ost anesthesiologists take little interest in the waste anesthetic gas disposal (WAGD) system beyond the connection of the anesthesia machine to the vacuum outlet, but there are new and serious reasons why anesthesiologists should be concerned about what happens “behind the walls,” especially if they are contemplating the purchase of new anesthesia machines.

The Medical Gas Professional Healthcare Organization (MGPHO) has reports of eight incidents of vacuum pump failure and pump fires, all associated with the procurement of new anesthesia equipment. These fires have presented as flames coming from the pump, burnt-through metal pump housings, pump exhaust fires and possibly internal pump fires. Although none has so far been reported to have altered patient outcomes, a risk is clearly present to the facility and hospital personnel.

The very limited information available indicates that one of several possible causes for these occurrences are the anesthesia machines and WAGD interfaces that cause the pumps to work much harder in the face of an increased ambient oxygen level within the WAGD system.

The one standard in the United States that deals with the subject of WAGD, the National Fire Protection Association’s NFPA 99-2002 Health Care Facilities Handbook, permits five different ways to implement a WAGD system and many variations within those basic implementations. The decision about which of these implementations to install is often driven primarily by a desire to simplify the engineering and a poorly understood perception of lowest cost. Efficacy and clinical function have simply been assumed.

The most common implementation of WAGD in the United States under NFPA 99-2002 is to pipe the waste gas into the medical-surgical vacuum system. This is not done under the European standard (EN737) as the vacuum levels are far too great. This approach is perceived to be inexpensive but means that, in a very large number of cases, waste anesthesia gases are passed through an oil-lubricated pump. Although the hazard of passing waste anesthetic gas, with its potentially high oxygen and nitrous oxide content, to an oil-containing pump must be instantly obvious, the practice is one of very long standing and appears to have been free of reported problems until recently.

The recent incidents appear to be the simple result of connecting new anesthesia systems to existing vacuum pipelines with no appreciation by anesthesiologists that the new systems have requirements different from those of the older machines and no appreciation by facility engineering that the new machines presented hazards different from those older machines.

More modern WAGD interfaces on new anesthesia systems bring with them a much higher flow requirement (25-50 lpm versus 6-9 lpm). Some anesthesia machines also expel their ventilator drive gas into the waste gas stream, unlike older machines. While this method is effective in eliminating a potential source of workspace contamination and does offer clinical benefits in some designs, this drive gas can be pure oxygen, potentially increasing the total quantity of oxygen entering the WAGD line. These machines can be specifically modified to operate from medical compressed air, but many anesthetizing locations are not provided with medical air outlets.

In a controlled study involving four of a potential 12 newer anesthesia machines connected to the WAGD system, the oxygen level in the exhaust of the oil-lubricated vacuum pump was found to reach 35 percent (which caused immediate termination of the study).

Changes in anesthesia machine design must drive corresponding changes in the piped WAGD systems to ensure that these very controllable hazards are reduced or eliminated. A WAGD implementation should be the one that best fits the machines and procedures used by the anesthesiology staff. Such implementation requires that the system provide a sufficient volume of flow, equipment that is safe with the gases they can expect to encounter and vacuum levels appropriate to ensuring control of the pressure in the breathing circuit. It is the anesthesiology staff members who can provide these details and who must ensure that any new WAGD implementation in their institution meets these criteria.

It is essential that the WAGD systems not become an obstacle to medical practice or conversely that medical practice create an uncontrollable hazard at the pump. This conflict can arise when new machines are to be connected to a legacy WAGD system already in place. The conflict is easily resolved by ensuring that before new anesthesia machines are put into clinical service, the WAGD system is examined for any possible hazards. At this writing, the hazards to be checked include the presence of oil in the WAGD producer (the vacuum pump) and the producer’s capacity to handle the higher flows required by the newer machines. Solutions may include using compressed air as a drive gas for ventilators on the newer machines, increasing the capacity of the vacuum pumps, avoiding the use of oil-lubricated pumps or the use of a nonflammable lubricant, or installing an EN737-style, low-vacuum WAGD system.

It should be stressed that these eight reported events are not known to be the result of any negligence on the part of either the anesthesia machine manufacturers or the facilities engineering community but rather an unintended consequence of more modern standards rubbing up against a traditional way of doing things.

If you require more information, a detailed engineering-based survey of waste anesthetic gas systems and the options available to the facility, engineer and user of these systems is available at no charge at <www.beaconmedaes.com>.

Bibliography:

CEN EN 737-2 1998 Medical gas pipeline systems, Part 2: Anaesthetic gas scavenging disposal systems — Basic requirements. Comite Europeen de Normalisation (European Standards Organization).

National Fire Protection Association. NFPA 99-2002 Health Care Facilities Handbook. Quincy, MA; 2002.

Mark Allen is Director of Marketing at Beacon Medaes in Charlotte, North Carolina.

David E. Lees, M.D., is Professor and Chair, Department of Anesthesiology, Georgetown University Medical Center, Washington, D.C. He is the immediate past ASA Liaison to the National Fire Protection Association.