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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.
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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.
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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.
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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.
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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.
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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.
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