design of anesthesia machines
oxygen supply systems
medical gas cylinders
In this chapter, we will discuss how our medical gases (oxygen, nitrous oxide, and air) get into the machine and what happens to them when they are in there. The pneumatic system of an anesthesia machine is subdivided into three smaller systems based on the amount of pressure seen in each one: the high-pressure system (concerning the gas cylinders on the back of the machine), the intermediate-pressure system (concerning gases from the pipeline or wall source), and the low-pressure system (flowmeters). We will discuss the first two in this chapter and will cover flowmeters in a separate chapter.
This system includes the gas cylinders, or tanks, on the back of the machine; how the cylinders are mounted onto the hanger yoke on the back of the machine; and what happens to the gas after it enters the machine.
Volumes and Pressure
We are all familiar with the “E” cylinders that are mounted on the backs of anesthesia machines and used for patient transport. An E cylinder of oxygen that is full contains 660 L of oxygen and is at a pressure of 2,200 psig. An E cylinder of nitrous oxide has 1590 L at a pressure of 745 psig.
An oxygen cylinder has a linear relationship between volume and pressure; for example, when the cylinder is half empty, at 330 L, the pressure will be around 1100 psig, and so on, until the tank is empty.
A nitrous oxide cylinder, when full, has some of its contents in a gaseous phase and some in a liquid phase. The pounds per square inch of a nitrous oxide cylinder will read 745 throughout most of its useful life. That is because there is a nonlinear relationship between volume and pressure in regards to nitrous oxide because part of it is in a liquid phase. As long as there is liquid in the tank contributing to the vapor pressure of the gas above it, the pressure gauge will not change.
That is why it is recommended to change nitrous oxide cylinders when the pressure begins to read below 745 psig. There can still be a good amount of nitrous oxide in the tank (up to 400 L) when the liquid nitrous oxide is gone, but there is no way of knowing except to take the tank off the back of the machine and weigh it. Even if you wanted to weigh the existing tank, when it is off the machine, it is easier in the long run to replace it with a full tank and not worry about how much nitrous oxide is left.
An oxygen cylinder is green, right? Have you ever seen one that is not green? Well, if you have been practicing anesthesia outside the United States, you have seen oxygen tanks of a different color. Medical gas colors can be different in different parts of the world. Although we rely on color very much in our specialty (gases, inhalational agents, oral airway sizes, and so on), color coding is not foolproof.
Did you know that it is not a law or a Food and Drug Administration regulation that oxygen is always in a green tank, nitrous oxide is always in a blue tank, or air is always in a yellow tank? It is a guideline but not a requirement. All that your local medical gas supplier needs to do to be legal is to properly label the tank for its contents. The authors once discovered an air tank attached to our machine that was gray, similar to a cylinder of carbon dioxide. Gas analysis proved the contents were in fact air, and the tank was labeled as medical air.
Correct Gas Placement
Now are you worried that you may put a gas cylinder onto a wrong yoke on the back of the machine? That is why it is important to read labels. Color coding is helpful, but we have seen that it can potentially mislead you.
Fortunately, there is another way to ensure placing the correct tank onto the correct yoke. On the stem of each gas cylinder (the stem is the chrome part on the very top of the cylinder) are a couple of little holes that line up with a couple of little pins right where the stem fits onto the yoke nipple. The pins and holes are in matching positions that are different for each gas. This is called the Pin Index Safety System (Figure 4-1). If the pins on the yoke do not line up with the holes on the cylinder stem of a corresponding tank, get another cylinder and check the yoke. Either the cylinder has an incorrect stem on it or the yoke pins have been damaged (Figure 4-2). To proceed with a case using that cylinder or yoke is not a good idea.
Figure 4-1 Pin Index Safety System interlink between the anesthesia machine and gas cylinder. (Reproduced with permission from Morgan GE, Mikhail MS, Murray MJ. Clinical Anesthesiology. 4th ed. New York, NY: McGraw-Hill; 2006. Figure 2-4.)
We all rely on anesthesia technicians and other ancillary staff to help us in our daily work. They may even know more about the machine than we do. But don’t count on it. A pilot doesn’t trust the baggage handler to check out the avionics of the plane, and we shouldn’t rely solely on staff to know more about the machine than we do. While it is true a pilot probably doesn’t know how to refuel his airliner, we should know how to replace a cylinder. If you don’t, make it a priority to learn how.
A full cylinder should come with a new washer taped or somehow attached to the stem. This washer is what goes between the cylinder and the yoke nipple. If it is not placed there, there will be a leak when the cylinder is opened. The exit of pressurized gas from a cylinder to the machine inlet results in heat, which can deform and flatten the washer. That is why a new washer should be used each time a cylinder is changed.
Be sure to take the old washer off, though! If more than one washer is used between the cylinder stem and the yoke nipple, the added space the extra washer creates can be enough to inadvertently disable the Pin Index Safety System by making the pins unable to reach the corresponding holes.
A machine should always have an oxygen cylinder on it. Some types of machines even have two yokes for oxygen. It is less important to have nitrous oxide and air cylinders on their yokes, of course, because they are not absolutely necessary to safely perform an anesthetic. So why isn’t there a leak from empty yokes when you are using pipeline gas?
There is a check valve for each yoke. When the pressure of the gas in the cylinder is greater than that of the gas in the machine (meaning when you have opened the cylinder), the check valve is opened by that pressure. When the pressure in the machine is greater than the pressure of an empty cylinder or there is no cylinder on the yoke, the check valve closes. But even with this check valve system, a small amount of gas from inside the machine can escape from around the closed check valve if the yoke is empty. Gas-specific yoke plugs are available that stop these kind of small leaks (Figure 4-3).