Report of a Ventilation Mode Selector Switch Failure
Stuart Tilman, MD, Sem Lampotantg, PhD and
Dietrich Gravenstein, MD
Department of Anesthesiology, University of Florida, Gainesville, Florida
The pre-use anesthesia apparatus checkout recommendation (AACR ) required
by the FDA prior to administering anesthesia ascertains, among other
elements, the integrity of the ventilator. We describe a case where the
checkout detected a previously unknown and invisible failure mode of the
ventilator in an Aestiva anesthesia machine (Datex-Ohmeda, Madison, WI).
We were alerted to a trauma patient’s arrival in our emergency department.
Because the on- call staff had not used the anesthesia machine in the trauma
room, the AACR was performed. The ventilator was checked by placing the
Aestiva into ventilator mode with a fixed rate and volume and using the
reservoir bag placed onto the Y-piece to simulate lungs, as described in
step 12 of the AACR. It was noticed that the anesthesia machine’s ascending
bellows were not returning fully to the initial volume and were continuing
to lose volume with each ventilatory cycle despite a fresh gas flow of 5
L/min. The ventilation mode selector switch was flipped to the manual
ventilation mode position and the machine examined. Despite a visual check
of the circuit, the seating of the soda lime canisters and the hoses, no
source for the leak could be found. Flipping the selector switch back to
mechanical ventilation mode again, failed to reproduce the leak. In a hurry
to begin but unwilling to start a case with a suspected variable leak in the
ventilation system we quickly confirmed the availability and functioning of
a self-inflating resuscitator-bag and troubleshot one additional time. With
the ventilation mode selector switch set to manual mode there was no leak
but on switching back to mechanical mode, the large leak reappeared. It was
then we noticed that the selector switch did not appear to be at its
mechanical limit. Pressure was applied to the lever and it moved to its
proper position, causing the leak to disappear. Having established the cause
of the variable leak, confirming it could be corrected and having back-up
equipment available, we began the case and finished without complication.
The machine was taken out of service following our case our Clinical
Engineering group disassembled the machine to identify the nature of the
failure mode.. Two actions occur when the lever is turned from bag to
machine ventilation: 1) the ventilator is turned on. This does not require
the lever to be at its mechanical limit. 2) the bag is isolated from the
breathing circuit. This action requires the lever to be at its mechanical
limit. Further examination found a viscous build up on the lever that
prevented it from reaching its mechanical limit. We speculate the source of
this accumulated material was the lubricant for the machine components or
cleaning solutions used between surgical cases.
Our case report illustrates the importance of performing the AACR. It is
being prepared for publication to notify users of the Aestiva anesthesia
machine at other institutions. The manufacturer was notified of this latent
error, to allow an engineering solution before a patient comes to harm.
