Barotrauma from Anesthesia Ventilators
Hazard [Health Devices Nov 1988;17(11):354-5]
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.
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
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
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
- 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).
- A misplaced one-way valve disk occluded the ventilator connection to
the breathing circuit (Health Devices 11:325-6, October 1982).
- Misconnection of a vacuum line directly to an anesthesia ventilator
scavenging port disabled the overflow (and pressure-relief) valve (Health Devices 16:178-9,
- 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,
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
- 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.
- Confirm proper ventilator operation in the pre-use
testing of an anesthesia system (see test 13 in ECRI's Pre-Use Checklist for
- 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.
- 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