The anesthesia machine has been developed over time from a basic gas delivery apparatus to an integrated system of components used to provide a safe anesthetic. It is the most important piece of equipment in the operating room. As an anesthesia technician, the more you know about the anesthesia machine, the more valuable an asset you will be to your department and your institution. If the machine fails, the operating room may be delayed or out of service, or worse, a patient could be harmed.

In this chapter, we will look at the components that make up today’s anesthesia delivery system. We will trace it from the gas supplies through the flowmeters, the vaporizers, the ventilator, and the breathing circuit to the patient and out through the waste gas scavenging system (Fig. 26.1).

As an anesthesia technician, you need to know where you can get information about the machines your institution uses. Each manufacturer has specific technical manuals for each type of machine. Although frequently online, it is beneficial to have at least one hardbound edition available for quick reference. This can be important if the online manual cannot be reached due to power failure or network outage. A more generalized in-depth resource is Dorsch and Dorsch’s Understanding Anesthesia Equipment. The University of Florida’s “Virtual Anesthesia Machine” is a good online resource for how machines work and for simulation and troubleshooting (http://vam.anest.ufl.edu/anesthesiamachine/index.html).

Oxygen (O₂), air, and nitrous oxide (N₂O) are provided from two sources: central pipeline supplies and gas cylinders mounted on the machines. Safety systems are designed into every step of gas passage through the machine to prevent the delivery of a hypoxic mixture of gas to the patient. Beginning with the gas supply, specialized connections bring the gas from each pipeline supply to the anesthesia machine. The first of these is the Diameter Index Safety System (DISS), in which color-coded gas hoses connect to the wall outlet of each gas with different diameter threaded connectors. The connector of one gas cannot be connected to a different gas outlet, as the diameters would be different. These connections are also used for the hose-to-machine connection. The pressurized gas hoses often have quick-release connections that are noninterchangeable. These allow for the quick hookup or release of the machine gas hoses, which facilitates moving the machine and performing a machine checkout. The checkout of anesthesia machines is done prior to each anesthetic and verifies the integrity of the machine. Chapter 27 discusses the checkout process.

The compressed gas tanks attach to the machine by specialized yokes. The Pin Index Safety System (PISS) prevents the placement of tanks onto the wrong yoke. Two pins protruding from the yoke assembly correspond to two holes on the tank’s valve stem. Each gas has a specific standardized pattern. Therefore, each gas cylinder can only be attached to that gas tank’s yoke (i.e., O₂ tank to the O₂ yoke). An incorrect gas tank will not engage and seal to the valve openings of another gas’s yoke.

Gas tanks, like the pressure hoses, are color coded. The US color-coding standard differs from some colors used internationally (Fig. 26.2).

As gas enters the machine, there is a back-check valve that keeps gas flowing only one way into the machine. If the machine has two O₂ tanks and both valves are open, the back-check valves prevent the equalization of pressure between the tanks (gas flowing from a full tank into an empty tank). The valves also prevent gas leakage out of the machine if the tanks or pipeline supply hoses are disconnected from the machine.

Oxygen E cylinder tanks are full at 1,900 psi (pounds per square inch) with 660 L of O₂. You can quickly estimate the volume of O₂ remaining in the tank by multiplying the current pressure in psi by 0.3 (e.g., 1,000 psi × 0.3 = 300 L). The volume of oxygen in the backup tank can be used to calculate how long oxygen can be provided to a patient if the wall source of oxygen is not functional. At an oxygen gas flow rate 10 L/min, 300 L would last about 30 minutes. It is important to note that some anesthesia machines use oxygen to drive the ventilator and may consume more oxygen than indicated by the oxygen flowmeter setting if the ventilator is in use.

After the gas enters from the tank, a pressure regulator reduces the pressure to approximately 45 psi. The gas pipeline pressure is usually 50-55 psi, but it can drop lower. This higher pressure from the pipeline allows the O₂ to flow preferentially from the pipeline and preserve the O₂ in the tank if the O₂ tanks are left open.

Within the anesthesia machine’s internal oxygen pathway is an oxygen supply failure alarm. This audible alarm is triggered if the internal oxygen pressure drops below a set value, usually 30 psi. A low pressure indicates a potential problem with the oxygen pipeline. Oxygen supply failure safety devices are also present within the machine before the gases pass into the flowmeters. These devices are designed to prevent hypoxic mixtures of gases from being delivered to patients in case of an oxygen supply failure. They reduce the chance of a hypoxic mixture but do not eliminate it. The safety devices are
triggered if the oxygen pressure drops below 20 psi.

There are two types of oxygen supply failure devices that react to a decrease in oxygen pressure. One shuts off the other gases, and the other proportionally reduces gas flow. Datex-Ohmeda machines have a pressure sensor shutoff (failsafe) device that will shut off the other gases. At pressures greater than 20 psi, the flow of these gases continues according to their specific flowmeter settings. In contrast, Dräger machines use a proportioning device, known as the oxygen failure protecting device (OFPD). As the oxygen pressure decreases, the OFPD decreases the nitrous oxide flow proportionally (if it is in use) until the pressure of both gases reaches zero.

The flowmeters on the anesthesia machine display the flow of the specified gas in liters per minute. Flowmeters can be either electronic or mechanical. The mechanical flowmeters typically use graduated cylindrical glass tubes with a bobbin that corresponds to the flow (Fig. 26.3). The volume of flow is read by reading the height of the bobbin with respect to the numbers on the glass tube. The flow is read from the center of a ball-shaped float but from the top of all other bobbin designs. The oxygen flowmeter is always downstream from the air and nitrous oxide flowmeters. In the United States, this corresponds to the oxygen flowmeter positioned to the right of the other two flowmeters on the anesthesia machine. Newer anesthesia machines may display the gas flow electronically on a liquid crystal display (LCD) screen (Fig. 26.4).

A mechanical or electronic linkage exists between the oxygen and nitrous oxygen flowmeter control valves. The linkage functions as a safety mechanism to prevent the delivery of an oxygen concentration less than 25% when using nitrous oxide. If a mechanical linkage is present, it connects these valves with a chain that does not allow the flow of nitrous oxide, unless the proportional flow of oxygen from the flowmeters results in at least 25% oxygen in the gas mixture. An electronic linkage uses a computer to determine the maximum safe nitrous oxide flow based upon the oxygen flow.

There is typically an auxiliary oxygen connection and flowmeter located on the anesthesia machine. This oxygen outlet usually has a barbed fitting that allows for the direct connection of nasal cannula or oxygen mask tubing. The flowmeter associated with this auxiliary oxygen connection is usually limited to 10 L/min.

The purpose of a vaporizer is to convert the volatile anesthetic medication from a liquid to a set concentration of gas for delivery to the patient.
Please see Chapter 28 for more information on vaporizers. Within the anesthesia machine, the vaporizers are located between the flowmeters and the common gas outlet. The vaporizer has a number of components that are important to understand to ensure proper maintenance and service. Each vaporizer is designed to hold only one specific volatile anesthetic medication. Because the vapor pressure of each volatile anesthetic is different, placing the wrong medication in the vaporizer could result in the inappropriate delivery of medication. Every vaporizer is clearly labeled on the outside with the name of the medication that should be placed into it. The filler port on the vaporizer is used for adding liquid anesthetic to the vaporizer. Usually, the filler ports are designed to only connect with the bottle of the appropriate volatile anesthetic medication. This minimizes the likelihood of filling the vaporizer with the wrong medication. The liquid anesthetic enters the vaporizing chamber of the vaporizer. The concentration of volatile anesthetic that is delivered to the patient is given in units of volume percentage. The output from the vaporizer is determined by turning the concentration dial on the vaporizer. The concentration delivered increases when the dial is turned in the counterclockwise direction. Vaporizers have a temperature compensation mechanism to ensure that the concentration dial reflects the output of the vaporizer despite changes in ambient temperature.