SpeakerSpeaker

Co-ordinatorCo-ordinator

Dr.Tipu SultanDr.Tipu Sultan

Dr.Shivali

Pandey(M.D.)

Dr.Shivali

Pandey(M.D.)

SURFACE TENSION

Surface tension forces tend to reduce the area of interface

and favour alveolar collapse.

According to Laplace law

Pressure =2T

=2×surface tension

R Radius

So alveolar collapse is directly proportional to surface tension

but inversely proportional to alveolar size. Collapse is more

likely when surface tension increases or alveolar size

decrease.

The surface tension of the alveoli is reduced by the surfactant which is secreted by type II pneumocyte .

Ability of surfactant to lower surface tension is directly proportional to its concentration with in alveolus.

In smaller alveoli Surfactant are more concentrate surface tension Prevent collapse

In larger alveoliSurfactant are

less concentratedRelative in

surface tension

Prevent over-distens of alveoli

PHYSICS OF VAPORIZATION

Vapor Pressure

If the container is kept at a constant temperature, a dynamic equilibrium is formed between the liquid and vapor phases so that the number of molecules in the vapor phase remains constant. These molecules bombard the walls of the container, creating a pressure. This is called the saturated vapor pressure.

Vapor pressure depends only on the liquid and the temperature.

Boiling point

A liquid’s boiling point is the temperature at which its vapor pressure is equal to the atmospheric pressure.

Gas concentration

Two methods are commonly used to express the concentration of a gas or vapor.

Partial pressure

Volumes percent (Vol%)

Partial pressure

The part of the total pressure due to any one gas in the mixture is called the partial pressure of that gas.

Volume percent

Volumes percent is the number of units of volume of a gas in relation to a total of 100 units of volume for the total gas mixture.

Volumes percent expresses the relative ratio of gas molecules in a mixture, whereas partial pressure expresses an absolute value.

Partial pressure/ Total pressure = Volumes percent

Gas and vapor concentrations delivered by a vaporizer are usually expressed in volumes percent.

Patient uptake and anesthetic depth are directly related to partial pressure but only indirectly to volumes percent.

Latent heat of vaporization

Defined as the number of calories required to change 1 g of liquid into vapor without a change in temperature.

Specific heat

Specific heat of a substance is the number of calories required to increase the temperature of 1 g of a substance by 1°C.

The specific heat for an inhaled anesthetic is important because it indicates how much heat must be supplied to the liquid to maintain a constant temperature when heat is being lost during vaporization.

Manufacturers select vaporizer component materials that have high specific heat to minimize the temperature changes associated with vaporization.

Thermal conductivity

Thermal conductivity is a measure of the speed at which heat flows through a substance. Vaporizers are constructed of metals that have relatively high thermal conductivity, which helps maintain a uniform internal temperature

PROPERTIES OF COMMON ANESTHETIC AGENTS

ANESTHETIC VAPORIZERS

A vaporizer is a device that changes a liquid anesthetic agent into its vapor and adds a controlled amount of that vapor to the fresh gas flow or the breathing system.

Ideal vaporizer

It should be simple, safe, satisfactory and more practical.

It should have low resistance to gas flow.

It should be temperature compensated for uniform vaporization.

It should permit a relatively constant concentration at different flow rates of carrier gases and thus it should have flow stability.

It should permit precise, accurate, controllable and predictable delivered concentration of the vapor to the patient.

Performance of vaporizer should not be affected by changes in fresh gas flow, volume of liquid, ambient temperature and pressure, decrease in temperature due to vaporization and pressure fluctuation due to mode of respiration.

It should be light weight with small liquid requirements.

Construction should be corrosion and solvent resistant.

It should have good quality control.

Vaporizer design

Concentration calibration

Variable bypass vaporizers

Electronic vaporizers

Vaporization methods

Flow over

Injection

Bubble through type

Temperature compensation

Mechanical thermocompensation

Supplied heat

Computerized thermocompensation

Location of vaporizer

Outside the breathing system

Inside the breathing system

Specific anesthetic agent

Agent specific

Multiple agent

GRAY AND NUNN CLASSIFICATION

Plenum vaporizers –

These are named after plenum system which means the system in which fresh air is forced into a chamber.

These vaporizers are used with fresh gas flow from anaesthetic machines.

Examples are : Boyle vaporizers, copper kettle, fluotek Mark 2, Fluotek Mark 3, Halox vaporizer, Pentec (for methoxyflurane), Enfluratec (for enflurane).

Vaporizers are meant for unidirectional gas flow and have a relatively high resistance to gas flow.

They are not suitable for use as draw over vaporizers or in circle system.

Inhalers or draw over vaporizers

Usually have a very low resistance to gas flow.

Examples are EMO vaporizer, Emotril, Tecota, Oxford miniature inhaler.

Simple vaporizers for use inside of circle anesthetic system

Examples are : Goldman vaporizer, Rowbotham vaporizer

High concentration can be maintained by surrounding the glass vaporizer with a controlled temperature water jacket or by incorporating copper kettle device.

Temperature compensation may also be done by including a temperature sensitive valve which controls the amount of carrier gas to enter into the vaporizer and the rest to bypass the vaporizer.

Should have good quality control.

Vaporizer filling devices are agent specific being coded to fit the safety filing port of the correct vaporizer and the anesthetic agent supply bottle. They prevent the risk of adding the wrong agent to the wrong vaporizer and decrease the extent of spillage. The fillers are colour, red for halothane, orange for enflurane and purple for isoflurane.

VAPORIZERS AND STANDARDS

The ASTM anesthesia workstation standard contains the following provision regarding vaporizers:-

1. The effects of variations in ambient temperature and pressure, tilting, back pressure, and input flow rate and gas mixture composition on vaporizer performance must be stated in the accompanying documents.

2. The average delivered concentration from the vaporizer shall not deviate from the set value by more than ±20% or ±5% of the maximum setting, whichever is greater, without back pressure.

3. The average delivered concentration from the vaporizer shall not deviate from the set value by more than +30% or -20% or by more than +7.5% or -5% of the maximum setting, whichever is greater, with pressure fluctuations at the common gas outlet of 2 kPa with a total gas flow of 2 L/minute or 5 kPa with a total gas flow of 8 L/minute.

4. A system that prevents gas from passing through the vaporizing chamber or reservoir of one vaporizer and then through that of another must be provided.

5. The output of the vaporizer shall be less than 0.05% in the “OFF” or “zero” position if the “zero” position is also the “OFF” position.

6. All vaporizer control knobs must open counterclockwise.

7. Either the maximum and minimum filling levels or the actual usable volume and capacity shall be displayed.

8. The vaporizer must be designed so that it cannot be overfilled when in the normal operating position.

9. Vaporizers unsuitable for use in the breathing system must have noninterchangeable proprietary or 23-mm fittings. Conical fittings of 15 mm and 22 mm cannot be used. When 23-mm fittings are used, the inlet of the vaporizer must be male and the outlet female. The direction of gas flow must be marked.

10.Vaporizers suitable for use in the breathing system must have standard 22-mm fittings or screw- threaded, weight-bearing fittings with the inlet female and the outlet male. The direction of gas flow must be indicated by arrows and the vaporizer marked “for use in the breathing system.

FACTORS THAT INFLUENCE VAPORIZER OUTPUT

Flow rate

The output of all variable-bypass vaporizers is less than the dial setting at low flow rates (<250 mL/min).

Insufficient turbulence is generated in the vaporizing chamber at low flow rates to upwardly advance the vapor molecules.

Extremely high flow rates, such as 15 L/min, the output of

most variable-bypass vaporizers is less than the dial setting.

This discrepancy is attributed to incomplete mixing and failure

to saturate the carrier gas in the vaporizing chamber.

Temperature

The output of contemporary temperature-compensated vaporizers is almost linear over a wide range of temperatures.

Automatic temperature-compensating mechanisms in the bypass chamber maintain constant vaporizer output with varying temperatures.

The wick systems are placed in direct contact with the metal wall of the vaporizer to help replace energy (heat) consumed during vaporization.

Materials that vaporizers are constructed of are chosen because they have a relatively high specific heat and high thermal conductivity.

Intermittent Back pressure

When assisted or controlled ventilation is used, the positive pressure generated during inspiration is transmitted from the breathing system back to the machine and the vaporizers.

Back pressure may either increase (pumping effect) or decrease (pressurizing effect) the vaporizer output.

Pumping Effect

Factors

This change is most pronounced when there is less agent in the vaporizing chamber, when carrier gas flow is low, when the pressure fluctuations are high and frequent, and when the dial setting is low.

Mechanism

Figure shows the vaporizer during exhalation. The relative resistances of the outlets form the bypass and vaporizing chamber determine the flows to each (items 3 and 4 in the figure).

Figure shows inspiration. Positive pressure at point C

prevents gas and vapor outflow. Pressure is transmitted to

points A and B. this results in gas being compressed in the

vaporizing chamber and bypass. Because the bypass has

a smaller volume than the vaporizing chamber, more gas

enters the vaporizing chamber so that the normal ratio

between the flow to the vaporizing chamber and that

through the bypass is disturbed. The result is an increased

flow to the vaporizing chamber, which then picks up

additional anesthetic vapor.

Figure shows the situation just after the beginning of

exhalation. The pressure at point C falls rapidly and gas flows

for the vaporizing chamber and the bypass to the outlet.

Because the bypass has less resistance than the vaporizing

chamber outlet, the pressure in the bypass falls more quickly

than that in the vaporizing chamber, and gas containing vapor

flows from the vaporizing chamber into the bypass. Because

the gas in the bypass now carriers vapor and the gas flowing

from the vaporizing chamber is still saturated the concentration

in the vaporizer output is increased.

Modifications to minimize the pumping effect

Alterations to the vaporizer

Keeping the vaporizing chamber small or increasing the

size of the bypass will decrease the effects of back

pressure.

Another methods is to employ a long, spiral or large

diameter tube to lead to the vaporizing chamber.

The extra gas forced into this tube and subsequently

returned to the bypass does not reach the vaporizing

chamber.

Alterations to the anesthesia machine

Pressurizing valve

Unidirectional valve

Pressure relief device

Pressurizing Effect

Factors

The effect is greater with high flows, large pressure fluctuations, and low vaporizer settings.

Mechanisms

Figure shows a vaporizer flowing free to atmosphere. The pressure in the vaporizing chamber and the bypass is P. As gas flows to the outlet, the pressure is reduced to R.

The number of molecules of anesthetic agent picked up by

each milliliter of carrier gas depends on the density of the

anesthetic vapor molecules in the vaporizing chamber.

Depends on the agent’s vapor pressure. The vapor pressure

depends solely on the temperature and is not affected by

alterations in the atmospheric pressure.

Figure shows the situation when an increased pressure is

applied to the vaporizer outlet and transmitted to the

vaporizing chamber. This increased pressure ill compress the

carrier so that there will be more molecules per milliliter. The

number of anesthetic vapor molecules in the vaporizing

chamber will not be increased, because this depends on the

saturated vapor pressure of the anesthetic and not on the

pressure in the container. The net result is a decrease in the

concentration of anesthetic in the vaporizing chamber and the

vaporizer outlet.

Interplay between pressurizing and pumping effects

The changes in vaporizer output caused by the pumping effect usually are greater than those associated with the pressurizing effect.

The pressurizing effect is seen with high gas flows and the pumping effect at low flows.

Carrier gas composition

When the carrier gas is changed rapidly from 100% oxygen to 100% nitrous oxide, a sudden transient decrease in vaporizer output occurs, followed by a slow increase to a new steady-state value.

Because nitrous oxide is more soluble than oxygen in the halogenated liquid within the vaporizer sump, when this switch occurs, the output from the vaporizing chamber is transiently reduced.[63] Once the anesthetic liquid is totally saturated with nitrous oxide, vaporizing chamber output increases somewhat, and a new steady state is established.

The steady-state plateau is achieved more rapidly with increased flow rates, regardless of the ultimate output value

Halothane output of a North American Dräger Vapor 19.n vaporizer with different carrier gases. The initial output concentration is approximately 4% halothane when oxygen is the carrier gas at flows of 6 L/min (A). When the carrier gas is quickly switched to 100% nitrous oxide (B), the halothane concentration decreases to 3% within 8 to 10 seconds. A new steady-state concentration of approximately 3.5% is then attained within 1 minute. See text for details.  

VAPORIZER MOUNTING SYSTEM

Permanent mounting

Permanent mounting means that tools are required to remove or install a vaporizer on the anesthesia machine.

Advantage

Less physical damage to vaporizers

Fewer leaks

Disadvantage

Machine may not have enough mounting locations to accommodate all of the vaporizers that are likely to be needed.

Malfunctioning vaporizer cannot easily be exchanged.

Detachable mounting

They allow the vaporizer to be mounted and removed without the use of tools.

Before mounting a vaporizer, the control dial must be in the OFF position, and any adjacent vaporizer must be turned OFF. The locking lever on the vaporizer should be unlocked. The vaporizer is fitted onto the mounting system and locked in position.

To remove a vaporizer, the control dial is turned OFF, and the locking lever moved to the unlock position. the vaporizer can then be lifted off the manifold.

Select-a-tec mounting system

The mounting system has channels for the fresh gas flow from the flowmeters toward the machine outlet. When a vaporizer is mounted and turned ON, two plungers push the valves down so that gas passes through the vaporizer.

If the mounting position is empty or vaporizer is turned OFF, the gas passes directly through the mounting system.

Advantage

Allowing a more compact machine.

Vaporizers can be easily removed and replaced, even during a case.

If malignant hyperthermia is a potential problem, the vaporizers can be removed. Better results than if the vaporizers remain on the machine in the OFF position.

Disadvantage

Partial or complete obstruction to gas flow.

Leaks may occur. A common leak source is an absent or damaged O-ring. Another cause is leaving the locking lever in the unlocked position.

SAFETY FEATURES

Newer generations of anesthesia vaporizers have minimized or eliminated many hazards once associated with variable-bypass vaporizers.

Agent-specific, keyed filling devices help prevent filling a vaporizer with the wrong agent.

Overfilling of these vaporizers is minimized because the filler port is located at the maximum safe liquid level.

Vaporizers are firmly secured to a vaporizer manifold on the anesthesia workstation. Thus, problems associated with vaporizer tipping have become much less frequent.

Contemporary interlock systems prevent the administration of more than one inhaled anesthetic.

Interlocking systems

Interlock (vaporizer exclusion) systems prevent more than one vaporizer form being turned ON at a time.

Older versions of the select-a-tec system that provide mounting positions for three vaporizers require that if only two vaporizers are fitted, then the center position must be occupied. If the center position is not occupied, the interlock that helps to ensure that only one vaporizer at a time can be turned ON is ineffective.

Later versions incorporate an additional interlock that helps to ensure that only one vaporizer can be turned ON at a time, even if the center position is not occupied.

When the vaporizer of choice is turned ON, a pain is forced into a notch on the concentration control know of each of the other vaporizer.

Hazards

Misfilling 

Vaporizers not equipped with keyed fillers have occasionally been misfilled with the wrong anesthetic liquid.

When vaporizer misfilling occurs, either an anesthetic overdose or underdose will result.

The use of supplemental monitoring such as a multigas analyzer may alert the user to such a problem.

Contamination 

Contamination of anesthetic vaporizer contents has occurred as a result of filling an isoflurane vaporizer with a contaminated bottle of isoflurane.

Tipping 

Tipping of a vaporizer can occur when it is incorrectly “switched out” or moved.

Excessive tipping can allow the liquid agent to enter the bypass chamber and cause an output with an extremely high concentration of agent.

Tec 4 is slightly more immune to tipping than the North American Dräger Vapor 19.n because of its extensive baffle system.

If either vaporizer is tipped, it should not be used until it has been flushed for 20 to 30 minutes at high fresh gas flow rates.

During this procedure, having the vaporizer concentration control dial set at a low concentration maximizes bypass chamber flow and will aid in removal of any residual liquid anesthetic in that area.

After this procedure, use of a multigas analyzer is strongly recommended.

Drager Vapor 20.n series vaporizers now have a transport (“T”) dial setting that helps prevent tipping-related problems. When the dial is placed in this position, the vaporizer sump is isolated from the bypass chamber.

Overfilling 

Liquid anesthetic enters the bypass chamber, and up to 10 times the intended vapor concentration can be delivered to the common gas outlet.

Side-fill systems largely prevent overfilling.

Underfilling 

The combination of a low vaporizer fill state (<25% full) and a high vaporizing chamber flow rate can result in a clinically significant and reproducible fall in vaporizer output.

Simultaneous Administration of Inhaled Anesthetics 

On some older anesthesia machines from Datex-Ohmeda (equipped with the Select-a-Tec three-vaporizer manifold) that did not use a vapor interlock system, two inhaled anesthetics could be administered simultaneously when the center vaporizer was removed.

On such machines, the left or right vaporizer needed to be moved to the central position if the central vaporizer was removed (as indicated by the manifold warning label). Once this was done, the vaporizer's interlock system would operate properly and allow only one agent to be administered at a time.

Leaks 

A loose filler cap is the most common source of vaporizer leaks.

Leaks can occur at the O-ring junctions between the vaporizer and its manifold.

To detect a leak within a vaporizer, the concentration control dial must be in the “on” position.

No vapour output

The most common cause of no vapor output is an empty vaporizer.

Incorrect vaporizer mounting.

An overfilled vaporizer

Blood entering the vaporizer

VARIABLE BYPASS VAPORIZERS

The Datex-Ohmeda Tec 4, Tec 5, and Tec 7, as well as

the North American Dräger Vapor 19.n and 20.n

vaporizers, are classified as variable-bypass, flow-over,

temperature-compensated, agent-specific, out-of-

breathing-circuit vaporizers.

These vaporizers are agent specific and out of circuit.

Variable-bypass vaporizers are used to deliver halothane,

enflurane, isoflurane, and sevoflurane, but not desflurane.

Basic operating principles

Generic variable-bypass vaporizer

Vaporizer components include the concentration control dial, the bypass chamber, the vaporizing chamber, the filler port, and the filler cap.

The concentration control dial is a variable restrictor, and it can be located either in the bypass chamber or in the outlet of the vaporizing chamber. The function of the concentration control dial is to regulate the relative flow rates through the bypass and vaporizing chambers.

More than 80% of the flow passes straight through the bypass chamber to the vaporizer outlet, and this accounts for the name “bypass chamber.” Less than 20% of the flow from the flow meters is diverted through the vaporizing chamber.

The mixture that exits the vaporizer is the combination of flow through the bypass chamber, flow through the vaporizing chamber, and flow of entrained anesthetic vapor.

Variable-bypass vaporizers have an internal mechanism to compensate for variations in ambient temperature.

In warm operating room - To compensate for this increased vapor pressure, the bimetallic strip of the temperature-compensating valve leans to the right, thereby decreasing resistance to flow through the bypass chamber and allowing more flow to pass through the bypass chamber and less flow to pass through the vaporizing chamber.

In cold operating room - The vapor pressure inside the vaporizing chamber is reduced. To compensate for this decrease in vapor pressure, the bimetallic strip leans to the left, which increases resistance to flow through the bypass chamber and causes more flow to pass through the vaporizing chamber and less to pass through the bypass chamber.

Simplified schematic of the Ohmeda Tec–type vaporizer.

ELECTRONIC VAPORIZERS

Computer calculates the carrier gas flow that needs to pass through the vaporizing chamber in order to produce the desired anesthetic agent concentration.

Another type of electronic vaporizer withdraws a calculated amount of liquid agent from the agent bottle and injects that liquid into the breathing system of fresh gas flow.

FLOW OVER VAPORIZERS

Here the carrier gasses are allowed to flow over the surface of the anesthestic liquid for vaporization.

The vaporization largely depends on the effective area of the gas liquid interface, and speed of gas flow over the liquid surface.

The distance of carrier gas flow above the liquid surface also influences vaporization.

INJECTION

Certain vaporizers control the vapor concentration by injecting a known of liquid anesthetic into a known volume of gas.

BUBBLE THROUGH TYPE VAPORIZERS

Carrier gas is allowed to bubble through the anaesthetic liquid.

Very efficient as it provides a good contact between the gas and liquid.

The vaporization may be greatly enhanced if the size of bubbles is small as it provides more surface area for the contact.

Fast moving bubbles may lower the vaporizer output concentration.

Depth of the liquid anaesthetic may influence the vaporization.

TEMPERATURE COMPENSATED VAPORIZERS

Maintain a constant vapor output with fluctuations in liquid anesthetic temperature.

Mechanical thermocompensation

By altering the splitting ratio so that the percentage of carrier gas that is directed through the vaporizing chamber is increased or decreased.

Supplied heat

An electric heater can be used to supply heat to a vaporizer and maintain it at a constant temperature.

Computerized thermocompensation

Thermocompensation may be accomplished by computer control.

Two methods have been used.

o The amount of agent injected into the breathing system or fresh gas flow may be altered.

o Second method is computerized control of the amount of carrier gas that flows through the vaporizing chamber.

PLENUM VAPORIZERS

These are named after plenum system which means the systemic in which fresh air is forced into a chamber.

These vaporizers are used with fresh gas flow from anaesthetic machines.

Examples are : Boyle vaporizers, copper kettle, fluotek Mark 2, Fluotek Mark 3, Halox vaporizer, Pentec (for methoxyflurane), Enfluratec (for enflurane).

Vaporizers are meant for unidirectional gas flow and have a relatively high resistance to has flow.

BOYLE VAPORIZERS

The Boyle anaesthetic apparatus is equipped with two vaporizers :-

o Ether

o Trichloroethylene

Vaporizer is of variable bypass, flow over or bubble through type.

The glass bottle is calibrated upto 300ml. there is a metal top incorporating the controls, namely, a lever and a plunger to regulate the concentration of anaesthetic liquid.

The level is turned from ‘off’ position towards ‘on’ position, so that the gases pass through the bottle.

If ‘off’ position the inlet and outlet of vaporizer are closed, so that the gas passes without entering into it.

Lever is lifted up, the gas flows down the chrome plated inlet tube of the vaporizer. It is designed as U tube.

Chrome plated plunger with a hood (cow) at the end is placed directly above the end of the deliver U tube.

Plunger is up, the gas passed under the hood and then down the surface of volatile liquid to the vaporizer outlet. But when the plunger is depressed, the hood may come under the surface of liquid anaesthetic and may form large bubbles through the liquid. This procedures maximum rate of vaporization.

COPPER KETTLE VAPORIZER

Made of copper because of its high heat capacity and thermal conductivity.

The system provides sufficient reservoir of heat to prevent a fall in temperature inside the vaporizing chamber.

On the top there is a filling port and a funnel and a draining port at the bottom. There is a transparent window to detect the anesthetic liquid inside the kettle. The temperature of the vaporizer is indicated by a thermometer.

Bubble through type the anaesthetic liquid to provide maximum vaporization. It is temperature compensated and placed out of the breathing system.

Originally it was used for chloroform anaesthesia, but other agents like ether and halothane may be used with satisfactory result.

OXFORD VAPORIZER

It is a portable ether inhaler with a temperature regulating device.

Ether, the volatile anaesthetic, is placed in ‘the inner container which is surrounded by chemical crystals like hydrated calcium chloride. All these are surrounded by hot water jacket. Once calcium chloride crystals have been melted by hot water, ether vapour will generate spontaneously. As the melted calcium chloride is cooled, it solidifies and releases the heat of crystallization. This heat is absorbed by the ether placed in the inner chamber and vaporization occurs.

Ether vapour is then mixed with oxygen in a mixing chamber and delivered to the patient through the breathing circuit.

GOLDMAN VAPORIZER

Mainly used for administration of halothane.

It is neither temperature and level compensated nor accurately calibrated.

Halothane concentration using this vaporizer is usually low and its output is mainly influenced by as flow rate.

The vaporizer essentially consists of a small glass pot attached to a metal head. On the side of the head there is an outlet of gases and on other side there is an inlet where carrier gases are received and/or divided partly in the vaporizing chamber and the rest to bypass channel.

At the top of the head there is a control device which can be rotated to alter the vaporizer output.

Maximum concentration using this vaporizer never exceeds 2% irrespective of the total gas flow. But agitation and splashing may increase the vapour concentration over 5%.

ROWBOTHAM VAPORIZER

It is a simple flow-over type of vaporizer.

It contains no wicks and is not temperature compensated.

It is intended for administering moderate concentrations of halothane.

Ether, chloroform or trichloroethylene can also be used.

Vapour strength is controlled by means of a lever stopcock.

Maximum concentration of halothane may be up to 3.1% with gas flow rate of 4 L/min.

FLUOTHANE VAPORIZER

It is a standard Boyle pattern vaporizer specially modified for use of halothane.

The vapour strength is controlled by means of a lever stopcock over a graduated scale.

EMO VAPORIZER

It is a variable bypass and flow-over (with wick) type of vaporizer.

It is temperature compensated.

May be used for vaporization of diethyl ether, halothane, chloroform and trichloroethylene.

It is portable, 23 cm in diameter, 24 cm in height and maximum 6.5 kg by weight.

In the EMO inhaler, air enters through the inlet and partly passes through the bypass into the mixing chamber and the rest enters the vaporizing chamber through its inlet.

At the inlet of vaporizing chamber there is a control valve which controls the air flow into the chamber. The vaporizing chamber has wicks around its walls and is surrounded by water jacket made of aluminium (Mark I) or stainless steel (Mark II and Mark III.

Water jacket contains 1250 ml distilled water and acts as a heat reservoir and reduces the temperature variation due to vaporization.

There is a thermocompensator small metal ‘bellows containing a liquid in the vaporizing chamber outlet.

If the temperature rises, the plunger attached to the bellows goes upwards following expansion of thermocompensator and this leads to less gas flow through the vaporizing chamber.

Temperature falls, the plunger comes down leading more gas flow into the vaporizing chamber.

There is an indicator to denote the level of the anaesthetic liquid and in full position about 400 ml of ether filler of the anaesthetic liquid, when depressed ether enters into the vaporizing chamber. There is a temperature indicator to denote that it is at proper working temperature.

The unit is very useful, as it is compact, cheap, portable, efficient and reliable. Air acts as a carrier gas and no compressed gas cylinders are needed.

HALOX VAPORIZER

It is designed on a principle similar to the copper kettle vaporizer, but in construction large mass of copper is replaced by a glass container for simplicity.

In this vaporizer only halothane is used and it is needed relatively in small amount.

TEC-5 VAPORIZER

Designed for use with halothane, enflurane,isoflurane, and sevoflurane.

Construction

Tec 5 vaporizer is shown in figure

Diagram of Tec 5 vaporizer in the ON position

The internal baffle system is designed to keep liquid from reaching the outlet if the vaporizer is tipped or inverted.

When the concentration dial is in the zero position, all of the gas from the flowmeters bypasses the vaporizer through the Select-a-tec bar.

When the dial is turned past zero, inflowing gas is split into two streams by the rotary valve. One stream is directed to the vaporizing chamber, the other through the bypass.

Gas flowing through the bypass flows down one side of the vaporizer and past, the thermostat, which is a bimetallic strip in the base.

Temperature in the vaporizer decreases, the thermostat permits less gas flow through the bypass so that more gas passes through the vaporizing chamber.

The gas flowing to the vaporizing chamber first passes through the central part of the rotary valve, after which it is directed through a helical channel then past a spiral wick that is in contact with the wick skirt, which dips into the liquid agent. Gas with vapor leaves the vaporizing chamber via a channel in the concentration dial rotary valve and flows to the outlet.

Evaluation

Greatest accuracy is at a fresh gas flow of less than 5L/minute and dial settings less than 3%. At higher flows and higher dial settings, there is a decrease in output.

The greatest accuracy is between 15°C and 35°C. The thermostat does not respond to temperature below 15°C and the output will be less than indicated on the dial. If the temperature is above 35°C the output will be unpredictably high.

Maintenance

The exterior of the vaporizer may be wiped with a damp cloth.

The manufacturer recommends that if the agent contains additives or stabilizing agents, the vaporizers should be drained every 2 weeks or when the level is low.

There are no additives or stabilizing agents, the vaporizer can be drained at less frequent intervals the vaporizer should be returned to a service center every 3 years.

TEC 6 VAPORIZER

Designed for use only with desflurane.

Construction

It is somewhat larger than the Tec 5 vaporizer.

The concentration dial at the top is calibrated form 1% to 18% in gradations of 1% upto 10% and 2% between 10% and 18%.

The amber warm up LED indicates an initial warmup period after the vaporizer is first connected to the mains power.

Once warm up is complete, the green operational LED is illuminated, indicating that the vaporizer has reached its operating temperature and the concentration dial can be turned ON.

A short tone sounds at the transition from warm up to operational.

The red no output LED flashes and an auditory alarm of repetitive tones sounds if the vaporizer is not able to deliver vapor.

This can be caused by an agent level less than 20ml, tilting the vaporizer more than 10 degrees power failure, or an internal malfunction.

The amber low agent LED accompanied by an audible alarm is illuminated if there is less than 50mL of agent in the vaporizer.

The amber alarm battery low LED illuminates to indicate that a new battery is required.

The liquid level indicator has a liquid crystal display (LCD) that indicates the amount of liquid in the vaporizer between 50 and 425mL.

There are 20 bars. A single bar corresponds to a volume of approximately 20mL.

When the unit is plugged in, the electronics go through a self test. For 2 seconds, the alarm sounds, and each LED and LCD illuminates.

Once the vaporizer is plugged in, the power is always ON, and the sump heaters are operational.

Desflurane is heated to 39°C, which is well above its boiling point, by two heaters in the base. An external heat source is needed, because the potency of desflurane requires that large amounts be vaporizes. In addition, the desflurane boiling point is near room temperature and depending on ambient temperature would make the output unpredictable.

The sump assembly holds the agent and includes the filling port, drain, heaters, and agent level sensor. It has a capacity of 425mL.

The temperature is monitored, and the associated electronics act as a thermostat.

Two heaters in the upper part of the vaporizer prevent agent condensation where the warm vapor meets the cold gas from the common manifold.

When the proper temperature is reached, the green operational LED illuminates.

A signal from the control electronics operates the solenoid interlock, allowing the dial and rotary valve to be turned. When the dial and rotary valve are turned, the shut off valve opens.

Fresh gas flow enters the vaporizer and encounters a fixed resistor that creates back pressure. The higher the fresh gas flow, the greater the back pressure generated.

Electromechanical devices maintain the agent vapor pressure at the variable resistor in the rotary valve at the same level as the fresh gas pressure at the fixed restrictor.

This pressure balance between the desflurane and the diluent flow compensates for changes in temperature, vapor pressure, or diluent flow rate.

Control electronics, which in turn alters the agent pressure at the variable resistor by opening or closing the agent pressure regulating valve to balance the pressures.

With this balance of pressures maintained, the concentration delivered by the vaporizer depends only on the ratio of fresh gas flow through the fixed restrictor and agent vapor flow through the variable resistor, which depends on the concentration dial setting.

Performance of Tec 6 vaporizer, with oxygen as the carrier gas

Evaluation

Vaporizer is calibrated for flows from 0.2 to 10L/minute.

Output is almost linear at the 3%, 7% and 12% settings, with slightly lower outputs at flows less than 5L/minute and slightly greater outputs at higher flows.

The vaporizer is designed to be used at ambient temperatures between 18°C and 30°C.

Output is within ±15% of the dial setting.

Fluctuating back pressure does not significantly affect the Tec 6 vaporizer output.

The output is decreased with air or nitrous oxide as the carrier gas.

Maintenance

Requires servicing every year.

ALADIN VAPORIZER

Designed for desflurane, isoflurane, sevoflurane, halothane, and enflurane.

Construction

Vaporizer consists of two parts. The electronic control mechanisms is in the anesthesia machine. The agent is in a portable cassette that is inserted into a slot in the anesthesia machine.

The concentration dial is on the anesthesia machine next to where the cassette is placed.

On the display screen the agent is identified by name and color code and the setting in numbers and on a triangular graph.

The cassette is shown in figure. It is color coded for the agent that it is designed to contain and is magnetically coded so that the machine can automatically identify which cassette has been inserted.

The liquid level indicator and the filling port are located on the front. The cassette holds upto 25ml when full. When the ball in the sight tube is at the bottom, the cassette contains 80L or less. The liquid level may also be displayed on the machine. When the only 10% of liquid remains in the cassette, an alarm message appears.

Fresh gas enters the vaporizer and is split between the bypass flow that is in the machine and the vaporizing chamber.

Diagram of Aladin cassette vaporizer

Sensors in the vaporizing chamber outlet and bypass monitor the flow through each.

Wicks in the vaporizing chamber increase the surface area.

Check valve in the inlet to the vaporizing chamber protects against agent backflow into the bypass.

The flow at the outlet of the vaporizing chamber is controlled by the central processing unit (CPU) in the anesthesia machine.

This receives information, including the fresh gas composition, anesthetic agent, dial setting, temperature in the rear of the cassette and flow.

Inside the vaporizing chamber, metal plates increase heat capacity, heat conductivity, and vaporizing surface, all of which improve temperature stability and vaporization.

When cassette temperature decreases to less than 18°C, a fan below it is activated to facilitate heat transfer to the vaporizer.

When the cassette is removed, two spring loaded valves automatically close the channels to and from the vaporizer.

When the cassette is inserted into the machine, these valves open to make the connections with the inflow and outflow channels.

In error situations, the valves operate to cut off anesthetic agent delivery.

Evaluation

Accuracy for all agents is ±10% of the setting or ±3% of the maximum dial setting (whichever is greater) with fresh gas flows from 200ml/minute.

The effects of sudden changes in fresh gas flow or carrier

gas composition, back pressure, and tipping were minimal.

If the temperature falls below 20°C or the fresh gas flow is

over 8L/minute, the vaporizer may be unable to produce

high concentrations and the message insufficient agent and

decreased flow will appear on the machine.

Maintenance

Cassettes surface can be cleaned with a cloth moistened in

a mild soap solution.

Cassette should be emptied before sending it to a service

center.

DRAGGER VAPOR VAPORIZER

Temperature and level compensated.

It is of variable bypass, flow-over (with wick) type.

It is agent specific for halothane, isoflurane or enflurane.

It is used outside the breathing system.

It is compact, lightweight and very accurate over a wide range of flow rates.

The concentration output of this vaporizer independent of the fresh gas flow rate from 300 ml/min to 7 L/min. Slight decrease of concentration at higher flow rates.

In ‘off’ position, fresh gas flow bypasses the vaporizer completely without any resistance.

With the position ‘on’, the concentration dial is to be turned counter clockwise to open the port for fresh gas inflow. Gas partly enters the vaporizing chamber to become saturated with anaesthetic liquid and the rest traverses the bypass. Both of them meet in the mixing chamber and then pass on to the outlet.

Each vaporizer is individually tested and the concentration calibrated up to 4% halothane.

It is designed to operate at a temperature range of 10-40°C.

In cases of 100% O2 the output concentration is 5-10%

higher and in 30% O2 and 70% N2O it is 5-10% lower.