The Anesthesia Gas Machine
Michael P. Dosch
University of Detroit Mercy Graduate Program in Nurse Anesthesiology
This site is http://www.udmercy.edu/crna/agm/.
Revised July 2012
ANESTHESIA GAS MACHINE> COMPONENTS & SYSTEMS> PROCESSING> VAPORIZERS
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.
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. Thus, the temperature of the remaining liquid will drop as vaporization proceeds, lowering VP, unless this is prevented.
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, Penlon Sigma, Aladin vaporizers (Aisys, ADU), and Tec 4, 5, 7 are classified as
|Classification||Tec 4, 5, 7, SevoTec, and Aladin (Aisys, ADU); Vapor 19, Vapor 2000||Copper Kettle, Vernitrol||Tec 6 (Desflurane)|
|Splitting ratio (carrier gas flow)||Variable-bypass (vaporizer determines carrier gas split)||Measured-flow (operator 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)||Bubble-through||Gas/vapor blender (heat produces vapor, which is injected into fresh gas flow)|
|Temperature compensation||Automatic temperature compensation mechanism||Manual (i.e., by changes in carrier gas flow)||Electrically heated to a constant temperature (39ºC; thermostatically controlled)|
|Calibration||Calibrated, agent-specific||None; multiple-agent||Calibrated, agent-specific|
|Position||Out of circuit||Out of circuit||Out of circuit|
|Capacity||Tec 4: 125 mL
Tec 5: 300 mL
Vapor 19: 200 mL
Aladin: 250 mL
|200-600 mL (no longer manufactured)||390 mL|
The vaporizer interlock ensures that
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 15 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;91:834-6 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
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. The check valves are why a negative pressure leak check was recommended by the 1993 FDA checklist (step 5), since it works for all machines. The ADU has check valves in the vaporizer control mechanisms.
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 (see Ehrenwerth and Eisenkraft Anesthesia Equipment 1993).
|Filling a keyed vaporizer. Click on the thumbnail, or on the underlined text, to see the larger version (35 KB).|
|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).
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 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.
Important safety features include:
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. Click on the thumbnail, or on the underlined text, to see the larger version (146 KB).|
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).
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 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.
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 (ADU/Aisys cassettes are the other). The dial must first be rotated to a "T" setting ("transport" or "tip") which is beyond zero (clockwise).
Aladin vaporizer. Click on the thumbnail, or on the underlined text, to see the larger version (185 KB).
Aladin vaporizer (Aisys, ADU) 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 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. 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;93:391-5.
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, ADU) is a variable bypass vaporizer.
|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;76:1338-41): 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. Click on the thumbnail, or on the underlined text, to see the larger version (144 KB).