise quickly (10-20 sec) to 90 cm H2O, a dangerously high level. Ventilator operation was greatly restricted during occlusion and produced different operating sounds. However, these sounds are not a reliable indication of occlusion in all circumstances.


  1. Test the exhalation valves of anesthesia absorbers; determine whether a valve disk will slide into the valve body without deliberate force. If it does, determine if the manufacturer offers a guard or other modification that can be installed in this valve to prevent occlusion by a misplaced disk. Most such units already have these guards installed. Check that they are in place.
  2. All disks, like all critical replacement parts (in anesthesia systems and in inventory) should be under tight controls, especially during servicing. The search for a missing disk should include examination of the exhalation valve body. Before discarding a defective disk, cut it in half to prevent its reuse.
  3. As part of a pre-use check of anesthesia systems, ventilate a simple lung simulator (even a hand-constricted reservoir bag to permit pressure buildup). This will reveal any occlusion or intermittent ventilation produced by a misplaced disk (see Health Devices 13:324, October 1984).
  4. Use a ventilation alarm that is activated not only by a disconnect but also by stacked breaths (which we observed in our testing). (We evaluated ventilation alarms in Health Devices 10:204-20, July 1981.)


  • Anesthesia Unit Absorbers, Carbon Dioxide [10-140]
  • Anesthesia Units [10-134]


  1. Dean HN, Parsons DE, Raphaely RC. Bilateral tension pneumothorax from mechanical failure of anesthesia machine due to misplaced expiratory valve. Anesth Analg 1971;50:195.

Cause of Device-Related Incident

Device factor: Improper maintenance, testing, repair, or lack or failure of incoming inspection

User error: Device misassembly

Mechanism of Injury or Death

Barotrauma; Suffocation

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