Anaesthesia vaporizers

Dr Rahul Varshney

Introduction

▪ A vaporizer (anesthetic agent or vapordelivery device) changes a liquidanesthetic agent into its vapor and addsa controlled amount of that vapor to thefresh gas flow to the breathing system.Up to three vaporizers are commonlyattached to an anesthesia machine, butonly one can be used at a time.

Historical aspects

▪ ‘Inhalational Anesthesia’ tried by early ‘clinicians’ from time immemorial

▪ Historical records show use of “Soporific sponges” soaked in

‘medicinal elixirs’

▪ Actual use of easily ‘vaporizable substances’ came much later, in 18th

century, 16th October 1846 to be exact

▪ WTG Morton used his ‘ Letheon’ inhaler - Ether inhaler first time toachieve surgical anesthesia, as a public demonstration, in the historyof mankind.

▪ Thus ether then chloroform again back to ether, Led to evolutionof various devices used for vaporization of these liquids

Historical aspects

▪ The main ‘reviver’ of ether was Kurt Schimmelbusch and his ‘mask’

▪ Contraption made with wires and layer of gauze pieces/used along with

‘open ether - drop by drop method’ for administration of ether.

▪ “Yankauer’s mask” in 1904, Flagg’s can/ KEM Bottle,

▪ More sophistication: Epstein Macintosh Oxford (EMO) vaporizer with Oxford

inflating bellows (OIB)

▪ “Anesthesia Machine” was invented.

Historical aspects:

▪ With deeper insights into physical principles, properties and laws

▪ Advances for development of more sophisticated devices

▪ As a result Oxford Miniature Vaporizer (OMV), Copper Kettle.

▪ halogenated compounds like halothane/halogenated ethers

▪ has produced the Tec series of vaporizers.

▪ Presently available modern vaporizers

▪ advanced in their construction capable of delivering precise, predictable

and calculated/ constant concentration of the Volatile anesthetic agent.

▪ Thus the humble beginning has evolved in to a precision perfect and

an analytical science.

Physical principles:

▪ Heat of Vaporization

▪ The Number of calories required to vaporize 1 ml. of the liquid

▪ Latent heat of vaporization

▪ The Number of calories needed to convert 1 gram of liquid to vapor without atemperature change

▪ Temperature of remaining liquid falls and may decrease rate of vaporization

▪ Specific heat

▪ The quantity of heat energy required to increase the temperature of a 1 gm. ofa substance/1 ml. of a liquid by 10 Celsius is called the Specific Heat of thesubstance/ liquid.

▪ Thermal conductivity

▪ Measure of speed with which heat flows through a substance.

There are a number of ways of classifying vaporizers:

Mechanism for adding anaesthetic vapour to the fresh gas flow

▪ Variable bypass

▪ Measured flow

The internal resistance of the vaporizer

▪ High: plenum vaporizers

▪ Low: draw-over

Temperature compensation

▪ High thermal conductivity and specific heat capacity of the jacket (a ‘heat sink’)

▪ Automatic adjustment of the splitting ratio:

1. Bimetallic strip

2. Bellows

3. Electronically controlled

Mechanism for adding anaesthetic vapour to the fresh gas flow

Volatile anaesthetics are too potent to be used at their saturated vapourpressure and must therefore be diluted to a safe concentration before beingdelivered to the patient. This is commonly achieved in one of two ways’

▪ Variable bypass vaporizers (e.g. most modern vaporizers, apart from the Tec6) split the fresh gas flow into two streams. One stream enters a vaporizationchamber and leaves fully saturated with anaesthetic vapour, whilst theremainder of the fresh gas bypasses this chamber. The two gas flows arereunited downstream to produce the desired final concentration. Altering theFGF does not alter the ratio between the flows in the two streams (splittingratio) and therefore does not alter the final concentration.

▪ Measured flow vaporizers (e.g. the Tec 6 desflurane vaporizer) use a separateheated and pressurized vapour stream that is precisely injected into the FGF.Increasing the FGF dilutes the output and therefore an automated mechanismcompensates for this.

Bypass vaporizer

The internal resistance of the vaporizer

▪ Draw-over vaporizers have low internal resistances to gas flow. The patient’sinspiratory effort is sufficient to draw fresh gas through the vaporizer and draw-over vaporizers are therefore useful in the field where pressurized gas may not beavailable. Mechanisms to improve the accuracy of anaesthetic delivery, such abaffles and temperature compensation increase resistance and are not usuallypresent in draw-over vaporizers, leading to unpredictable performance. Examples,the Goldman, the Oxford Miniature Vaporizer (OMV) and Epstein and Macintosh ofOxford (EMO) vaporizers. These vaporizers are used within the breathing system.

▪ Plenum vaporizers in contrast rely on pressurized gas flow rather than thepatient’s inspiratory effort. They have a high internal resistance and are used withcontinuous flow anaesthetic machines. A plenum vaporizer should saturate all gasthat passes through the vaporization chamber in order to achieve a consistentoutput, even at high FGFs. Examples: Boyle’s bottle, the Copper kettle, the Tec 5series and the Aladin cassette. These vaporizers are used outside the breathingsystem.

Temperature compensation

▪ Latent heat of vaporization, when Left unchecked, the temperature of theremaining liquid anaesthetic will fall significantly, along with its saturatedvapour pressure and therefore lead to a reduction in the output of thevaporizer.

▪ The first method used to compensate for the latent heat of vaporization is touse a heat sink, such as a water bath (Boyle’s bottle) or a large mass ofcopper. Modern vaporizers are still made of large masses of metal for thispurpose.

▪ Invariably though, there will be some drop in temperature within the vaporizeras it is used. To maintain a constant output, this drop in temperature andsaturated vapour pressure of the anaesthetic must be compensated for. Thisis achieved by the use of devices such as bimetallic strips, bellows orelectronic control.

The concentration of anaesthetic produced by the

vaporizer depends on the fraction of fresh gas that is

diverted into the vaporizing chamber. This fraction is

governed by the calibrated control dial. The proportion

bypassing divided by the proportion entering the

vaporizing chamber is known as the splitting ratio.

In order to ensure that the end concentration is controlled

only by the splitting ratio and not by variations in the

amount of anaesthetic leaving the vaporizing chamber, the

diverted gas must always become fully saturated with

vapour before it re-joins the bypass gas. This is achieved

using wicks that increase the surface area for evaporation

of the anaesthetic liquid and baffles that direct the

incoming gas down closer to the surface of the liquid.

These features significantly increase the internal

resistance of the vaporizer.

Bimetallic strips

The bimetallic strip consists of strips two different metals joined together.

The metals have different coefficients of thermal expansion, and they are

wound into a coil. As the temperature increases, one metal will expand

more than the other, causing the coil to loosen. Similarly, the coil will

tighten as the temperature decreases. At the centre of the coil is a

pointer, which moves across a calibrated dial as the coil tightens or

loosens so that the temperature can be read.

▪ Advantages

Cheap.

▪ Disadvantages

Limited accuracy and slow response times.

Characteristics of Ideal VAPORIZER

▪ Performance not affected by changes in

▪ FGF,

▪ Volume of liquid agent,

▪ Ambient temperature & pressure,

▪ Decrease in temperature & pressure

▪ Low resistance to flow

▪ Light weight with small liquid requirement

▪ Economical and safe to use

▪ Corrosion and solvent-resistant

Features of modern vaporizer

▪ Variable bypass

▪ Fresh gas splits into bypass gas and carrier gas

▪ Flow over

▪ Carrier gas flows over the surface of the liquid volatile agent in the vaporizing chamber

▪ 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

▪ Out of circuit

Property TEC 4, Vapor

19n, 2000,

Aladin

TEC 5 TEC 7 Vapor 19n Vapor 2000 D Vapor

TEC 6 Des.

Principle of

vaporization

Flow over, Flow over Flow over Flow over Flow over Gas-vapor blender

Carrier gas flow Variable bypass Variable bypass Variable bypass Variable bypass Variable bypass Dual circuit

Capacity mls.

With dry wicks

With wet wicks

135

100

300

225 225200

140

360

280

D-vapor 300

TEC 6: 425

Thermo-

compensation

Automatic Automatic Automatic Automatic Automatic Thermostatically controlled at

39 0C.

Position Out of circuit Out of circuit Out of circuit Out of circuit Out of circuit Out of circuit

specificity Agent-specific Agent-specific Agent-specific Agent-specific Agent-specific Agent-specific

Low flow suitability Not very good Good Very Good Good Very Good Very Good

Comparative properties

OLD VAPORIZERS

MORTON’S ETHER INHALER

Draw over, flow over with wicks, concentration not calibrated,

temperature not compensated, agent specific.

OPEN DROP METHOD

Draw over, flow over without wicks, concentration not calibrated, temperature not compensated, multiple agent.

BOYLES BOTTLE (1920)

Plenum type, variable bypass, flow over or bubble through, concentration

poorly calibrated, temperature not compensated, agent specific, out of circle.

Advantages• Could be used with several different anaesthetic agents.

• Full saturation of the vapour chamber gas flow was

possible.

Disadvantages• No temperature compensation so volatile output fell as

the reservoir cooled.

• The concentration of anaesthetic delivered to the patient

was imprecise.

• Tipping Boyle’s bottle could lead to dangerous rises in

anaesthetic concentrations

Used with early continuous flow anaesthetic machines to deliver ether, trichloroethylene or chloroform.

GOLDMANS VAPORIZER (1959)

Plenum or Draw over type ,variable bypass, flow over, temperature not

compensated, concentration poorly calibrated, multiple agent, both inside

and outside circle.

Advantages• Small and cheap.

• Simple to use and service.

• Lightweight and portable.

• Restricted output prevents halothane overdosing.

Disadvantages• Variable output that is difficult to measure.

• No temperature compensation.

• Unsuitable for use with less potent anaesthetic agents, because it is

inherently inefficient.

• There is a risk of anaesthetic agent spillage into the breathing system.

Oxford miniature vaporizer (OMV)

Variable bypass, draw-over vaporizer, not actively temperature compensated, but it

does incorporate an ethylene glycol heat sink, low resistance

Advantages• Portable.

• Robust and easily serviceable.

• Most volatile agents can be used by simply switching the interchangeable

dials.

• When the control dial is switched off, volatile agent cannot easily spill into the

breathing circuit if the vaporizer is tilted or inverted.

• An ethylene glycol heat sink buffers temperature changes, to an extent.

• Metal mesh wicks help increase the output of the vaporizer.

• Acceptable accuracy over a range of flow rates and tidal volumes.

Disadvantages• Not temperature compensated.

• Small 50 ml reservoir empties quickly.

The OMV remains in current use as part of the British military’s Triservice apparatus for delivering anaesthesia in

the field, typically with isoflurane, but also sevoflurane.

EPSTIEN MACINTOSH OXFORD (E.M.O.) (1952)

Draw Over, Concentration calibrated, Flow over, Temperature compensated

by water jacket and agent specific, can be used any where.

The accurate and precise delivery of ether irrespective of temperature

AdvantagesTemperature compensation.

Reliable and generally safe.

DisadvantagesBulky and heavy (it weighs 10 kg).

Requires high gas flow to deliver anaesthetic agents accurately.

The pumping effect of positive pressure ventilation may lead to dangerous

surges in volatile output.

Designed specifically for use with ether, which is now obsolete in the developed

world.

TEC - 2

Plenum type, concentration poorly calibrated, flow over with

wicks, temperature compensated, out of circle and agent specific.

TEC - 3

Plenum type, variable by pass, flow over with wicks, temperature

compensated, concentration calibrated, out of circle, agent

specific.

TEC - 4

Plenum type, variable bypass, flow over with wicks, temperature compensated,

concentration calibrated, out of circle, agent specific.

NEWER VAPORIZER

TEC 5

Plenum type, concentration calibrated, variable bypass, flow over with wicks,

out of circle, agent specific with keyed filling.

TEC - 7

Concentration calibrated, plenum type, Variable bypass, Flow over with wicks,

Temperature compensated, out of circuit, agent specific

▪ The latest model of the TEC series

▪ It delivers Isoflurane, Sevoflurane, Enflurane and Halothane efficiently

▪ Accommodates 225 mL of anesthetic agent.

▪ Non-spill system limits movement of liquid agent

▪ if the vaporizer is tilted or inverted

▪ helping to protect internal components.

Advantages• Easy to use and reliable.

• Properly calibrated modern variable bypass vaporizers are accurate to +/- 15% of the dial setting

for all flows between 200 ml.min-1 and 15 l.min-1 at 21°C.

• This type of vaporizer does not require a power source.

Disadvantages• High internal resistance so must be used ‘out of circle’.

• The heat sink makes the vaporizer heavy – another reason why this type of vaporizer is not

suitable for use in the field.

• There are no alarms to indicate that the level of liquid anaesthetic inside the vaporizer is low.

• Temperature compensation only works within a reasonable range of ambient temperatures.

• If the vaporizer is used in an extremely hot or cold environment it will deliver anaesthetic

unreliably.

Problems of Desflurane

▪ Desflurane is much more volatile than all the other inhalationals.

▪ Its boiling point is low -- only 22.80 C, so most of it gets evaporated at normal room temperatures

▪ Vapor pressure of desflurane at 200 C is 664 mm Hg.

▪ While that of Enflurane, isoflurane, halothane are 172, 240, 244 mm Hg. respectively

▪ At 1 atmosphere and 200 C , 100mL/min flow passing through vaporizing chamber would carry

▪ 735 mL/min. of desflurane

versus

▪ 29, 46 and 47 mL/min of enflurane, Isoflurane and halothane respectively.

▪ Under these conditions to produce 1% of desflurane,

▪ we need 73 L/min Fresh Gas Flow

As compared

▪ to 5 L/min for other anesthetics, to pass through vaporizer

• In the above figure, note different vapor pressure-temperature relationships between common

volatile agents

• Desflurane falls outside the grouping

• Hence, Not surprisingly, special vaporizer is required for desflurane.

Specifically designed to deliver Desflurane

Described as a gas/vapor blender than as a vaporizer.

It is heated electrically to 350 C

Pressurized Device with a pressure of 1550 mmHg (2 atm)

Electronic monitors of vaporizer function

FGF does not enter vaporization chamber, instead Desflurane

vapor enters the path of FGF

Percentage control dial regulates flow of Desflurane into FGF

Dial calibration is from 1% to 18%

Provided with back up 9 volt battery

Datex-Ohmeda Tec 6 Vaporizers for Desflurane (1989)

The pressure in the vapor circuit is electronically

regulated to equal the pressure in the fresh gas

circuit.

At a constant fresh gas flow rate, the operator

regulates vapor flow by use of a conventional

concentration control dial.

When the fresh gas flow rate increases, the

working pressure increases proportionally.

At a specific dial setting, at different fresh gas

flow rates, vaporizer output is constant because

the amount of flow through each circuit is

proportional.

Advantages

• Comparable accuracy to variable bypass Tec 5 vaporizers; +/- 15% of dialled setting.

• Unaffected by ambient temperature because the desflurane is heated.

• Automatically compensates for variation in FGF.

• Has visual and audible alarms to alert the anaesthetist that the vaporizer is almost

empty or that there is no output.

Disadvantages

• Requires an electrical power supply.

• Requires time to warm up before it is operational.

Safety

• As with other Tec vaporizers, it is very difficult to fill the Tec 6 vaporizer with an

anaesthetic other than desflurane due to the key system for filling. There is also a

colour coding system that helps prevent filling of vaporizers with the wrong

anaesthetic.

• The Tec 6 design prevents desflurane liquid spilling into the FGF if the vaporizer is

tilted or inverted.

Schematic diagram of the TEC 6 vaporizer.

There are two mechanisms that govern the release

of desflurane vapour into the FGF.

1. The first is the dial that is located on top of the

vaporizer that is set to a desired concentration by

the anaesthetist.

2. The second is a valve that maintains the set

concentration, in response to changes in the FGF

(if the FGF increases then the rate of desflurane

release must also increase to maintain a constant

concentration). This is achieved by a differential

pressure transducer which compares the pressure

in the desflurane circuit with that in the FGF circuit.

When the FGF is increased, its pressure also

increases and this is detected by the transducer. A

microprocessor then opens the valve enough to

increase the amount of desflurane that is injected.

The opposite occurs when the FGF is reduced.

Aladin Cassette Vaporizer System

▪ A Novel system

▪ Single vaporizer capable of delivering 5 different anaestheticagents

▪ It is designed for use with Datex-Ohmeda S/5 ADU and similar machines.

▪ FGF is divided into bypass flow and liquid chamber flow

▪ Liquid chamber flow conducted into agent specific, color coded cassette in which volatile anesthetic is vaporized

▪ Machine accepts only one cassette at a time

▪ Magnetic Labeling

Advantages

• Automated recognition of the agent inserted.

• On-screen data showing agent levels and anaesthetic usage.

• Automated, electronically monitored and controlled FGF, temperature and pressure

compensation.

• No risk of spillage of anaesthetic agent into the bypass channel.

• Cassette can be carried safely in any orientation.

Disadvantages

• Specific to a particular branded anaesthetic machine.

• Anaesthetic delivery requires electrical power.

▪ Similar to tec 4,5 vaporizers.

▪ The interlock on Dräger machines continues to function if any

vaporizers are removed.

▪ There is no outlet check valve - the tortuous inlet arrangement

protects from the pumping effect.

▪ No anti-spill mechanism.

▪ Should not be tipped more than 45.

DRAGER 19.1

Drager 2000

• Is one of two tippable vaporizers

(ADUcassettes are the other).

• The dial must first be rotated to a "T"

setting ("transport" or "tip") which is

beyond zero (clockwise).

• Tortous in let protects against pumping

effect.

Safety features

▪ Color specific (for each agent)

▪ Keyed fillers bottles

▪ Low filling port

▪ Vaporizers are locked into the gas circuit, thus ensuring they are seated correctly.

▪ Secured vaporizers Interlocks

▪ less ability to move them about minimizes tipping

▪ Only one vaporizer is turned on

▪ Trace vapor output is minimized when the vaporizer is off

▪ Concentration dial increases output in all when rotated counterclockwise.

Filling system

▪ Bottle Keyed System

▪ Funnel Fill System

▪ Keyed Filling System

▪ Quick-Fill System

▪ Easy-Fill System

▪ Desflurane Filling Systems

Quick fill system

FUNNEL FILL

▪ Vaporizers may be filled by a conventional

funnel-fill mechanism, in which the liquid

anesthetic is simply poured into a funnel in

the vaporizer.

▪ Complication is filling with wrong agent.

KEYED FILL

In this system, an agent-specific filler tube is

used, one end of which slots into a fitting on the

vaporizer, and the other end slots into a collar on

the bottle of anesthetic. The fitting on the

vaporizer and the collar on the bottle are specific

to each agent.

▪ The bottle has a permanently

attached, agent-specific filling

device that has three ridges that fit

into slots in the filler.

QUICK FILL

EASY FIL

▪ A color coded bottle adaptor is attatched to bottle and then fitted into the vaporizer.

▪ A drain plug is there for draining vaporizer.

Hazards

1. Incorrect Agent

2. Tipping

3. Overfilling

4. Reversed Flow

5. Control Dial in Wrong Position

6. Leaks

7. Vapour Leak into the Fresh Gas Line

8. Contaminants in the Vaporizing Chamber

9. Physical Damage

10. No Vapor Output

11. Projectile

Hazards

▪ Tipping

▪ If tipped >45 degrees-liquid can obstruct the outlet valves

▪ Treatment: Flush for 20-30 min at high flow rates with dial set at high concentration

▪ Overfilling May result in high output

▪ Fill only up to max filling line

▪ Fill only when the vaporizer is off

▪ Leaks

▪ Relatively common due to malposition or loose filler cap.

▪ Not detected with standard checklist perform negative pressure check

Hazards

▪ Misfilling

▪ Vaporizers not equipped with keyed filling lead to misfiling.

▪ Contamination

▪ It occurs by filling a vaporizer with contaminated anesthetic bottle.

▪ Underfilling

▪ Leads to decreased vaporizer output.

▪ Simultaneous Inhaled Anesthetic Administration

▪ Happened in old machines with no interlock system

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