vaporizers by dr. ahmed mostafa assist. prof. of anesthesia and i.c.u

VaporizersBy

Dr. Ahmed MostafaAssist. Prof. of anesthesia

and I.C.U

Physics review of vaporization

• Saturated vapor:

The vapor which is in equilibrium with its liquid.

• Vapor pressure:

- It is the pressure exerted by vapor molecules on the wall of the container.

- Depends on:

- The physical properties of the individual agent.

- Temperature.

Once equilibrium condition has been met, the anesthetic would be at its saturated vapor pressure (at that temperature).


• Saturated vapor pressure:

- It is the pressure exerted by saturated vapor molecules on the wall of the container.

• Saturated vapor concentration:

- It is the volume present % of a saturated vapor in a gas mixture at a given temperature.

If the temperature is increased to point that the vapor pressure is equal to the atmospheric pressure, all of the liquid converts to the vapor phase, and the temperature would be equal to the liquid's boiling point.


• Boiling point:

- It is the temperature at which Saturated vapor pressure of a liquid is equal to the ambient atmospheric pressure.

- It is pressure dependent.

- The boiling points for some anesthetic agents are:

Desflurane (Suprane) 22.8 0C.

Isoflurane (Forane) 48.5 0C.

Halothane (Fluothane) 50.2 0C.

Enflurane (Ethrane) 56.5 0C.

Sevoflurane (Sevorane, Ultane) 58.5 0C.


• Different anesthetic agents have different volatilities which means that they have different saturated vapor pressures (SVP) and that the most volatile agents have the highest saturated vapor pressure. Boiling points decrease with decreasing pressure, for example and increasing altitude.

Vaporizers

Value:

- Convert the liquid-form of the anesthetic to vapor phase.

- Add a controlled adjustable amount of this vapor to the anesthetic circuit. (How???)

Output Regulation:

Methods:

1.Variable Bypass Vaporizer:

1.Variable Bypass Vaporizer:

the total gas from the anesthesia machine flow meters is split with some gas flowing into the vaporizing chamber, picking up anesthetic agent molecules, while a larger gas pass bypasses the chamber completely. Vaporizer outflow is based on the re- mixing of the two gas pass and results in administration to the patient of the anesthetic concentration indicated on the dial.

The ratio of the bypass gas to that entering the vaporizing chamber is called the splitting ratio.

Splitting ratio =

2. Measured Flow Vaporizer:

2. Measured Flow Vaporizer:

Measured flow is sent by separate O2 flow meter to pass to the vaporizer. In order to dilute this otherwise lethal anesthetic concentration, output from that gas flow pass is combined with gas passing from main flow meters, perhaps containing nitrous oxide and O2 with the resultant gas mixture exhibiting proper anesthetic concentration for the patient.

3. Electronic Vaporizers:

• In an electronic vaporizer, the volume of carrier gas necessary to

produce the desired agent concentration may be determined by a computer that 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 or fresh gas flow. The amount of liquid that is injected is adjusted to achieve the desired anesthetic agent concentration.

Methods of vaporization1.Flow-over (Plenum(Latin derivative, opposite to vacuum)):

- A stream of carrier gas passes over the surface of the liquid. Increasing the area of the carrier gas-liquid interface can enhance the efficiency of vaporization.

- Most of the vaporizers used in anesthesia employ this vaporization method.

Methods of vaporization2. Draw- over:

- Gas is drowning over into the vaporizer by the patient’s inspiratory effort creating sub-atmospheric pressure.

- It is cheap, easy to use and portable.

- Not used now a day.

Methods of vaporization2. Draw- over:

Methods of vaporization3. Injection:

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

Methods of vaporizationHow to increase Vaporization?

• By increasing surface area of vaporization. (How??)

Wicks:

• Metal or fabric wicks are present in the vaporizing chamber, one of its ends is immersed into the anesthetic liquid, while the other end project up into the chamber.


• The liquid anesthetic moves up the wicks by capillary action.

• Advantages:

- Large surface area.

- Level compensation (Compensate for drops in liquid anesthetic level).


Bubbling:

• The carrier gas is bubbled through the liquid anesthetic.

Temperature Compensation (Tec)

Methods of temperature compensation:

1.Physical:

• The vaporizer is made of a large mass of copper (i.e. Copper Kettle vaporizer) which has: High thermal conductivity and High specific heat.

• Some vaporizer use water as a heat reservoir because it has specific heat capacity.



2. Manual mechanism:

• The output vapor concentration is adjusted manually by rotating the control dial according to the temperature of the liquid anesthetic as measured by a built in thermometer.



3. Mechanical (Automatic) Thermo-compensation:

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




• As the vaporizer cools, the thermal element restricts the bypass flow, causing more carrier gas to pass through the vaporizing chamber. The opposite can occur if the vaporizer becomes too warm. (How ???)




I. Bimetallic strip:

The strip consists of two strips

of different metals which expand

at different rates




II. Metal bellow:




III. Metal rod:



4. Electrical:

• An electric heater can be used to supply heat to a vaporizer and maintain it at a constant temperature. (e.g. Desflurane vaporizer)



5. Computerized Thermo-compensation:

• Thermo-compensation may be accomplished by computer control.

Effect of intermittent back pressure (The pumping effect)

The pumping effect is due to positive pressure ventilation or use of the oxygen flush valve. It can increase vaporizer output. Modern vaporizers are relatively immune (older vaporizers were certainly not immune).


• Methods of decreasing the pumping effect:

1.Unidirectional valve between the vaporizer outlet and the common gas outlet.

2.Smaller vaporizing chambers.

3.Tortuous (long spiral) inlet chambers.



4. Pressure valve which inserted at the downstream of the vaporizer to insure that the pressure in the vaporizer is constant and greater than the pressure at the ventilator.

5. Exclude wicks from the area where the inlet tube joins the vaporizing chamber.

Factor affecting the performance of the vaporizers

1. The saturated vapor pressure.

2. The temperature.

3. The splitting ratio.

4. The surface area of the gas liquid interface.

5. The rate of fresh gas flow:

• The vaporizer output concentration is almost not affected by changes in the fresh gas flow rates between 250 mL/min. to 15 L/min.


6. The pumping effect.

7. Effects of Altered Barometric Pressure:

• Vaporizers automatically compensate for changes in ambient pressures (i.e. altitude changes).

8. The position of vaporizers in relation to a circle system:

• A vaporizer may be placed either outside or inside the circuit.


Vaporizer Inside circuit(VIC)

Vaporizer outside circuit(VOC)

Position of the vaporizer -In line with patient’s ventilation.-Low internal resistance.

-In line of fresh gas flow.-High internal resistance.

Vaporization depends on -Fresh gas flow.-Minute volume of the patient.

-Fresh gas flow.-Efficiency of the vaporizer.

Used safely with Spontaneous ventilation. Controlled ventilation.

Efficiency Low efficiency. High efficiency.

Examples Draw-over vaporizers Plenum vaporizers

Vaporizer mounting system

1. Permanent Mounting:

Tools are required to remove or install a vaporizer on the anesthesia machine.

- Advantages:

a- Less physical damage to vaporizers and fewer leaks.

b- Vaporizers are always filled in the vertical position.

- Disadvantages:

a- The machine may not have enough mounting locations to accommodate all of the vaporizers that are likely to be needed.

b- A malfunctioning vaporizer cannot easily be exchanged, especially while anesthesia is being administered.


2. Detachable Mounting (The Select-a-tec system):

- Are standard on most new anesthesia machines.

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

Advantages:

- The anesthesia machine can have fewer mounting locations, 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.

- This gives better results than if the vaporizers remain on the machine in the OFF position.


Disadvantages:

- Partial or complete obstruction to gas flow

- Leaks.

- Several cases have been reported where there was failure to deliver anesthetic agent associated with problems with mounting systems.

- In another case, a switch malfunction caused the fresh gas flow to be directed to the wrong vaporizer .This resulted in delivery of fresh gas with no vapor to the breathing system.


3. Interlock Devices:

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

- Checking the interlock device should be included with the anesthesia apparatus checkout procedure.

Features of modern vaporizers

• General features:

Variable bypass.

Flow over.

The vaporization is improved by using wicks.

Thermal compensation:

• Either by automatic or computerized mechanisms.

Pumping effect: By different mechanisms.

Vaporizer outside the circuit.


• Safety features and hazards:

Agent specific.

Don’t allow 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.



Don’t allow cross filling or trans-filling.

Simultaneous inhaled agent administration: If removing the central vaporizer from a group of

three, move the remaining two so that they are adjacent.



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.

Note: 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.



Don’t allow leaks from the vaporizer to the circuit:

Leaks are relatively common, often due to malposition of vaporizers on the back bar, or loss of gaskets, and these leaks may not be detected with the standard checklist unless the negative pressure check is performed.



Breath by breath gas analysis:

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

Electronic failure.

Concentration-calibrated Vaporizers

• Tec 5:

http://upload.wikimedia.org/wikipedia/commons/b/b7/Vaporizer.jpg


• Tec 5:

http://www.wemed1.com/Products/spec.asp?ItemNumber=ANV-O-FLUROTEC5&Code=

http://www.wemed1.com/Products/spec.asp?ItemNumber=ANV-O-ISOTEC5&Code=

http://www.wemed1.com/Products/spec.asp?ItemNumber=ANV-O-SEVOTEC5&Code=


• Tec 5:

The control dial is at the top. When the concentration dial is in the zero

position, all of the gas from the flow meters bypasses the vaporizer.

Temperature compensation: Automatic by bimetallic strip.


• Tec 5:

The greatest accuracy is between 15°C and 35°C. The thermostat does not respond to temperatures 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.

Hazards:


• Tec 6:(Desflurane vaporizer)


• Tec 6 (Desflurane vaporizer):

designed for use only with desflurane because it has the following unique physical properties:

Boiling point is 22.8 0C.

Its vapor pressure is 669 mmHg.



Its potency is low (MAC is 6-9%). So more vaporization is required which in turn causes more cooling effect.

Desflurane is potentially valuable because it has a low blood gas solubility coefficient of 0.45 at 37 0C.

Recovery from anesthesia is more rapid than with other potent inhaled anesthetics.



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

The power cord attachment and battery case are on the bottom. The battery provides power for the alarms and liquid crystal level indicator. The power cord exits at the side.



The vaporizer casing is warm to the touch when it is connected to the electrical supply, which will cause slight heating of adjacent equipment.

The vaporizer is calibrated for flows from 0.2 to 10 L/minute



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

Hazards:

Leak into the fresh gas with the vaporizer turned OFF.



When filling the vaporizer, the bottle must be gripped tightly when it is rotated downward. If not gripped tightly, the bottle may be dropped when it is released under pressure at the lower position.



Principles:



Principles:

In the Tec 6 vaporizer, the fresh gas pathway is shown above in red and his pressure controlled by a resistor (R1 above) which establishes a working pressure.



Principles:

Liquid desflurane is contained in the sump and is held at 39oC.Since vapor pressure is dependent on temperature, the most appropriate value for vapor pressure in the sump corresponds to about 1300 mm Hg.



Principles:

Note above the sump shutoff valve which will remain closed until the vaporizer reaches operating temperatures at which point the peak concentration control valve (R2) is activated to the on position.



Principles:

The pressure regulating valve which is reduces the pressure from the vaporizing chamber to about 1.1 atm (74 mm Hg) [fresh gas flow rate equals approximately 10 L/min].



Principles:

Desflurane vapor output is adjusted by setting the concentration control valve. Desflurane vapor coming through the vaporizing circuit mixes with fresh gas near the outlet.



Principles:

The two circuits appear physically separate; however, interaction between the circuits occurs in a regulated manner.



Principles:

Notes the differential pressure transducer and control electronics which influence the pressure regulating valve setting and the sump shutoff valve state.



Principles:

The purpose of the differential pressure transducer and associated electronics is to ensure equivalence in the working pressure between a fresh gas circuit and the vaporizing circuit.



Principles:

In this type of system then, the vaporizer output will be constant since fresh gas flow and vapor flow will be regulated in a coordinated, proportional manner.


• Tec 7:


• Tec 7:

Approximately 300 mL of liquid is needed to fill a vaporizer with dry wicks. Approximately 75 mL is retained in the wicks when the vaporizer is drained.

The greatest accuracy is between 15°C and 35°C.


• Tec 7:

Hazards:

The Tec 7 vaporizers are intended to be operated in the upright position. If a vaporizer is inverted, it should be connected to a scavenging system, the dial set to 5%, and the vaporizer purged with carrier gas at 5 L/minute for 5 minutes.


• Tec 7:

Hazards:

The vaporizer could be overfilled if it is not filled in the upright position. The dial must not be turned ON during filling to prevent overfilling.


• Aladin Cassette Vaporizer:



The vaporizer system used in the Datex-Ohmeda S5/Anesthesia Delivery Unit (ADU)

Is unique in that the single electronically controlled vaporizer is designed to deliver five different inhaled anesthetics including halothane, isoflurane, enflurane, sevoflurane, and desflurane.



The vaporizer consists of a permanent internal control unit housed within the ADU and an interchangeable Aladin agent cassette that contains anesthetic liquid.



The Aladin agent cassettes are color-coded for each anesthetic agent, and they are also magnetically coded so that the Datex-Ohmeda ADU can identify which anesthetic cassette has been inserted.

The cassettes are filled using agent-specific fillers.


• Vapor 19.1, 19.3:


• Vapor 19.1, 19.3:

Output is independent of fresh gas flow in the range of 0.3 to 15 L/minute.

The Vapor 19.1 with a keyed filler will not overfill, even when the vaporizer is in the ON position and the bottle adaptor is loosened.


• Vapor 19.1, 19.3:

Hazards:

If a filled Vapor 19.1 is tilted, liquid agent may spill into the control device whether the vaporizer is turned ON or OFF. This can result in either an increase or decrease in delivered concentration.


• Vapor 19.1, 19.3:

If the vaporizer is tipped more than 45°, it should be flushed with a flow of 10 L/minute with the concentration dial at the highest setting for at least 20 minutes.

Thank you

Dr. Ahmed Mostafa

vaporizers by dr. ahmed mostafa assist. prof. of anesthesia and i.c.u

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