KEYWORDS
design and ergonomics of anesthesia machines
ASA monitoring standards
In this book, we will discuss different types of anesthesia equipment. When we say “anesthesia equipment,” we mean the anesthesia machine itself, as well as the major monitoring devices. The trend is currently to have both machine and monitors integrated in an “anesthesia delivery system” or “anesthesia work station.” Regardless of how the equipment is packaged, it is still useful to think of the components as separate entities in order to understand them. So in this book, when we talk about the “anesthesia machine,” we mean the device that delivers oxygen and other gases, delivers inhalational anesthetics, and ventilates the patient. When we talk about “monitors,” we generally mean the devices that measure physiologic, chemical, and pharmacologic information.
We will begin by reviewing what is found on a generic, modern anesthesia machine. It will be analogous to talking about airplanes; all airplanes fly, but there is a wide variety of designs, incorporating different engines, wings, and so forth, but all airplanes have things such as engines and wings. This is also how it is for anesthesia machines. They all do the same thing, but there are differences in how the same thing is done and differences in how machines are designed and function.
PURPOSE OF AN ANESTHESIA MACHINE
What is the main purpose of an anesthesia machine? This is a question we often ask medical students doing an anesthesia rotation. Naturally, many of them say something like “to deliver anesthesia gas to a patient.” In our minds, that ability is not the main purpose of an anesthesia machine.
The main purpose of an anesthesia machine is to deliver oxygen to a patient. See the list below. Everything else is secondary. Everything about an anesthesia machine is built around the purpose of delivering oxygen to a patient. All the fail-safe systems of a machine concern the prevention of the delivery of a hypoxic mixture, not the delivery of anesthetic agents.
Okay, so what is the second most important function of an anesthesia machine? The students will then answer “to deliver anesthesia gas to a patient.” But again they are wrong. After oxygenation, the next most important function or purpose of an anesthesia machine is to provide a means of positive-pressure ventilation. If we must ensure the patient receives oxygen, we must also have a means of being able to force oxygen into a patient because apnea is a major effect of anesthetics. Merely delivering oxygen to an apneic patient’s mouth will not help too much.
Okay, so now comes the anesthesia delivery part, right? Yes, the third main function of an anesthesia machine is to deliver inhalational anesthetic agents to a patient. You should remember these functions in their order of importance as we go along.
MAIN PURPOSES OF AN ANESTHESIA MACHINE
1. Oxygen delivery
2. Means of positive-pressure ventilation
3. Delivery of inhalational anesthetics
GENERIC ANESTHESIA MACHINE
Let’s pretend we are at an imaginary anesthesia machine dealer. We walk around, looking at all the shiny new models, enjoying that “new anesthesia machine smell,” looking under the hoods, and so forth. They all may look different and have different options, but what kinds of things will we find on every one we look at?
Gases
There will be the capability to deliver at least two gases on an anesthesia machine: oxygen and nitrous oxide. Virtually all machines will also have medical air. (What about sevoflurane, desflurane, and isoflurane? Those things are not gases; more on this later on.)
When we say there is the capability to deliver specific gases, there will also be means of supplying the gases to the machine and controlling the flow rate and concentration of these gases.
Sources of Gases
A generic anesthesia machine will have the ability to obtain the gases from two different sources. One is from a gas pipeline, or “wall source.” The other source is from medical gas cylinders. There will be built-in safety devices that will keep us from accidentally hooking a gas up to the wrong inlet of the machine so we will not inadvertently be delivering nitrous oxide when we think we are delivering oxygen. Additionally, there will be gauges, either electronic or analog, to tell us the pressure in psig (pounds per square inch) on the gauge of each source of each gas.
Flowmeters
Either electronic or glass flowmeters for each gas will be present, with control knobs for us to regulate both the concentration of gases and the flow rate in liters per minute.
Hypoxia Fail-Safe Devices
There are several different mechanisms that are built in to anesthesia machines that keep us from giving a hypoxic mixture of gases.
Vaporizers
After all, it is an anesthesia machine. There has to be a way of delivering inhalational anesthetic agents to the patient in a controllable, safe, efficient manner.
Reservoir Bag
This is the “squeeze bag” that we use to manually ventilate a patient. We must have a means of being able to deliver positive-pressure ventilation to the patient.
Ventilator
A means of automatically ventilating a patient is present on a generic anesthesia machine.
Circle Anesthesia Circuit
A circle circuit is by far the most commonly used anesthesia breathing circuit. This is the conduit from the machine to the patient that delivers oxygen and anesthetics and allows ventilation. There are several parts of a circle circuit as well, which are discussed in a later chapter.
Carbon Dioxide Absorber
A circle circuit means that there is rebreathing of carbon dioxide. Although it is true that there are types of anesthesia machines that are more like a critical care ventilator (semi-open circuit), virtually all anesthesia machines are semi-closed in function, meaning that there is rebreathing. So somehow we must be able to rid the rebreathed gas of carbon dioxide. A carbon dioxide absorber uses a chemical absorbent to scrub the rebreathed gas of carbon dioxide.
Waste Gas Scavenger
What happens to all of the inhalational agent that the patient is administered? It must go somewhere. It is true that there is rebreathing of the agent, but eventually it leaves the machine. A scavenger system allows the waste gas to leave the machine without polluting the working environment of the operating room (OR), sparing us from long-term exposure to the anesthetics.
Oxygen Flush System
For a multitude of reasons, at some time the clinician needs to fill the machine quickly with oxygen or increase the pressure inside the machine. An oxygen flush button allows that to happen.
Auxiliary Flowmeter
Not every anesthetic is a general anesthetic. We often administer oxygen via nasal cannula during neuraxial anesthetics or monitored anesthesia care (MAC). An extra oxygen flowmeter allows us to do so easily.
Battery Backup
Does your machine have a battery? If so, when was it last tested? Again, most modern anesthesia machines will have some sort of electrical backup supply.
Electrical Equipment
The anesthesia machine is an electrical machine. It has wires and cords, and somewhere it will have at least one circuit breaker. There will be power outlets on the back of the machine for plugging in accessory equipment and monitors. Many will have USB and Ethernet interfaces for use with an automated record system, for example.
Components of a Generic Anesthesia Machine
Gases
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