waste gas evacuation systems
operating room pollution
Let’s say you are performing an anesthetic. You have the fresh gas flow (FGF) at 2 L/min. We know that the FGF enters the circle circuit, joining the exhaled gas that will be rebreathed. But eventually, what happens to all the gas and vapor we control with our flowmeters and vaporizers? It has to go someplace. It has to leave the machine; otherwise, the machine would explode.
There is an exhaust for the machine. It is called the scavenger system. But it is more than merely the tail pipe for the anesthesia machine. It has a role in protecting you and the rest of the people in the OR. This assembly takes all the gas that leaves the machine and directs it out of the operating room (OR) so we are not exposed to the waste anesthetics. A properly adjusted scavenger system’s output is the same as the FGF.
TYPES OF SYSTEMS
There are two main types of scavenger systems, classified by whether medical suction is used to evacuate the exhaust gas. An active system requires wall or pipeline suction to evacuate the waste gas. A passive system relies on the upstream flow of gas coming out of the machine to passively flow out of the system, similar to how water flows through and out of a garden hose (a garden hose doesn’t need suction at the end of it to pull the water out of it).
As stated, an active system must be attached to some form of suction or evacuation unit, which would be the facility wall or pipeline suction. In most instances, there are separate plug-ins on the wall for scavenging suction and patient suction. Somewhere in the scavenger apparatus, there will be a means of adjusting the negative pressure from the wall, so the suction will not be too strong or too weak to be effective.
Active systems need some form of negative pressure relief to ensure the suction will not be so strong as to create a vacuum within the anesthesia machine itself. An active system also needs some type of positive pressure relief in case suction is not adequate or is disrupted so positive pressure will not build up in the anesthesia machine. In fact, one of the possible hazards of scavenger systems is barotrauma to the patient if the scavenger suction is not strong enough or is not working at all.
A passive system is a low-pressure system that relies on the passive flow of gas out of the machine and is connected to some sort of egress out of the room so as to not pollute the OR (which is, remember, the whole purpose of the scavenger in the first place).
Unlike active systems, there is no risk of developing too much negative pressure, so a negative-pressure relief valve is not needed. However, there is the possibility of high pressure building up if the passive flow is impeded, so a high-pressure relief valve is necessary.
The exit of the waste gas from the anesthetizing location can be as simple as a hole in an outside wall to a ventilation duct with a fan to assist the gas out of the area.
We have just discussed the two types of scavenger systems, active and passive. Now we need to talk about the two different types of interfaces. What is an interface? An interface classifies the scavenger system’s relationship to its environment. A scavenger system can be open or closed to its environment.
An active system can either have an open or closed interface. A passive system can have only a closed interface. If you are confused, don’t worry. All of this will be explained simply.
An open interface is a scavenger system that is actually open to the room environment (Figures 10-1 and 10-2). The scavenger itself will have holes or slits in the apparatus itself that allow protection of the patient to both positive and negative barotrauma. The scavenger will have what looks like a flowmeter device as part of its construction with a bobbin inside. The goal is to keep the bobbin within the factory-set lines or zone by adjusting the strength of the wall suction. This allows for the proper amount of suction, individualized for each anesthetizing location. This is important because suction strength can vary from room to room or location to location. Remember in our discussion about active systems we said that such a system required a means of both negative pressure relief and positive pressure relief. In an open interface, this is achieved because of the open system itself. If suction is too strong, the unit will entrain room air to satisfy the suction power instead of sucking it out of the machine. If suction is too weak, the unit will let the excess volume vent into the room instead of causing increased pressure in the anesthesia machine and potential barotrauma to the patient (Figure 10-3).