18: The Anesthesia Machine and Vaporizers

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CHAPTER 18 The Anesthesia Machine and Vaporizers

James Duke, MD, MBA

1 What is an anesthesia machine?

A more modern and correct name for an anesthesia machine is an anesthesia delivery system. The job of the first anesthesia machines was to supply a mixture of anesthetizing and life-sustaining gases to the patient. A modern anesthesia delivery system performs these functions and also ventilates the patient and provides a number of monitoring functions. The most important purpose is to help the anesthesiologist keep the patient alive, safe, and adequately anesthetized. Anesthesia machines have become rather standardized. Currently there are two major manufacturers in the United States: Drager and Datex-Ohmeda.

2 Describe the plumbing of an anesthesia machine to create an overview of its essential interconnections

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

image A gas delivery system that supplies at its outlet a defined mixture of gases chosen by the anesthesiologist
image The patient breathing system, which includes the patient breathing circuit, absorber head, ventilator, and often gas pressure and flow monitors
image A scavenger system that collects excess gas and expels the gas outside the hospital, reducing the exposure of operating room personnel to anesthetic gases

3 What gases are ordinarily available on all anesthesia machines and what are their sources?

Oxygen (O2) and nitrous oxide (N2O) are available on almost every anesthesia machine. Most commonly the third gas is air, but it can also be helium (He), heliox (a mixture of He and O2), carbon dioxide (CO2), or nitrogen (N2). If the third gas does not contain O2 (as do air and heliox), it is possible to deliver a (dangerous) hypoxic mixture to the patient.

Usually the gas source for anesthesia machines in hospitals is from a centralized wall or pipeline supply. An emergency backup supply of gases is stored in tanks called E-cylinders attached to the rear of the anesthesia machine. These tanks should be checked daily to ensure that they contain an adequate backup supply in case of pipeline failure.

4 Since the flow rates of N2O and O2 are controlled independently, can the machine be set to deliver a hypoxic mixture to the patient?

Different machine manufacturers have different ways of protecting the patient from hypoxic mixtures. The Drager’s ORMC (oxygen ratio monitor and controller) senses the oxygen flow rate and controls N2O flow pneumatically. Datex-Ohmeda’s Link-25 system mechanically links the O2 and N2O flow knobs to ensure that the proportion of N2O to O2 remains in a safe range as the N2O flow is increased.

5 What is a regulator? How does it control the flow of gas?

Oxygen and air in the E-cylinders is pressurized up to approximately 2200 psig, but the anesthesia machine needs to work with gas at an initial pressure of about 50 psig, slightly less than the pressure of the gas from the wall supply. A regulator performs this function. Each gas is regulated separately. Gas from both the cylinder and the wall supply encounters a check valve that selects the gas source with the highest pressure for use by the machine. Under normal circumstances the wall supply is used preferentially, and the tank supply is spared for use if the wall supply fails.

6 How does the hospital piped gas supply compare to the use of tank gas?

For practical purposes, wall gases are continuous in volume availability, as long as the central oxygen supply is refilled. Wall gas pressures are typically about 55 psig. Tank pressure is generally regulated by the first-stage regulator to 45 psig. The anesthesia machine preferentially chooses the source with the highest pressure. As long as everything is working correctly, the wall supply is used rather than the tank supply. Use of the wall supply oxygen is preferable because it is available in greater volume, is cheaper, and preserves the tank supply for use only in emergency situations.

7 The hospital supply of oxygen is lost. The gauge on the O2 tank reads 1000 psi. How long will you be able to deliver oxygen before the tanks are empty?

Contemporary anesthesia machines have two sources of gases: the wall outlet and E-cylinders attached to the machine itself. The cylinders are color coded and usually left shut off, being saved for use in an emergency, but a normally functioning anesthesia machine and normal wall outlet oxygen pressures will use the wall outlet preferentially.

A full green E-cylinder of O2 has a pressure of 2000 psi and contains about 625 L of O2. Since the O2 is a compressed gas, the volume in the tank correlates linearly with the pressure on the gauge. Therefore a pressure of 1000 psi means that the O2 tank has about 312 L of gas left.

Besides delivering oxygen flow directly to the patient, oxygen also powers the ventilator bellows, and the patient’s minute ventilation approximates the driving flow of the bellows. Thus, if a patient is receiving an oxygen flow of 2 L/min and a minute ventilation of 8 L/min, 10 L of oxygen will be drained from the oxygen tank every minute. A tank with 312 L remaining will last for 31 minutes at this rate. To reduce consumption of the tank oxygen, decrease the oxygen flow rate and hand-ventilate; also request that additional oxygen tanks be brought to the room.

8 A new tank of N2O is installed, and the pressure gauge reads only about 750 psig. Why is the pressure in the N2O tank different from the pressures of other gases?

Air and O2 are compressed gases and cannot be compressed to liquids in a room temperature environment because the critical temperature (the temperature at which a gas can be compressed into a liquid) is exceeded. However, at room temperature N2O condenses into a liquid at 747 psi. E-cylinders of N2O contain the equivalent of about 1600 L of gas when full, whereas E-cylinders of O2 and air hold only about 600 L. The pressure in the N2O tank remains constant until the N2O has been vaporized. About only a quarter of the initial volume of N2O remains when the cylinder pressure drops, although accurate estimation would require weighing the cylinder, knowing the empty (tare) weight of the cylinder, and determining the moles of N2O remaining.

In contrast, being a compressed gas, the volume of gas remaining in an oxygen or air cylinder is directly proportional to the pressure. The pressure of a filled cylinder is 2200 psig. A pressure reading of 1100 would suggest that the tank is half full or has about 300 L remaining.

9 List the uses of O2 in an anesthesia machine

image Contributes to the fresh gas flow
image Provides gas for the O2 flush
image Powers the low O2 alarm
image Controls the flow of nitrous oxide
image Powers the fail-safe valves
image Is the driving gas for the ventilator

10 Describe the safety systems used to prevent incorrect gas connections at the wall and cylinders

image All wall-supply gas connectors are keyed so only the O2 supply hose can be plugged into the O2 connector on the wall, the N2O hose into the N2O outlet, and so on. This is known as a Diameter-Index-Safety-System (DISS).
image The gas cylinders are keyed using a 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!
image These safety systems should be backed up by a monitor that measures the delivered concentration of oxygen in a mixed gas. It is this monitor that is most critical in preventing delivery of a hypoxic gas mixture.

11 In addition to the distinctions described previously, what other ways are gases distinguished to help prevent human error?

image First, the O2 flow knob is distinctively fluted. Knobs for other gases are knurled.
image Second, a color code exists such that each gas knob, flowmeter, tank, and wall attachment bears the corresponding color for its associated gas. In the United States O2 is green, air is yellow, and N2O is blue. International standards differ.

12 There are two flowmeters for each gas on an anesthesia machine. Couldn’t you safely get away with only one?

Two flowmeters are used to increase the range of flows over which an accurate measurement can be obtained. The flow tubes in anesthesia machines are always placed in series so all the gas flows sequentially through both tubes. To measure flows accurately in the ranges used for low flow or even closed-circuit anesthesia (200 to 1000 ml/min), two flow tubes are essential. Flowmeters, also known as Thorpe tubes, are also specific to the gas for which they have been designed and are not interchangeable with other gases.

13 Why are the flowmeters for air, O2, and N2O arranged in a specific order?

Reasons include U.S. (NIOSH) government standards, manufacturer’s conventions, and safety. Requiring the O2 knob to be in the same relative position on all anesthesia machines decreases the risk of the anesthesiologist turning the wrong knob. In the United States the O2 flowmeter must always be on the right, closest to the point of egress into the common gas manifold, just proximal to the anesthesia vaporizers. With the O2 flowmeter in that position, most leaks will tend to selectively lose gases other than O2. This configuration is least likely to deliver a hypoxic gas mixture. Again, the best way to detect a hypoxic gas mixture is through use of an oxygen analyzer.

14 What is meant by a fail-safe valve?

The fail-safe valve device is designed to cut off the flow of all gases except O2 when the O2 pressure falls below a set value, usually about 25 psig (Figure 18-1).

image

Figure 18-1 Effect of fail-safe device and oxygen failure pressure device.

15 Would it be safer to leave the tank O2 supply on your machine turned on so, if the pipeline O2 failed, the machine would automatically switch immediately to the backup tank supply?

No. First, when all equipment is functioning properly, the disadvantage of leaving the tank on is that, if a failure in wall O2 should occur, your machine will use gas from the tank, unknown to you. You may not recognize the problem until the machine (and tank) is totally out of O2 and the low O2 pressure alarm begins to sound. At this point you must scramble to find O2 quickly.

The second reason allows for equipment failure, and there are two parts to the explanation:

1. When no gas is flowing, it is possible for pressure to be maintained in the gauge despite a leak where the tank is connected to the yolk. Thus it is possible to have a full indication on the pressure gauge and an empty tank. The pressure in the tank should be checked after the wall supply is disconnected and the system pressure bled down, and then the tank should be turned off.
2. If wall O2 pressure drops too low, the tank could be drained, supplying the anesthesia machine rather than saving tank O2 for emergencies. A second check valve prevents the tank O2 from entering the wall supply plumbing should the wall supply fail. If this valve fails, the tank could, for the short period until the tank empties, backfill the hospital system, helping to supply O2 to ward patients.

16 How long can you continue to deliver O2 when the wall supply fails?

The E-cylinders that supply O2 to most anesthesia machines hold 600 L when full. If the ventilator is not in use (remember that O2 powers the ventilator), the O2 flowmeter indicates how much O2 is being used. With a flow of O2 of 2 L/min, there is approximately 300 minutes (or 5 hours) of O2 available. If the ventilator is in use, the additional gas required for this purpose is roughly equal to the ventilator minute volume, and the length of time the tank supply will last will be significantly decreased below the previously estimated 5 hours for a full tank. It should be noted that manually ventilating the patient in this instance would preserve the oxygen supply. Reducing the oxygen flow would also extend the life of the tank.

17 What physical principles are involved in the process of vaporization?

The saturated vapor pressure of the volatile anesthetic, which varies with temperature, determines the concentration of vapor molecules above the liquid anesthetic. The heat of vaporization is the energy required to release molecules of a liquid into the gaseous phase. The liquid phase draws external heat during vaporization, or it will become cooler as molecules leave and enter the gaseous phase. To address this problem, vaporizers are constructed of metals with high thermal conductivity. High thermal conductivity ensures that there is a conduit of heat so the heat required for vaporization is constantly restored from the environment and the rate of vaporization of the volatile anesthetic is independent of changes in the temperature of the vaporizer.

18 What does it mean when it is said that a vaporizer has variable bypass? What is the effect of having such a vaporizer turned on its side?

The vaporizers are located downstream from the flowmeters. Fresh gas from the flowmeters enters the vaporizer and is divided into two streams. Most of the gas enters the bypass chamber and is not exposed to the volatile agent. The remaining gas enters the vaporizing chamber and becomes saturated with anesthetic. The concentration dial determines the proportion of gas flow that enters each of the two streams. These then reunite near the vaporizer outlet. The fresh gas leaving the vaporizer contains a concentration of vapor specified by the concentration dial.

Should a variable bypass vaporizer be turned on its side, liquid anesthetic may spill from the vaporizing chamber to the bypass chamber, effectively creating two vaporizing chambers and increasing vaporizer output. This could create toxic levels of volatile anesthetic being delivered to the patient. However, most but not all modern vaporizers have mechanisms minimizing this effect.

19 What does temperature compensation mean?

During vaporization the liquid anesthetic will cool, drawing heat from the metal of the vaporizer, which draws heat from the operating room. As liquid anesthetic cools, the saturated vapor pressure decreases, as does vaporizer output. Temperature compensation means the vaporizer has mechanisms for adjusting the output to compensate for temperature.

20 What is the pumping effect?

Positive pressure can be transmitted back into the vaporizer during ventilation of the patient. The positive pressure can briefly cause gas to reverse flow within the vaporizer, allowing gas to periodically reenter the vaporizing chamber. The result of the pumping effect is increased vaporizer output beyond that indicated on the concentration dial. Modern vaporizers have mechanisms to reduce but not eliminate the pumping effect.

21 How does altitude affect modern vaporizers?

The effect of the change in barometric pressure on volume percent output can be calculated as follows: x′ = x (p/p′), where x′ is the output in volume percent at the new altitude (p′), and x is the concentration output in volume percent for the altitude (p), when the vaporizer is calibrated. Consider the following example: A vaporizer is calibrated at sea level (p = 760 mm Hg), taken to Denver (p′ = 630 mm Hg), and set to deliver 1% isoflurane vapor (x). The actual output (x′) is 1% (760/630) = 1.2%. Remember that partial pressure of the vapor, and not the concentration in volume percent, is the important factor in depth of anesthesia. Note that 1% at sea level (760 mm Hg) is 7.6 mm Hg and that 1.2% at Denver (630 mm Hg) is 7.6 mm Hg; thus, regardless of altitude, the clinical effect is unchanged.

22 What happens if you put the wrong agent in a vaporizer calibrated for another agent?

The incorrect agent in an agent-specific vaporizer typically delivers either an overdose or underdose. The most important factor in determining the direction of error is the vapor pressure or the agent. If an agent with a high vapor pressure is put into a vaporizer meant for a less volatile agent, the output will be excessive. If an agent with a vapor pressure lower than the agent intended for the vaporizer is accidentally used, the anesthetic output will be lower than anticipated. Increasingly, vaporizers and bottles of volatile anesthetics are keyed to prevent incorrect addition of volatile anesthetics into vaporizers.

23 What is different about the desflurane vaporizer?

Desflurane has a vapor pressure of 664 mm Hg at 20° C. In other words the boiling point of this agent is approximately at room temperature. Desflurane is also less potent than other common agents (minimum alveolar concentration [MAC] = 6%), and up to 18% volume percent may be delivered. Passively vaporizing this volume of agent will result in large temperature variations that require compensation. The desflurane vaporizer actively heats the liquid agent to 39° C. At this temperature the vapor pressure of the agent is approximately two atmospheres. This has proven a practical means of accurately delivering an anesthetic with such a high vapor pressure.

24 What prevents turning on two vaporizers simultaneously?

Modern anesthesia machines have an interlock system or an interlocking manifold that allows only one vaporizer to be turned on at a time. However, in anesthesia machines that allow for three vaporizers, the center spot must be occupied, or the interlocking manifold will not be operational.

25 At an altitude of 7000 feet, you have to set the vaporizer to deliver more desflurane than you would expect given the published minimum alveolar concentration of that agent. Explain why this does not happen with vaporizers for other anesthetic agents

Conventional vaporizers (for halothane, isoflurane, and sevoflurane) are altitude compensated. The altitude compensation occurs because the diverting valve is functionally located at the outlet of the vaporizer, a variation in design that minimizes the pumping and pressurizing effects. The output of these vaporizers is a constant partial pressure of agent, not a constant volume percent. The desflurane vaporizer does not divert a portion of the fresh gas flow through a vaporizing chamber but rather adds vapor to the gas flow to produce a true volume percent output. Because it is the number of molecules of agent (the partial pressure) that anesthetizes the patient, conventional vaporizers provide the same anesthetizing power at any altitude. The desflurane vaporizer delivers the set volume percent regardless of altitude, which represents a partial pressure (anesthetizing power) that is 24% less than the same concentration at sea level. Thus a correspondingly higher percentage of desflurane must be delivered to achieve MAC at 7000 feet.

26 A patient with malignant hyperthermia needs to be anesthetized. Should the vaporizers be removed from the anesthesia machine?

Datex-Ohmeda vaporizers are easily removed simply by releasing a latch and lifting the vaporizer from the machine. On the Drager anesthesia machines it is necessary to remove two Allen screws to release the vaporizers. Then unless the vaporizer is being replaced by a second one, it is necessary to install a bypass block to the empty vaporizer slot. These tasks are easily accomplished by anyone capable of manipulating an Allen wrench, but Drager recommends that their vaporizers be changed only by authorized service personnel. However, flushing the machine with oxygen for several minutes should remove all agent (except from rubber parts in the absorber and circle). The anesthesia provider should ensure that the user of the machine cannot accidentally turn on a vaporizer.

KEY POINTS: The Anesthesia Machine and Vaporizers image

1. Anesthesia machines are an integrated system that not only delivers anesthetic gases but also monitors both itself and the patient.
2. When compressed, some gases condense into a liquid (N2O and CO2), and some do not (O2 and N2). These properties define the relationship between tank volume and pressure.
3. Anesthesia machines must have a backup supply of oxygen in case the wall oxygen fails.
4. Flowmeters accurately measure only the gas for which they were explicitly calibrated.
5. The output of traditional vaporizers depends on the proportion of fresh gas that bypasses the vaporizing chamber compared with the proportion that passes through the vaporizing chamber.
6. The desflurane vaporizer actively injects vapor into the fresh gas stream, whereas all traditional vaporizers use a passive variable bypass system.

27 What is a scavenger?

Except in a close-circuit situation, gas is always entering and leaving the anesthesia breathing circuit. The exhaust gas is a mixture of expired gas from the patient and excess fresh gas that exceeded the patient’s needs but contains anesthetic agent. To reduce exposure of operating room personnel to trace amounts of anesthetic agents, it is considered appropriate to capture and expel this anesthetic-laden gas from the operating room environment. The device used to transfer this gas safely from the breathing circuit into the hospital vacuum system is called a scavenger. Because of the periodicity of breathing, gas exits the breathing circuit in puffs. The scavenger provides a reservoir for the exhaust gas until the exhaust or vacuum system, which works at a constant flow rate, can dispose of the gas. The scavenger must also prevent excess suction or an occlusion from affecting the patient breathing circuit. It does this by providing both positive and negative relief valves. Thus, if the vacuum system fails or is adjusted to too low a rate, back pressure exits through a positive-pressure relief valve. (Granted it contaminates the operating room, but that problem is minimal compared to blowing the patient’s lungs up like a balloon.) If the vacuum is adjusted too high, a negative-pressure relief valve allows room air to mix with the exhaust gas, preventing buildup of more than a 2.5 mm Hg suction at the breathing circuit.

SUGGESTED READINGS

1. Brockwell R.C., Andrews J.J. Inhaled anesthetic delivery systems. In: Miller R.D., editor. Miller’s anesthesia. ed 6. Philadelphia: Elsevier Churchill Livingstone; 2005:273-316.

2. Dorsch J.A., Dorsch S.E. Understanding anesthesia equipment, ed 5. Philadelphia: Lippincott Williams & Wilkins, 2008.

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