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Draw over vaporizer - Sometimes called ‘vaporizer-in-circle/circuit’ or ‘VIC’, although in practice they are not used with circle systems. plenum originates from the Latin for ‘full’ or ‘pressurized’. Sometimes called ‘vaporizer-out-of-circle/circuit’ or ‘VOC’.
As anaesthetic liquid changes state and becomes a vapour, it absorbs heat from its surroundings, which provides the energy to break bonds between the liquid molecules. This energy is known as the latent heat of vaporization. Copper was used for the housing of the vaporizer because it has both a high heat capacity and high thermal conductivity
you may see your local barista using a bimetallic strip thermometer to measure the temperature of the steamed milk.
▪ A vaporizer (anesthetic agent or vapor
delivery device) changes a liquid
anesthetic agent into its vapor and adds
a controlled amount of that vapor to the
fresh gas flow to the breathing system.
Up to three vaporizers are commonly
attached to an anesthesia machine, but
only one can be used at a time.
▪ ‘Inhalational Anesthesia’ tried by early ‘clinicians’ from time immemorial
▪ Historical records show use of “Soporific sponges” soaked in
▪ 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 to
achieve surgical anesthesia, as a public demonstration, in the history
▪ Thus ether then chloroform again back to ether, Led to evolution
of various devices used for vaporization of these liquids
▪ 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.
▪ 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.
▪ 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 a
▪ 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. of
a substance/1 ml. of a liquid by 10 Celsius is called the Specific Heat of the
▪ 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
▪ High thermal conductivity and specific heat capacity of the jacket (a ‘heat sink’)
▪ Automatic adjustment of the splitting ratio:
1. Bimetallic strip
3. Electronically controlled
Mechanism for adding anaesthetic vapour to the fresh gas flow
Volatile anaesthetics are too potent to be used at their saturated vapour
pressure and must therefore be diluted to a safe concentration before being
delivered to the patient. This is commonly achieved in one of two ways’
▪ Variable bypass vaporizers (e.g. most modern vaporizers, apart from the Tec
6) split the fresh gas flow into two streams. One stream enters a vaporization
chamber and leaves fully saturated with anaesthetic vapour, whilst the
remainder of the fresh gas bypasses this chamber. The two gas flows are
reunited downstream to produce the desired final concentration. Altering the
FGF does not alter the ratio between the flows in the two streams (splitting
ratio) and therefore does not alter the final concentration.
▪ Measured flow vaporizers (e.g. the Tec 6 desflurane vaporizer) use a separate
heated and pressurized vapour stream that is precisely injected into the FGF.
Increasing the FGF dilutes the output and therefore an automated mechanism
compensates for this.
The internal resistance of the vaporizer
▪ Draw-over vaporizers have low internal resistances to gas flow. The patient’s
inspiratory 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 be
available. Mechanisms to improve the accuracy of anaesthetic delivery, such a
baffles and temperature compensation increase resistance and are not usually
present in draw-over vaporizers, leading to unpredictable performance. Examples,
the Goldman, the Oxford Miniature Vaporizer (OMV) and Epstein and Macintosh of
Oxford (EMO) vaporizers. These vaporizers are used within the breathing system.
▪ Plenum vaporizers in contrast rely on pressurized gas flow rather than the
patient’s inspiratory effort. They have a high internal resistance and are used with
continuous flow anaesthetic machines. A plenum vaporizer should saturate all gas
that passes through the vaporization chamber in order to achieve a consistent
output, even at high FGFs. Examples: Boyle’s bottle, the Copper kettle, the Tec 5
series and the Aladin cassette. These vaporizers are used outside the breathing
▪ Latent heat of vaporization, when Left unchecked, the temperature of the
remaining liquid anaesthetic will fall significantly, along with its saturated
vapour pressure and therefore lead to a reduction in the output of the
▪ The first method used to compensate for the latent heat of vaporization is to
use a heat sink, such as a water bath (Boyle’s bottle) or a large mass of
copper. Modern vaporizers are still made of large masses of metal for this
▪ Invariably though, there will be some drop in temperature within the vaporizer
as it is used. To maintain a constant output, this drop in temperature and
saturated vapour pressure of the anaesthetic must be compensated for. This
is achieved by the use of devices such as bimetallic strips, bellows or
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.
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.
Limited accuracy and slow response times.
Characteristics of Ideal VAPORIZER
▪ Performance not affected by changes in
▪ 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
▪ Only calibrated for a single gas, usually with keyed fillers
▪ Out of circuit
Property TEC 4, Vapor
TEC 5 TEC 7 Vapor 19n Vapor 2000 D Vapor
TEC 6 Des.
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
With dry wicks
With wet wicks
TEC 6: 425
Automatic Automatic Automatic Automatic Automatic Thermostatically controlled at
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
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.
• Could be used with several different anaesthetic agents.
• Full saturation of the vapour chamber gas flow was
• No temperature compensation so volatile output fell as
the reservoir cooled.
• The concentration of anaesthetic delivered to the patient
• Tipping Boyle’s bottle could lead to dangerous rises in
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.
• Small and cheap.
• Simple to use and service.
• Lightweight and portable.
• Restricted output prevents halothane overdosing.
• Variable output that is difficult to measure.
• No temperature compensation.
• Unsuitable for use with less potent anaesthetic agents, because it is
• 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
• Robust and easily serviceable.
• Most volatile agents can be used by simply switching the interchangeable
• 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.
• 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
Reliable and generally safe.
Bulky 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
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
TEC - 4
Plenum type, variable bypass, flow over with wicks, temperature compensated,
concentration calibrated, out of circle, agent specific.
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
▪ 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.
• 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.
• 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
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
▪ 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
▪ to 5 L/min for other anesthetics, to pass through vaporizer
• In the above figure, note different vapor pressure-temperature relationships between common
• 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
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
• 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.
• Requires an electrical power supply.
• Requires time to warm up before it is operational.
• 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
• 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
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 anaesthetic
▪ It is designed for use with Datex-Ohmeda S/5 ADU and similar
▪ 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
• Automated recognition of the agent inserted.
• On-screen data showing agent levels and anaesthetic usage.
• Automated, electronically monitored and controlled FGF, temperature and pressure
• No risk of spillage of anaesthetic agent into the bypass channel.
• Cassette can be carried safely in any orientation.
• 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.
• 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
▪ 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.
▪ Bottle Keyed System
▪ Funnel Fill System
▪ Keyed Filling System
▪ Quick-Fill System
▪ Easy-Fill System
▪ Desflurane Filling Systems
Quick fill system
▪ Vaporizers may be filled by a conventional
funnel-fill mechanism, in which the liquid
anesthetic is simply poured into a funnel in
▪ Complication is filling with wrong agent.
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.
▪ A color coded bottle adaptor is attatched to
bottle and then fitted into the vaporizer.
▪ A drain plug is there for draining vaporizer.
1. Incorrect Agent
4. Reversed Flow
5. Control Dial in Wrong Position
7. Vapour Leak into the Fresh Gas Line
8. Contaminants in the Vaporizing Chamber
9. Physical Damage
10. No Vapor Output
▪ 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
▪ Overfilling May result in high output
▪ Fill only up to max filling line
▪ Fill only when the vaporizer is off
▪ Relatively common due to malposition or loose filler cap.
▪ Not detected with standard checklist perform negative pressure check
▪ Vaporizers not equipped with keyed filling lead to misfiling.
▪ It occurs by filling a vaporizer with contaminated anesthetic bottle.
▪ Leads to decreased vaporizer output.
▪ Simultaneous Inhaled Anesthetic Administration
▪ Happened in old machines with no interlock system