Cause of Device-Related Incident
Device factors; External factors; Support system failures; User errors

Clinical Specialty or Hospital Department
Anesthesia; Clinical/Biomedical Engineering; Obstetrics and Gynecology; OR / Surgery

Device Factors
Design / labeling error; Device failure; Improper maintenance, testing, repair, or lack or failure of incoming inspection

Document Type
Hazard Reports

External Factors
Medical gas and vacuum supplies

Mechanism of Injury or Death
Suffocation; Underdose

Support System Failures
Poor prepurchase evaluation

Tampering and/or Sabotage
*Not stated

User Errors
Failure to perform pre-use inspection; Inappropriate reliance on an automated feature

UMDNS
Anesthesia Units [10-134]; Anesthesia Unit Ventilators [10-145]; Breathing Circuits, Anesthesia [10-139]

Anesthesia Ventilators with Descending Bellows: The Need for Appropriate Monitoring



Hazard [Health Devices Oct 1996;25(10):391]

Problem

Several healthcare facilities have requested guidance regarding the use of anesthesia ventilators with descending bellows. These facilities are concerned that, because a descending bellows will cycle up and down even when a leak or disconnection is present in the breathing circuit, caregivers may assume that the patient is being adequately ventilated when this is not the case.

Although no formal guidelines or current standards restrict the use of descending bellows, ECRI believes that this type of bellows can present a risk to the patient and should be used only when appropriate monitoring measures are taken.

Background

Automatic ventilators include bellows that expand (fill with gas) during expiration and are compressed (to deliver gas) during inspiration in response to cycling ventilator drive pressure. Some units have ascending (rising or upright) bellows, which are attached to the base of the bellows housing and expand upward. Other units have descending (hanging) bellows, which are suspended from the top of the bellows housing and expand downward.

For an ascending bellows to expand, the expiratory pressure provided by the patient's passive lung and chest wall recoil must be sufficient to overcome the weight of the bellows. Conversely, the weight of a descending bellows assists expansion, allowing the bellows to expand—and actually draw in gas—even when no expiratory pressure is produced by the patient. Thus, when a leak or disconnection is present in the breathing circuit, a descending bellows will still expand (drawing air from the atmosphere through the leak and into the bellows) and be compressed as the ventilator cycles.

In the United States, anesthesia systems with descending bellows ventilators have not been sold since the early 1980s.

Discussion

Caregivers often regard the up-and-down movement of the bellows, which may be visible if the bellows housing is translucent, as an indicator of the proper cycling of the automatic ventilator. However, when a ventilator with descending bellows is used, this assumption may be incorrect. As mentioned above, if the breathing circuit between a patient and a descending bellows becomes disconnected or develops a leak, the bellows will continue to fill and empty as the ventilator cycles. Observers may interpret the movement of the bellows as an indication that the patient's lungs are being ventilated properly. In contrast, with the same disconnection or leak, an ascending bellows will either 1) partially deflate and cycle at a lower position, indicating some ventilation compromise, or 2) fully deflate and stop moving, definitively indicating that automatic ventilation has ceased.

Various breathing circuit monitors currently in routine use can help detect leaks or disconnections in systems containing a descending bellows. However, such monitors are not foolproof. For example, an airway pressure monitor is often relied on as a disconnect monitor, but it can be fooled if there is sufficient flow resistance (as would be caused by partial blockage at the site of a disconnection) to produce a back pressure greater than the low-pressure alarm limit set on the monitor. An expired volume (Vexp) monitor equipped with a standard sensor, which is routinely placed in the expiratory limb of a breathing circuit, can likewise be fooled. If a circuit disconnection occurs, a descending bellows will continue to draw gas through the expiratory limb with each ventilator cycle, and this intermittent gas flow will be erroneously sensed as normal function by the monitor.

Monitors that better detect inadequate ventilation (with either type of bellows) are available, but are not in universal use. For example, a carbon dioxide (CO2) monitor can be used to reliably detect leaks and patient disconnections. Also, facilities can use Vexp monitors that have small, low-dead-space, lightweight sensors that can be placed at the patient's Y-piece. In this position, the sensor can detect the cessation of gas delivery to the patient. However, the mere presence of such monitors will not ensure that the patient is adequately ventilated. For example, if anesthesia care providers forget to (or choose not to) turn on the monitors, or if they do not enable the alarms, set the limits properly, or believe that sounding alarms signify a condition that needs to be addressed, the monitors will not perform their intended function.

Recommendations

ECRI believes that the use of descending bellows is acceptable only if the facility can ensure that the following conditions are met:

  1. Staff, including occasional users, are fully informed of the limitations of this equipment and are trained in its use.
  2. Appropriate monitors with apnea alarm capabilities are used with the system. Specifically, either or both of the following should be used: a CO2 monitor or a Vexp monitor with a sensor placed at the patient's Y-piece. Additional monitors that should be used with the system include an inspired oxygen monitor, an airway pressure monitor, appropriate physiologic monitors, and, perhaps, gas/agent monitors.
  3. Care is taken to ensure that the monitors are turned on, that their alarms are enabled, and that alarm limits are set to appropriate values.

If these conditions cannot be met, then descending bellows on anesthesia ventilators should be replaced. For some systems, the ventilator can be removed and replaced with an ascending bellows ventilator; for others, the entire anesthesia system may need to be replaced. Contact the manufacturer for options.

When determining which option to choose, keep in mind that many anesthesia systems with descending bellows 1) may be nearing, or have already passed, the end of their useful life, 2) may lack some of the safety features of newer machines, and 3) may be difficult to maintain as parts for the older units become scarce. Therefore, replacing just the ventilator or equipping a system with additional monitors may not be cost-effective in the long run.

UMDNS Terms

  • Anesthesia Units [10-134]
  • Anesthesia Unit Ventilators [10-145]
  • Breathing Circuits, Anesthesia [10-139]

Cause of Device-Related Incident

Device factors: Design/labeling error; Device failure; Improper maintenance, testing, repair, or lack or failure of incoming inspection

User errors: Failure to perform pre-use inspection; Inappropriate reliance on an automated feature

External factor: Medical gas and vacuum supplies

Support system failure: Poor prepurchase evaluation

Mechanism of Injury or Death

Suffocation; Underdose


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