The anesthesia machine

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What is an anesthesia machine?

A more modern and correct name for an anesthesia machine is anesthesia delivery system . The job of the first anesthesia machines was to supply a mixture of anesthetizing and life-sustaining gases to the patient. Modern anesthesia delivery systems perform these functions, as well as ventilating and monitoring the patient. The most important purpose is to help the anesthesiologist and anesthesia provider keep the patient safe and adequately anesthetized. Currently, there are two major manufacturers available in the United States: Dräger and GE Healthcare (owner of Datex-Ohmeda).

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Describe the “plumbing” of an anesthesia machine.

Leaving out the safety features and monitors, the anesthesia machine is divided into three sections:

• The gas delivery system, which supplies at its outlet a chosen, defined mixture of gases

• The patient breathing system, which includes the breathing circuit, carbon dioxide absorber, ventilator, and often gas pressure and flow monitors

• The scavenger system, which collects excess gas and expels it outside of the hospital, thereby reducing exposure of the operating room personnel to anesthetic gases

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What gases are commonly available on anesthesia machines? What are their sources?

Oxygen, nitrous oxide, and air are available on almost every anesthesia machine. Usually the gas source for an anesthesia machine is from a centralized wall or pipeline supply. An emergency backup supply for each medical gas is stored in a compressed gas cylinder called an E-cylinder , and is attached to the rear of the anesthesia machine. These gas cylinders should be checked daily to ensure they contain an adequate backup supply in case of central pipeline failure.

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List the uses of oxygen in an anesthesia machine.

• Contributes to the fresh gas flow

• Provides gas for the oxygen flush valve

• Used as a driving gas for bellow ventilators: Ascending bellow ventilators, still used by modern GE machines, use oxygen as a driving gas. Pressure inside the bellows will always be slightly higher than that in the housing chamber (by 1–2 cm H ₂ O) because of the weight of the bellows itself. This is important because if there was a leak within the bellows, any net gas flow would be out of (not into) the bellows, and would not change the composition of the inspired gas.

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Because the flow rates of air, nitrous oxide, and oxygen are controlled independently, can the machine ever be set to deliver a hypoxic gas mixture to the patient?

In a word, no. Both Dräger and GE machines include several safety features, which prevent the provider from delivering a hypoxic gas mixture to the patient. First, software in modern anesthesia machines prevents the provider from digitally prescribing a hypoxic mixture. Further, anesthesia machines have built in “fail-safe devices” to safeguard the patient from the delivery of hypoxic gas mixtures. These “fail-safe devices” are machine specific and include an internal electric gas mixing device (GE) or a sensitive oxygen ratio controller system (Dräger). These devices either proportionally reduce or completely shut off flow from other gases, if the oxygen supply pressure decreases too much. The GE device has a circuit board that relies on pressure sensors and resistors to monitor and control flow, whereas Dräger uses a mechanical device that uses resistors and valves, and controls flow through a mechanical-pneumatic link between the two gas lines. Both devices ensure that the ratio of nitrous oxide to oxygen is such that fraction of inspired oxygen (F _i O ₂ ) remains greater than 25%, provided the anesthesia gas lines have not been swapped and are properly connected ( Fig. 26.1 ).

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What other mechanisms exist for preventing the administration of a hypoxic gas mixture?

• In older machines with flowmeters, the oxygen flow knob is larger and distinctively fluted. Knobs for the other gases are smaller and knurled.

• A color code exists such that the color for each gas knob, flowmeter, tank, and wall attachment are all consistent. In the United States, oxygen is green, air is yellow, and nitrous oxide is blue. International standards may differ.

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What is a pressure regulator ? What is a check valve ? How do these control the flow of gas into the anesthesia machine?

The medical gases stored in the E-cylinders are under high pressure (i.e., 2000 pressure per square inch gauge [psig] for oxygen, 2000 psig for air, and 750 psig for nitrous oxide), all of which are too high for the anesthesia machine, which requires a pressure of approximately 50 ± 5 psig. The function of the pressure regulator is to accept a high-pressure gas from its input, reduce its pressure, and then output that gas at a much lower pressure. Each gas E-cylinder has a separate pressure regulator to ensure its output is 45 psig. The pipeline gas supply has a pressure of approximately 50 to 55 psig and does not need to flow through a pressure regulator.

Each pipeline gas supply and their respective E-cylinder will then converge together before connecting to the anesthesia machine. However, just before their convergence, gas from the pipeline and the E-cylinder, each flow through their own check valve. This valve ensures that gas can only flow in one direction, as determined by the pressure gradient across the valve. The pressure regulator and the two check valves allow gas to flow preferentially from the pipeline gas supply and then from the E-cylinder as a backup. For example, normally the oxygen pipeline supply pressure is 50 to 55 psig and the pressure from the oxygen E-cylinder after the pressure regulator is 45 psig. Because of the check valves, oxygen will not flow from the supply pipeline into the E-cylinder and oxygen will only flow from the supply pipeline to the anesthesia machine. If the supply pipeline were to fail, its pressure would drop and oxygen would then flow from the E-cylinder to the anesthesia machine. Oxygen from the E-cylinder would not be able to flow retrograde because of the check valve on the hospital’s oxygen supply pipeline.

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How does the hospital pipeline (central) gas supply compare with the use of compressed gas cylinders?

For practical purposes, wall gases are continuous in volume availability, assuming the central supply is refilled. As mentioned earlier, pipeline gas pressures are typically 50 to 55 psig and the E-cylinder tank pressure is regulated by the “first-stage” pressure regulator to 45 psig. Because of the use of “one-way” check valves, gas will preferentially flow from the source (E-cylinder vs. hospital pipeline) with the highest pressure (45 psig for E-cylinder vs. 50–55 psig for hospital pipeline) to the anesthesia machine. Provided everything is working correctly, the wall supply is used rather than the tank supply. Use of wall supply oxygen is preferable because it is available in greater volume, is cheaper, and preserves the tank supply for emergency situations.

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Imagine the central supply of oxygen is lost. The gauge on the oxygen tank reads 1000 psi. How long will you be able to deliver oxygen before the tank is empty?

Contemporary anesthesia machines have two supply sources for medical gas: the pipeline supply from the wall and E-cylinders attached to the back of the machine itself. The cylinders are color coded and should be kept off, unless there is a failure of the hospital pipeline supply.

• Cylinder Colors (in the United States)

• Oxygen—Green

• Nitrous Oxide—Blue

• Air—Yellow

• Carbon Dioxide—Gray

• Nitrogen—Black

A full, green E-cylinder of oxygen has a pressure of 2000 psig and contains about 625 L of oxygen. Because the oxygen is a compressed gas, the volume in the E-cylinder correlates linearly with the pressure on the gauge. Therefore a pressure of 1000 psig means that the oxygen E-cylinder has about 312 L of gas remaining.

The oxygen supply to the anesthesia machine may be used for two purposes: (1) to oxygenate the patient, and (2) to pneumatically drive the ventilator bellows. When oxygen is used for both, a large percentage will be lost to drive the bellows (corresponding to the patient’s minute ventilation). Thus if a patient is receiving an oxygen fresh gas flow (FGF) of 1 L/min in the breathing circuit, with a minute ventilation of 9 L/min, 10 L of oxygen will be drained from the oxygen E-cylinder every minute. An E-cylinder with 312 L remaining will last for about 30 minutes at this rate. To minimize consumption of oxygen from the E-cylinder, it is recommended to turn off the bellow-driven ventilator and begin hand ventilating the patient.

One of the advantages of a piston-driven ventilator, as opposed to a bellow-driven ventilator, is that the piston is driven by electricity instead of oxygen, greatly reducing the amount that might be otherwise wasted driving the bellows. In this case, given the earlier scenario, only oxygen used by the FGF is consumed and you would have about 300 minutes of oxygen supply, instead of 30 minutes.

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A new E-cylinder of nitrous oxide is installed, and the pressure gauge reads only 750 psig. Why is the pressure in the nitrous oxide cylinder different than the others?

Air and oxygen are compressed gases. Under normal circumstances, gases can be compressed into their liquid form provided this transformation is conducted below that specific gases’ critical temperature (the temperature at which a gas can be compressed into a liquid). Oxygen and air cannot be compressed into liquids at their storage temperature (i.e., room temperature) because at room temperature, the critical temperature is exceeded. Therefore they exist in gas form within their respective E-cylinders.

The relationship between the volume of gas in a cylinder and the pressure displayed on its gauge is linear because of the ideal gas law (P (pressure) × V (volume) = n (no of moles) × R (gas constant) × T (temperature)). As a result, the volume of gas remaining in an oxygen or air cylinder is directly proportional to the gauge pressure. The pressure of a full air or oxygen cylinder is approximately 2000 psig. A pressure reading of 1000 psig would suggest that the tank is half-full.

Nitrous oxide, however, condenses into a liquid at 747 psig. Therefore it exists as a liquid at room temperature. E-cylinders of nitrous oxide contain, in liquid form, the equivalent of about 1600 L of gas when full. The pressure in the cylinder will remain constant, until all of the liquid nitrous oxide has been vaporized into gas form. This point is reached when approximately 25% of the initial volume of nitrous oxide remains in the cylinder. Only then does the pressure displayed on the gauge begin to decrease below 750 psig. An accurate estimation of the volume remaining in the cylinder before this point requires weighing the cylinder and subtracting the empty (tare) weight of the cylinder.

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Describe the safety systems used to prevent incorrect central and cylinder gas connections to the anesthesia machine.

• All central supply gas connectors are keyed so, for example, only the oxygen supply hose can be plugged into the oxygen connector on the wall, the nitrous oxide hose into the nitrous oxide outlet, and so on. This is known as the Diameter Index Safety System (DISS)

• The gas cylinders are keyed using the Pin Index Safety System (PISS—no kidding!) so that only the correct tank can be attached to the corresponding yolk on the anesthesia machine (assuming that the pins have not been sheared off!)

• These safety systems should always include an oxygen analyzer on the inspiratory limb that measures the delivered oxygen concentration to the patient. This is the most important safety feature of the anesthesia machine in preventing a delivery of a hypoxic gas mixture

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