Historical picture

The Anesthesia Gas Machine

Michael P. Dosch CRNA PhD, Darin Tharp CRNA MS
University of Detroit Mercy - Nurse Anesthesia
This site is https://healthprofessions.udmercy.edu/academics/na/agm/index.htm.

Revised Jan. 2024


Processing: Vaporizers

Physical principles

Vapor pressure Molecules escape from a volatile liquid to the vapor phase, creating a "saturated vapor pressure" at equilibrium. Vapor pressure (VP) increases with temperature. VP is independent of atmospheric pressure, it depends only on the physical characteristics of the liquid, and its temperature. So, even although evaporation proceeds at a rate governed by liquid temperature and is independent of altitude (barometric pressure), individual vaporizer types may or may not function the same at altitude.

Just as our skin cools when we step out of the shower, the temperature of the remaining liquid will drop as vaporization proceeds, lowering VP, unless this is prevented. The latent heat of vaporization is the number of calories needed to convert 1 g of liquid to vapor, without temperature change in the remaining liquid.

Specific heat is the number of calories needed to increase the temperature of 1 g of a substance by 1 degree C. Manufacturers select materials for vaporizer construction with high specific heats to minimize temperature changes associated with vaporization.

Thermal conductivity - a measure of how fast a substance transmits heat. High thermal conductivity is desirable in vaporizer construction.


Dräger Vapor 19.1, Vapor 2000 & 3000, Penlon Sigma, Aladin vaporizers (Aisys, Avance), and Tec 4, 5, 7 are classified as

Variable bypass
Fresh gas flow from the flowmeters enters the inlet of any vaporizer which is on. The concentration control dial setting splits this stream into bypass gas (which does not enter the vaporizing chamber), and carrier gas (also called chamber flow, which flows over the liquid agent).
Flow over
Carrier gas flows over the surface of the liquid volatile agent in the vaporizing chamber, as opposed to bubbling up through it (as in the older copper kettle and Vernitrol)
Temperature compensated
Equipped with automatic devices that ensure steady vaporizer output over a wide range of ambient temperatures
Only calibrated for a single gas, usually with keyed fillers that decrease the likelihood of filling the vaporizer with the wrong agent
Out of circuit
Out of the breathing circuit, as opposed to (much) older models such as the Ohio #8 (Boyle's bottle) which were inserted within the circle system.

The copper kettle and Vernitrol (strictly museum pieces at this point) are measured-flow, bubble-through, non-temperature compensated, multiple agent, and out of circuit.

The Tec 6 desflurane vaporizer is not a variable bypass vaporizer, it is a gas-vapor blender.

Vaporizer Models
Classification Tec 4, 5, 7, SevoTec, and Aladin (Aisys, Avance); Vapor 19, Vapor 2000 & 3000 Tec 6 (Desflurane)
Splitting ratio (carrier gas flow) Variable-bypass (vaporizer determines carrier gas split) Dual-circuit (carrier gas is not split)
Method of vaporization Flow-over (including the Aladin for desflurane, which does not require added heat like the Tec 6) Gas/vapor blender (heat produces vapor, which is injected into fresh gas flow)
Temperature compensation Automatic temperature compensation mechanism Electrically heated to a constant temperature (39ºC; thermostatically controlled)
Calibration Calibrated, agent-specific Calibrated, agent-specific
Position Out of circuit Out of circuit
Capacity Tec 4: 125 mL
Tec 5: 300 mL
Vapor 19: 200 mL
Aladin: 250 mL
390 mL

Vaporizer interlock

The vaporizer interlock ensures that

Operating principles of variable bypass vaporizers

Variable bypass vaporizer

Variable bypass- principle. Click on the thumbnail, or on the underlined text, to see the larger version.

Total fresh gas flow (FGF) enters and splits into carrier gas (much less than 20%, which becomes enriched- saturated, actually- with vapor) and bypass gas (more than 80%). These two flows rejoin at the vaporizer outlet. The splitting ratio of these two flows depends on the ratio of resistances to their flow, which is controlled by the concentration control dial, and the automatic temperature compensation valve.

Effect of flow rate: The output of all current variable-bypass vaporizers is relatively constant over the range of fresh gas flows from approximately 250 mL/min to at least 10 L/min, due to extensive wick and baffle system that effectively increases surface area of vaporizing chamber. All sevoflurane vaporizers are less accurate (due to the low vapor pressure of the agent) at high fresh gas flows (> 10 L/min) and high vaporizer concentration settings typical after induction, where they deliver less than the dial setting (Anesth Analg 2000;91:834 notes that this tendency is accentuated if the vaporizer is nearly empty). Clinically this is relatively unimportant, since we titrate to effect (end tidal agent concentration) and use overpressure.

Effect of ambient temperature: The output of modern vaporizers is linear from 20-35 degrees C, due to

  1. Automatic temperature compensating devices that increase carrier gas flow as liquid volatile agent temperature decreases
  2. Wicks in direct contact with vaporizing chamber walls
  3. Constructed of metals with high specific heat and thermal conductivity

Effect of intermittent back pressure transmitted from breathing circuit: The pumping effect is due to positive pressure ventilation or use of the oxygen flush valve. It can increase vaporizer output. Modern vaporizers are relatively immune (older vaporizers were certainly not immune) due to check valves between the vaporizer outlet and the common gas outlet, smaller vaporizing chambers, or tortuous inlet chambers. Any of these design features prevent gas which has left the vaporizers from re-entering it.

Effect of altitude

The effect of altitude on vaporizer performance is controversial. Dorsch and Dorsch (Understanding Anesthesia Equipment 5th ed. 2008) state that one should consult the operator's manual. Some sources state that variable bypass types need not be adjusted for moderate changes in barometric pressure, but the Tec 6 must be dialed up beyond the desired dose at higher altitudes. Other sources disagree. Note that nitrous oxide is less useful as altitude increases, since it becomes more difficult to supply adequate pO2 using N2O when total atmospheric pressure declines. See:

How to fill vaporizers

Filling keyed vaporizer Filling a keyed vaporizer. Click on the thumbnail, or on the underlined text, to see the larger version (35 KB).
Filling funnel vaporizer Filling a funnel-type vaporizer. Click on the thumbnail, or on the underlined text, to see the larger version (28 KB).

For either funnel or keyed filler types, fill the vaporizer only to the top etched line within the sight glass. Do not hold the bottle up on a keyed filler until it stops bubbling (this will overfill the chamber, particularly if the concentration control dial is "on", or if leaks are present). The only current vaporizer which can be filled while it is operating is the Tec 6 Desflurane (but even that vaporizer is safer to fill in the "off" position). But why remember this particular exception? It's safest to fill any vaporizer only when it is "off."

How much liquid agent does a vaporizer use per hour?

Ehrenwerth and Eisenkraft (1993) give the formula:
3 x Fresh gas flow (FGF) (L/min) x volume % = mL liquid used per hour

Or one can determine the volume (mL) of saturated vapor needed to provide 1% (ie 4000 x 0.01 = 40 mL); then use Avogadro's hypothesis, the molecular weight, the liquid density, and molar volume (22.4 L at 20 degrees C) to determine how many mL of liquid become 40 mL vapor per minute. Typically, 1 mL of liquid volatile agent yields about 200 mL vapor. This is why tipping an older vaporizer is so hazardous- it discharges liquid agent into the control mechanisms, or distal to the outlet. And minute amounts of liquid agent discharged distal to the vaporizer outlet result in a large bolus of saturated vapor delivered to the patient instantaneously.

Hazards and safety features of contemporary vaporizers


Safety features

Important safety features include:

Vaporizers- current models

Vapor 2000

Vapor 2000. Click on the thumbnail, or on the underlined text, to see the larger version (68 KB).

The Vapor 2000 is one of two tippable vaporizers (Aisys cassettes are the other). The dial must first be rotated to a "T" setting ("transport" or "tip") which is beyond zero (clockwise as viewed from above).


Vapor 3000

Vapor 3000. Click on the thumbnail, or on the underlined text, to see the larger version.

The Vapor 3000 is also tippable. The dial must first be rotated to a "T" setting ("transport" or "tip") which is beyond zero (clockwise).


Tec 7

Tec 7. Click on the thumbnail, or on the underlined text, to see the larger version.



Aladin vaporizer

Aladin vaporizer. Click on the thumbnail, or on the underlined text, to see the larger version (185 KB).

Aladin vaporizer (Aisys) Cassettes containing each volatile liquid anesthetic are inserted into a port containing the central electronic control mechanism, which recognizes the contents of the cassette and dispenses agent into the stream of fresh gas flow. Because each cassette is only a liquid sump without control mechanisms, they can be tipped in any orientation without danger, and they are maintenance free. The cassette and the control mechanisms are checked as part of the electronic equipment checklist daily. The Aladin will not deliver volatile agent without mains power or battery backup, and adequate oxygen (or air) pressure. The output of older vaporizers varies slightly with changes in fresh gas mixture, whereas the Aladin compensates for this automatically. The ADU electronic gas mixer features a low agent alarm for desflurane (but NOT the other agents!), the hypoxic guard system takes the desflurane concentration into account along with nitrous oxide, and the desflurane cassette works without added heat (it uses waste heat from the electronics). The Aisys has low agent alarms on all vaporizers. The cassettes are extremely light, and may be removed with one hand. For a study of this vaporizer's performance, see Anesth Analg 2001;93:391.

Gas/vapor blenders

Tec 6

Tec 6 vaporizer. Click on the thumbnail, or on the underlined text, to see the larger version (132 KB).

Tec 6 desflurane vaporizer: Because of the volatility of this agent, new systems were designed to contain, transfer, and vaporize it. The saturated vapor pressure at room temperature (20 degrees C) is 664 torr (87% of one atmosphere). This means that desflurane is nearly boiling at room temperature. The vaporizer is a gas/vapor blender, not a variable bypass type. Note that not all desflurane vaporizers are Tec 6 type. The Aladin cassette (Aisys, Avance) is a variable bypass vaporizer.

Tec 6 operating principles Tec 6 operating principles. Click on the thumbnail, or on the underlined text, to see the larger version (57 KB).

Classification (from Anesth Analg 1993;76:1338): electrically heated, dual circuit gas/vapor blender, constant-temperature, agent specific, and out-of-circuit. Function: Heats agent to 39 degrees C, which produces a vapor pressure of around 1550 mm Hg. Electronic controls inject pure vapor into the fresh gas flow from the flowmeters, controlled by the concentration control dial, and a transducer (which senses the fresh gas flow rate, and adjusts the vapor output accordingly). Requires electrical power (it shuts off in power failures!), and has alarms; two unusual aspects compared to other contemporary vaporizers. In use it is similar to variable bypass vaporizers: it fits in the interlocks, and is mounted on the back bar in a similar way. It is accurate at low flows (ie considerably less than 1 L/min total FGF). The operator's manual states that it may be filled during use (but it is safer to turn it off when filling). A mark on a liquid crystal display indicates when the liquid level is one bottle low (250 mL). The user must replace a battery which powers the alarms periodically. There is an alarm for low liquid level. The unit requires a warm-up period.

Checkout procedure for the Tec 6

Tec 6 alarm panel

Tec 6 alarm panel. Click on the thumbnail, or on the underlined text, to see the larger version (144 KB).

  1. Press and hold the mute button until all lights and alarms activated.
  2. Turn on to at least 1% and unplug the electrical connection. A "No Output" alarm should ring within seconds. This tests battery power for the alarms. This step is crucial in relation to the quick emergence characteristics of this agent- any interruption in its supply must be noted and responded to at once.

Vaporizers- older variable-bypass models

Tec 4 vaporizers

Tec 4 vaporizers. Click on the thumbnail, or on the underlined text, to see the larger version (120 KB).

Ohmeda Tec 4, 5 With the center vaporizer removed (if three are mounted side by side), one can activate both outer vaporizers simultaneously (in machines manufactured after 1995, this fault is corrected). Vaporizer outlet has check valve.

Tec 5 vaporizers Tec 5 vaporizers. Click on the thumbnail, or on the underlined text, to see the larger version (146 KB).
Tec 5 vaporizers including Sevotec 5

Sevotec 5 vaporizer (right). Click on the thumbnail, or on the underlined text, to see the larger version (25 KB).

The Sevotec 5 is used in a similar fashion to the other Tec 5 vaporizers. Note that in December 1997 the product labeling for sevoflurane was changed to allow fresh gas flow as low as 1 L/min (for not greater than 2 MAC-hours). Note also that sevoflurane may very well be safe at fresh gas flows of 1 L/m (or less) indefinitely, particularly when used with absorbents lacking strong bases (Anaesth Intensive Care. 2019;47:223 at doi 10.1177%2F0310057X19843304, Anesth Analg 2021;132:993).


Penlon Sigma Delta- Sevoflurane

Penlon Sigma Delta sevoflurane vaporizer (right). Click on the thumbnail, or on the underlined text, to see the larger version (15 KB).

Penlon Sigma is similar to the Tec vaporizers, and can be found on either type (GE, Dräger) of machine. The Penlon Sigma Delta sevoflurane vaporizer fits on Dräger machines.


Vapor 19.3

Vapor 19.3 vaporizer. Click on the thumbnail, or on the underlined text, to see the larger version (144 KB).

Dräger Vapor 19.1 is similar to Tec 4, 5: all are variable bypass types. The interlock on Dräger machines continues to function if any vaporizers are removed, but one must attach a short-circuit block to prevent leaks if any vaporizer is removed. There is no outlet check valve- the tortuous inlet arrangement protects from the pumping effect.


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