International standards have changed the way these systems are designed and installed.

Engineers have issues with Waste Anesthetic Gas Disposal (WAGD) systems.

The assumptions engineers have been using to design WAGD systems are being upset by some changes in the anesthesia machines that are being connected. Clients are calling with complaints and, in extreme cases, serious problems.

North American medical gas designers have over the years largely come to share a common set of assumptions about WAGD:

  • Dumping the WAGD into the medical vacuum source is cheap. This is assumed to be true in equipment terms and also in the designer’s own time — no line sizing or pump sizing needs to be done other than what must be done for the vacuum anyway.

  • Any oxygen will arrive at the pump sufficiently diluted to render it harmless.

  • Any other problem will be handled by someone else.

    These assumptions have lead to the most common implementation of WAGD in North America today — a WAGD terminal or two at each anesthetizing location piped into the medical vacuum line and hence into the medical vacuum pump. Many of these vacuum systems are then connected to an oil-lubricated pump.

    In no area of medical gases are international practice and North American practice more widely divergent. International standards (ISO) are the result of much effort in Europe toward perfecting WAGD. They have made some interesting strides forward.

    European systems are designed to be intrinsically safe. That means that the system protects the patient, no matter what the anesthesiologist does or doesn’t do. ISO systems cannot expose the breathing circuit to the intense vacuum in the medical vacuum piping.

    In these systems, WAGD is always implemented with a dedicated producer and a dedicated piping network. They do not use any components that are incompatible with the waste gases or with oxygen.

    In addition, the systems are designed around low-cost, low-horsepower regenerative blowers instead of relatively expensive pumps, and the terminal inlets used are very simple. This makes the systems relatively inexpensive, no small consideration in today’s environment.

    Unexpected Results

    It appears that ISO assumptions about WAGD are no longer merely of curious interest. The major anesthesia machine manufacturers operate globally and these manufacturers have begun to bring anesthesia systems originally designed in Europe into the North American market, along with their ISO assumptions about WAGD. The result is that two unexpected consequences are being reported:

  • Fires in vacuum pumps.

  • Vacuum pumps running excessively.

    There have been no reports of fires caused by WAGD for many years. However, beginning in the last half of 2002 and continuing through to the present, there have been several reports of fires in pumps. The reports range from flashes at the exhaust through to one case of explosive destruction of a pump. In all the cases reported so far, the pumps were in dual-use WAGD and medical vacuum service, and all were oil-lubricated.

    As for excessively running pumps, in several cases, vacuum pumps in dual-use and in dedicated WAGD service are running much harder than expected.

    Through the cooperation of Datex-Ohmeda, a well-known manufacturer of anesthesia systems, we were able to determine a possible set of causes for these circumstances. These bear directly on the assumptions under which WAGD have been designed and installed:

  • There are now in the U.S. market some anesthesia systems with modified interface valves which require larger inflows than previous machines. The “traditional” interface valve would draw something like 6-9 liters per minute when in operation, whereas these newer interface valves are calibrated to draw 40 lpm at 12 inches of vacuum (and could, of course, draw more at higher vacuum levels). This is consistent with what is expected under ISO. However, the standard sizing assumptions used in North America for WAGD assume only 1 scfm (28.3 lpm) per anesthetizing location.

  • Some anesthesia systems are now venting their ventilator drive gas (the gas used to power the ventilator itself and not breathed by the patient) into the waste gas stream. In older machines, this drive gas was vented into the room. This gas may be pure oxygen, and may be venting the entire duration of the case. The potential for exposure of the pump to elevated oxygen levels is therefore increased.

  • While at first blush it may seem irresponsible for the anesthesia machine manufacturers to make these changes without notice, it’s important to understand that what they have done is entirely consistent with the rules for WAGD as the National Fire Protection Association (NFPA) has them today.

  • NFPA has recommended that WAGD systems be compatible with oxygen for many years. Although it has been watered down somewhat over the years, NFPA 99 still carries a warning about the mixing of WAGD and medical vacuum (see NFPA 99, 2002, A- and also NFPA “Health Care Facilities Handbook,” 2002 edition, commentary on NFPA does not stipulate an inflow, but does require that the pump used in such a dual-use system “be adequate to handle the volume” (NFPA 99, 2002 The new machines clearly will work on any system that complies with these rules.

  • As these newer machines come online, the first anyone knows of any change is when the facility begins experiencing pump problems. The problems will worsen as these new machines gradually replace older anesthesia systems still in use. Thus, it is essential that designers and operators change their outdated assumptions about WAGD.

  • And there is plenty of scope for change. NFPA 99 permits at least five different WAGD implementations. They are: 1) Dual-use WAGD/medical vacuum (high vacuum); 2) WAGD into a dedicated pump (high to moderate vacuum); 3) WAGD into a blower or fan (low vacuum); 4) WAGD into a venturi-driven inlet which is driven by some system other than medical air (instrument air is ideal); and 5) WAGD handled passively, e.g., by ventilation and air changes (see Chapter 5, Environmental Systems).

    Selecting the Correct WAGD Implementation

    What factors should be weighed when selecting a WAGD implementation? There are several, and the weighting to be given to each will vary from facility to facility.

    Effectiveness. Will the system do the job of keeping the workspace free of waste gas? In reality, efficacy has less to do with the type of system selected than the design and installation of the system.

    Patient safety. Will the system protect the patient and ensure the anesthesiologist’s control of the procedure? Here, the low vacuum implementations are to be preferred over the high vacuum implementations due to the intrinsic safety implied in a lower vacuum.

    Cost. Which system is least expensive to implement and operate? Evaluating this is complex and the result varies dramatically between facilities. In general, low-vacuum systems are less expensive than are high-vacuum systems. Low-vacuum systems are also typically lower maintenance than are high-vacuum systems.

    One of the assertions often made is that WAGD dumped into a medical vacuum system is “free” since the medical vacuum “has to be there anyway.” When the average WAGD inlet only flowed 6-9 liters, there were many cases where this was at least in part true. With WAGD flowing at 40 liters, it is true far less often. The additional capacity required and the additional operating hours mean that the cost of WAGD produced by a medical vacuum pump is considerably higher than has been assumed.

    A simple rule-of-thumb test can be applied: Size the medical pump without the WAGD and select a pump of appropriate capacity. Add in the WAGD requirement (use 1.5-2 scfm per location). If the pump selected has sufficient capacity to handle the additional volume, an argument at least can be made that WAGD produced this way is relatively low-cost. If the pump selected does not have the spare capacity, and thus a larger pump must be selected, a dedicated system will almost certainly be less expensive in every way.

    Technology. Is the technology otherwise preferred for the medical vacuum source acceptable for WAGD? If not, can another option be equally acceptable? In cases where a technology preferred for use with the medical vacuum is oxygen-sensitive and there is not an equally acceptable oxygen-safe alternative, this will rule out mixed use. The same limitation may also render unacceptable certain pumps in a dedicated pumped system. In such a case, the limited technology options may be a powerful argument in favor of a low-vacuum system.

    Design complexity. How difficult is the system to design and what are the chances of problems? Whatever can be said against a dual-use system, it is undoubtedly the simplest to design. Correspondingly, low-vacuum systems offer the greatest range of advantages for the user, but are clearly the most complex to design and are also outside the experience of most North American designers. Low-vacuum systems are also the most complex to commission.