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

Clinical Specialty or Hospital Department
Anesthesia

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

Document Type
Hazard Reports

External Factors
*Not stated

Mechanism of Injury or Death
Barotrauma

Support System Failures
Failure to train and / or credential

Tampering and/or Sabotage
*Not stated

User Errors
Incorrect control settings

UMDNS
Anesthesia Unit Ventilators [10-145]; Breathing Circuits, Ventilator [15-003]

Barotrauma from Anesthesia Ventilators



Hazard [Health Devices Nov 1988;17(11):354-5]

Problem

ECRI investigations of barotrauma associated with the use of anesthesia ventilators emphasize the need to bring two problems to the attention of users. The first problem concerns an operating characteristic of anesthesia ventilators with rising bellows. The second problem can develop if the airway hose from the breathing circuit to the ventilator is occluded.

Discussion

High Pressures from Rising Bellows

Unlike earlier (descending) bellows, the rising bellows must be filled before patient ventilation can be started. If the ventilator is on and in its inspiratory phase when the flush valve is used to fill the bellows (either at the start of a case or, for example, following disconnection to suction the patient), the patient may be exposed to high pressures in the breathing circuit until the ventilator cycles into exhalation. The problem can be avoided by filling the bellows only when the ventilator is turned off.

When the ventilator is in its inspiratory phase, driving gas (at a pressure of up to 65-75 cm H2O) enters the bellows assembly to press the bellows down to deliver a breath. If the flush valve is activated to fill an empty bellows during this phase, the driving gas will keep the bellows from rising until the pressure in the breathing circuit exceeds the driving pressure. This exposes the patient to the maximum available driving system pressure, which some patients will not be able to tolerate.

If the bellows is filled before the ventilator is turned on or when it is in standby mode, no driving gas pressure is applied to the bellows assembly, so the bellows can be filled with a minimal rise in breathing circuit pressure. This is the only acceptable way to avoid delivering the ventilator's maximum pressure.

Most anesthesiologists recognize the danger of high breathing circuit pressures from using a flush valve in conjunction with an anesthesia ventilator. However, because the bellows does not rise when users attempt to fill it in the described situation, users may not recognize the high pressures being generated. A user-adjustable high-pressure alarm will give some measure of safety. However, if the alarm is set too high or the user does not respond to it quickly, the protection is lost.

Breathing Circuit Occlusion

If the airway hose from the breathing circuit to the ventilator is occluded, this eliminates all pressure-limiting features from the patient portion of the breathing circuit. (When using an anesthesia ventilator, all pressure-limiting and alarm functions are typically in the ventilator.) Fresh gas flow from the anesthesia unit to the patient circuit can cause a rapid pressure increase because there is no place for the gas to escape.

A typical fresh anesthesia gas flow (4-6 L/min) can raise the breathing circuit pressure to a hazardous level (90 cm H2O or more) in 10-20 sec. The anesthesiologist's attention may easily be diverted to other functions (e.g., obtaining a manual blood pressure measurement) for this period of time, exposing the patient to these pressures before the problem is recognized and corrective action is taken. If the flush valve is activated when this airway hose is occluded, pressures will rise so high and so quickly that barotrauma can occur before the operator can respond.

Occlusion of this section of airway hose is highly unlikely in normal use. However, ECRI has reported on several different types of occlusions that occurred in ventilator circuits:

  1. A ventilator hose fitting was occluded because of a molding error that resulted in mold flash across the lumen (Action Item, Health Devices Alerts, Accession No. 03079, 1981).
  2. A misplaced one-way valve disk occluded the ventilator connection to the breathing circuit (Health Devices 11:325-6, October 1982).
  3. Misconnection of a vacuum line directly to an anesthesia ventilator scavenging port disabled the overflow (and pressure-relief) valve (Health Devices 16:178-9, May 1987).
  4. A breathing circuit filter (such as those often used in anesthesia ventilator hose circuits) became occluded when water entered it and wet the entire filter medium (Health Devices 12:25-6, November 1982).

The occlusion problems described are not unique to any particular manufacturer or anesthesia ventilator model. Occlusion can occur with all types of anesthesia ventilators, not just the rising-bellows type.

In some cases, occlusion may occur unexpectedly and suddenly (e.g., item 2 above), and there may be no way of preventing the incident. Constant vigilance and readiness to respond may minimize the hazard to the patient. More often, occlusion is likely to be present when the system is set up (e.g., item 1) or to develop over time (e.g., a filter, such as the one described in item 4, that gradually occludes with use). Appropriate pre-use testing must be conducted to identify occlusions present at the time of setup so that they can be corrected before a patient is connected. Gradual occlusions will result in increasing breathing circuit pressure, which must be investigated and corrected.

Recommendations

  1. Always fill the bellows of an anesthesia ventilator before turning the unit on (at the beginning of a case or after an interruption of the case). Reactivate alarms and reset alarm limits as appropriate; some units will revert to default limits when turned on.
  2. Confirm proper ventilator operation in the pre-use testing of an anesthesia system (see test 13 in ECRI's Pre-Use Checklist for Anesthesia Units.
  3. Make sure that all users are familiar with the hazards of activating the flush valve when an anesthesia ventilator is in use and the importance of maintaining constant vigilance and readiness.

UMDNS Terms

  • Anesthesia Unit Ventilators [10-145]
  • Breathing Circuits, Ventilator [15-003]

Cause of Device-Related Incident

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

User error: Incorrect control settings

Support system failure: Failure to train and/or credential

Mechanism of Injury or Death

Barotrauma


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