Vaporizers are one of our interfaces with the anesthesia machine. We continually adjust them throughout each case we do. But do we really know what we are doing each time we move the dial?
Vaporizers are very complicated on the inside, much more than they may look to us. Inside each one there are things going on that pertain to gas laws, specific heat, vapor pressure, and all that stuff from chemistry we thought we would never see again. They are also categorized by certain qualities that may not make sense to us at first.
In this chapter, we will discuss how a vaporizer works, what each phrase in its classification means, and the hazards of vaporizers.
Think about a can of gasoline. When it is open, you can smell the fumes emanating from the can. Another name for the fumes would be vapor. The fumes we smell are not a “gas” (three-letter word, not the short word for gasoline). A true “gas” is a chemical that has already reached its boiling point in the setting in which it finds itself. Oxygen, carbon dioxide, nitrogen, and so on are true “gases” in our environment because they exist at a temperature above their boiling points. That is why it is incorrect (but nevertheless common) to refer to inhalational agents as “gas” because they are not true gases at the temperature of an operating room (OR); they exist in a state below their boiling points. (Desflurane at room temperature is almost at its boiling point, however). The only commonly used “gases” found in an anesthesia machine are oxygen, air, and nitrous oxide.
Now think about a can of gasoline on a hot day. Gas cans nowadays are made out of plastic, not metal like in the past. On a hot day, a full can of gasoline can actually expand and deform itself. This is because the gasoline fumes, or vapors, are exerting a pressure against the sides of the gas can. This is called, simply enough, vapor pressure. This is an example of how above every liquid in a closed container, there exists a vapor pressure. In this situation, the vapor pressure is enough to distend the sides of the plastic can.
The molecules of hydrocarbons that make up what we call gasoline are leaving the surface of the liquid and floating up into the above space while at the same time molecules of gasoline are settling back down into the liquid from the vapor in the space above. The liquid and vapor are in equilibrium with each other.
A liquid and a vapor in an enclosed space will always find its own equilibrium (Figure 6-1). The amount of substance that is in liquid versus vapor phase ...