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.

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.

Classification

Dräger Vapor 19.1, Vapor 2000, Penlon Sigma, Datex-Ohmeda S/5 ADU Aladin vaporizers, and Datex-Ohmeda Tec 4, 5 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 copper kettle and Vernitrol)

Temperature compensated

Equipped with automatic devices that ensure steady vaporizer output over a wide range of ambient temperatures

Agent-specific

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 are measured-flow, bubble-through, non-temperature compensated, multiple agent, and out of circuit.

Vaporizer interlock

The vaporizer interlock ensures that

• Only one vaporizer is turned on
• Gas enters only the one which is on
• Trace vapor output is minimized when the vaporizer is off
• Vaporizers are locked into the gas circuit, thus ensuring they are seated correctly.

Operating principles of variable bypass vaporizers

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) using 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 are 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 is recommended by the FDA checklist (step 5), since it works for all machines. The S/5 ADU has check valves in the vaporizer control mechanisms.

How to fill vaporizers

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 "on", or if leaks are present). The only current vaporizer which can be filled while it is operating is the Tec 6 (Desflurane).

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 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

Hazards

• Incorrect agent
• Tipping
  • If tipped more than 45 degrees from the vertical, liquid agent can obstruct valves.
  • Treatment: flush for 20-30 minutes at high flow rates with high concentration set on dial. Please note that this is the recommended treatment for the Tec 4 vaporizer. The correct approach for other models differs, so their individual operating manuals must be consulted.
• Simultaneous inhaled agent administration
  • If removing the central vaporizer from a group of three on an Ohmeda Modulus machine, move the remaining two so that they are adjacent. On models which were manufactured prior to 1995, removing the center vaporizer of three defeats the interlock, and allows the outer two vaporizers to be turned on simultaneously.
• Reliance on breath by breath gas analysis rather than preventive maintenance
  • Problem: failure of temperature compensation device may result in a rapid onset, high output failure of the vaporizer
  • Failure of renewable components such as seals and O-rings may have the same effect
• Overfilling
  • May be prevented by following the manufacturer's guidelines for filling: fill only to the top etched line on the liquid level indicator glass, and fill only when the vaporizer is off. Anaesthesia 2002;57:288
• Leaks
  • Leaks are relatively common, often due to malposition of vaporizers on the back bar (Anaesthesia 2002;57:299-300), or loss of gaskets, and these leaks may not be detected with the standard checklist unless the negative pressure check is performed.
  • Tec 6 vaporizers can also leak liquid while being filled, if the desflurane bottle is missing the white rubber O-ring near its tip. This can be mistaken for a defective vaporizer (Anesth Analg 2003;96:1534-5)
• Electronic failure
  • As vaporizers incorporate electronics, they are susceptible to electronic failure. Two case reports in 2000 detail ADU vaporizers failing due to "fresh gas unit failure", and from copious emesis soaking the machine (Anaesthesia 2000;55:1214-5, Anaesthesia 2000;55:1215).

Safety features

Important safety features include:

• Keyed fillers
• Low filling port
• Secured vaporizers (less ability to move them about minimizes tipping)
• Interlocks
• Concentration dial increases output in all when rotated counterclockwise (as seen from above)

Current models

Variable bypass vaporizers

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

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

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 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 vaporizer. Click on the thumbnail, or on the underlined text, to see the larger version (36 KB).

Penlon Sigma is similar to the Tec vaporizers, and can be found on either type (Ohmeda, Dräger) of machine. The Penlon Sigma Delta sevoflurane vaporizer fits a SelectaTec interlock bar for 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 Ohmeda 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. The Dräger site has a description of the Vapor 19, with operating principles and clinical guidelines.

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 Aladin 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).

Datex-Ohmeda Aladin vaporizer 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 S/5 ADU features a low agent alarm for desflurane, the hypoxic guard system takes the desflurane concentration into account along with nitrous oxide, and the desflurane cassette works without added heat. 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.

Gas/vapor blenders

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

Tec 6 Suprane^TM (desflurane) vaporizer: Because of the volatility of this agent, it requires new systems 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.

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). It may be filled during use. 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. Datex-Ohmeda has moving pictures on their site to help in troubleshooting the Tec 6.

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).

Injectors

The Siemens vaporizer (for the Kion; click on "Anaesthesia Systems", then Kion on the left) is a concentration-calibrated injector. By its nature, it needs no thermal compensation, since it does not vaporize agent.

Siemens vaporizer operating principle. Click on the thumbnail, or on the underlined text, to see the larger version (27 KB).

A calibrated throttle valve is opened or closed by the user. The more it is closed, the greater the pressure exerted by the fresh gas flow on the surface of the liquid anesthetic. This pressure tends to force liquid to atomize at the injector nozzle. The number of molecules of liquid injected is proportional to the resistance to gas flow at the throttle valve (controlled by the concentration-control dial). The liquid droplets vaporize in the flowing fresh gas stream. Thus, since the liquid is not vaporizing (at least within the vaporizer), no thermal compensation is required.

A desflurane vaporizer for the Kion is not available.