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:
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:
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:
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.
