Dear SIRS:

The APSF is pleased to announce an important new technology initiative which will appear as a regular column in the APSF Newsletter. This feature is entitled “Dear SIRS” and refers to the Safety Information Response System. The purpose of this column is to expeditiously communicate technology-related safety concerns raised by our readers, with input and responses from manufacturers and industry representatives. This process was developed by Drs. Michael Olympio, Chair of the Committee on Technology, and Robert Morell, Editor of this newsletter. Dr. Olympio is overseeing the column and coordinating the readers’ inquiries and the responses from industry. “Dear SIRS” makes its debut in this issue with an important safety concern raised by James M. Berry, MD, and Steve Blanks, CRNA, from Vanderbilt University Medical Center.

Misplaced Valve Poses Potential Hazard

We have discovered what we believe to be an error in the assembly of 9 Datex-Ohmeda Aestiva anesthesia machines recently delivered and installed which, if not detected, has the potential to cause injury to anesthetized patients.

The problem is in the AGSS (active gas scavenging system) option which produces, when the evacuation hose becomes occluded, sustained airway pressures (PEEP) of up to 40 cm. This condition is exacerbated by high fresh gas flows and when mechanical ventilation is in use.
The AGSS is designed to have an opening in the bottom of a plastic receiver, providing relief of both positive and negative excess pressures. In what appears to be an assembly error, a negative pressure relief valve (similar to a circle system one-way valve) was installed in this opening (see photos 1 and 2). This provided relief of excess negative pressure (too much evacuation suction), but no positive pressure relief. This valve is used appropriately in the passive system, but the passive system also has a positive pressure relief in the upper portion of the receiver unit. Our best explanation is that the receiver assembly is composed of a top and a bottom section. We were (possibly) delivered units with the top receiver half from an active system and the bottom half from a passive system.

No injury has occurred in our institution as a result of this incident, but this discovery followed a number of our anesthesia staff reporting unusual ventilator behavior which we subsequently traced to low evacuation flows. The most dramatic incident was when, after a bed rolled over a gas scavenging hose, the Aestiva ventilator immediately stopped cycling in mid-exhalation. The evacuation hose (color coded purple) is equipped with a needle valve which is adjusted to provide moderate flows (as shown by a small ball indicator on the side of the absorber assembly). The presence of the mis-assembled scavenging receiver may be detected either by a temporary ventilator failure when the evacuation hose is manually occluded (hand crimped) or by gross fluctuations of the ball indicator during normal ventilator cycling.

The Datex-Ohmeda company has been prompt and thorough in their response to this incident. They are in the process of identifying AGSS equipped units that may have been assembled incorrectly and are currently pursuing action to remedy this situation.

James M. Berry, MD
Professor
Steve Blanks, CRNA
Chief CRNA
Nashville, TN

Manufacturer Provides Prompt Response

(In Reply)

Datex-Ohmeda would like to thank the APSF Newsletter for the opportunity to respond to the letter by Berry and Blanks.

The authors have correctly identified the root cause of the rising airway pressure encountered in some of their Aestiva anesthesia machines. There are two options for gas scavenging in the Aestiva. One is passive and the other, the option used by the authors, is an active system that uses vacuum to remove the scavenged gases from the patient circuit and from the ventilator drive system.

The AGSS units that were assembled incorrectly had a valve normally used for the passive systems installed in the base of the AGSS unit. The presence of this valve prevented excessive gases within the AGSS from escaping when the vacuum outflow was occluded, as described by the authors.

Datex-Ohmeda has identified the cause of the incorrect assembly and has instituted changes in the assembly process to avoid a repeat of this error. In addition, there is now an additional test of the AGSS to verify that all units have the correct valves in place. Datex-Ohmeda has also identified the entire population of AGSS units that may have been assembled incorrectly, has identified the location of the entire suspect population, and has begun an active Field Action to check the AGSS units, verify their proper manufacture and assure that any occlusion of the AGSS exhaust hoses will not produce a rise in the system airway pressure in the future.

Michael Mitton
Director of Clinical Affairs
Datex-Ohmeda, now a part of GE Medical Systems