Vaporizers - principles
Historical: Plenum Vaporizers
Boyles Bottles Copper Kettle Vernitrol ---Vaporizers of high resistance that depended on compressed gas pressure to drive the gas through
Vaporizer methods - Bubble through
Bubble through - no longer used
Pic of Aladin Vaporizer
(Built in thermocompensation)
Liquid Used/Hour
3 x FGF x Vol% = ml liquid Used /Hour
Vaporizer is an ....?
A vaporizer is an instrument designed to change a liquid anesthetic agent into its vapor and add a controlled amount of this vapor to the fresh gas flow
What is only way to get an accurate value of concentration of inspired gases?
Agent analyzer - only way to get accurate value
Latent Heat of Vaporization =
Latent Heat of Vaporization = The amount of heat energy necessary to convert 1 gram of liquid into vapor.
How does the Tec 6 Vaporizer manage the unique characteristics of Des?
To manage this: Heated to 39°C. Pressure 1500 mmHg. Output is controlled by a differential pressure transducer. Desflurane vapor joins fresh gas flow in the common manifold.
Saturated Vapor pressure (SVP) =
Saturated Vapor pressure (SVP) =The highest partial pressure that can be exerted by a gas at a given temperature
Vapor =
Vapor - Gaseous phase of an agent which is normally a liquid at room temperature and atmospheric pressure
Vapor Pressure =
Vapor Pressure - Molecules that have broken away from the surface of a liquid forming vapor bombard the walls of the container creating pressure.
Vaporizer dial setting = ?
Vaporizer dial setting = concentration of agent delivered to the breathing system
Volumes %
Volumes % = volume of solute/volume of solution x 100 Partial pressure/total pressure of all gases x 100
Pressurizing Effect
When constant increase pressure is applied to the vaporizer outlet, carrier gas is compressed and there will be more molecules in the gas state per ml of liquid in the vaporizing chamber. Net result is decrease in the concentration of anesthetic delivered. (Mostly seen during high flow and low vaporizer dial settings.)
Specific Heat =
Specific Heat - The quantity of heat required to raise the temperature of 1 gram of a substance by 1 °C
What factors affect vaporizer output?
Splitting ratio. Surface area of the liquid/gas interface. Temperature. Time carrier gas in contact with agent. Gas flow rate. -High flow rates, gas leaving vaporizing chamber is less saturated. Boiling point and VP of the agent -Higher the BP the less vapor output. Barometric pressure -Lower BP = higher vapor output.
Temperature Compensation - Supplied heat
Supplied heat - Electric heater is used to supply heat Used to supply constant temperature
Vapor pressure isi ___?___ dependent
Temperature
What are the methods of Temperature compensation
Temperature compensation: Thermocompensation Supplied heat
Liquid temperature __?__ as vaporization proceeds.
decreases
Ways to Minimize the Pumping Effect
-A check valve between the vaporizer and the common gas outlet -Other methods previously used: Keeping the vaporizing chamber small. Increasing the size of the bypass area. Long spiraling tube leading to vaporizing chamber
Other characteristics of current vaporizers
-Agent specific: Vaporizer is designed for a single agent -In or Out of circuit: Out of circuit - vaporizer placement is between flow meters and common gas outlet In circuit - vaporizer is placed between common gas outlet and patient
Aladin Vaporizer Cassettes
-Electronic control mechanism is in the anesthesia machine. -Agent is in the portable cassette. -Magnetic sensor identifies the cassette
What do the Tec 6 alerts LED lights indicate?
-LED lights that indicate: < 20 ml. power failure. warm up and when temperature has been reached. -Audible alarm
1 ml of liquid volatile agent yields ....?
1 ml of liquid volatile agent yields about 200 ml anesthetic vapor
Vaporizer methods- Flow over Method = How can efficacy be enhanced?
A stream of carrier gas flows over the surface of the liquid. Efficacy can be enhanced by ↑ the area of the carrier gas-liquid interface. -Baffles -Spiral tracks *lengthen the pathway of the gas over the liquid. -Wick *liquid moves up the wick by capillary action.
As the temperature of the liquid falls, a __?___is created so that heat flows from __?__ to __?___
As the temperature of the liquid falls, a gradient is created so that heat flows from the surroundings into the liquid.
Effects of Rebreathing
At lower FGF, exhaled gases add agent to inspired gas Significant difference between vaporizer setting and inspired concentration INCREASED minute volume = INCREASED rebreathing of exhaled gases
Boiling point =?
Boiling point - the temperature at which the vapor pressure is equal to the atmospheric pressure.
Why do we need the Desflurane Vaporizer (Tec 6)?
Boiling point is near room temp Evaporates quickly. Cools and loses energy quickly. Cannot draw heat from environment fast enough to maintain stable temperature. Output unpredictable
Pumping Effaect - pic
Can occur with positive pressure ventilation &/or pressing Oxygen flush valve.
Vaporizer safety features
Color coded Transport mode Check valve Agent specific filling systems Interlock
Computerized thermocompensation=
Computerized thermocompensation = -Either controls amount of agent injected or -Amount of fresh gas flow
Concentration Calibrated
Concentration Calibrated: Variable bypass The total gas flow from the flowmeters goes through the vaporizer Variable resistance proportioning valve controls splitting ratio Picks up a predictable amount of vapor, then flows to the common gas outlet. Designed for vaporizers to be located between flowmeters and common gas outlet
Characteristics of vaporizers other than Des vaporizer:
Concentration calibrated Variable bypass Flow over Temperature compensated Agent-specific Out of circuit
What are the characteristics of Drager Vapor 19.1, Vapor 2000, Penlon Sigma, GE Healthcare Aladin and Tec 4, 5 & 7 vaporizers?
Concentration calibrated Variable bypass Flow-over (some have wicks & baffles) Temperature compensated Agent specific Out of circuit
Tec 6
Concentration calibrated. Injection. Thermocompensated by supplied heat or electrically heated dual circuit gas-vapor blender. FGF is sensed by pressure transducer and adjusts resistor. Control dial adjusts pressure regulator valve which adjusts amount of vapor joining FGF.
Materials and their specific heat largest to smallest
Copper > Aluminum> Brass> Steel>Glass
Method of Regulation of Vapor Output - Electronic Vaporizers/Injection
Electronic Vaporizers/Injection= A known amount of liquid agent or pure vapor is injected into the gas stream to produce the desired concentration Tec6 (Des) vaporizer
Temperature Compensation in the Aladin Vaporizer for Desflurane
Electronic circuitry takes into account the FGF rate (and type, ie: O2/air or O2/N2O) and temperature of the agent. This is used to control a flow valve in the outflow side of the vapor chamber path to allow the correct amount of desflurane to be added to the fresh gas. This is not much different from how the Aladin vaporizers for sevoflurane or isoflurane work. The main difference is how the electronics predict the performance of Desflurane in the vaporizer. The Desflurane cassette also has a fan that switches on to direct heat from the anesthetic machine onto the vapor chamber if it gets too cold (from vaporizing desflurane). Ref. http://gasexchange.com/questions/how-does-the-desflurane-vaporizer-work/
Eventually, equilibrium is established so that the heat lost to vaporization is ___?___
Eventually, equilibrium is established so that the heat lost to vaporization is matched by the heat supplied from the surroundings. At this point, the temperature ceases to drop.
Flow over gas flow Vaporizer Off = Vaporizer On =
Flow over Vaporizer Off - all gas goes through the bypass. Vaporizer On - incoming gas flow is split into two portions through the bypass thorough the vaporizing chamber. The ratio is called the "splitting ratio."
Why do we want a high specific heat?
This causes it to change temperature more slowly and therefore it doesn't cool as quickly as the liquid inside vaporizes and leaves the vaporizing chamber.
Measured Flow
Measured Flow - Flow metered or flow meter controlled All gas from flowmeters flows through the vaporizer then is mixed with additional flow from a separate flowmeter =. No longer available Not agent specific Operator had to know the agent VP, atmospheric pressure, temperature, total flow rate and vaporizer flow rate
Other features of the Tec 6 how much can it hold? when can it be filled? Safety filling feature?
Holds 460 ml agent. -Due to low potency Can be filled while in use A stop is built in to prevent dialing hypoxic mixture -70% nitrous oxide + 12% Suprane SAF-T-FILL to prevent evaporative loss during filling
Back pressure may do what to vaporizer output?
May either increase or decrease vaporizer output
Vaporizer hazards
Incorrect agent -HLH -LHL Tipping -Do not use if it has been tipped! -Use transport mode Overfilling Interlock malfunction
Increase Thermal Conductivity = ?
Increase Thermal Conductivity The higher the TC, the better the substance conducts heat.
Increase in Specific Heat = ?
Increase in Specific Heat Temperature changes more gradually for materials with a high specific heat than for those with a low specific heat.
It's desirable to ___?____ temperature changes when vaporizer is in use. How is this done?
It's desirable to minimize temperature changes when vaporizer is in use. Increase in Specific Heat Increase Thermal Conductivity
What do you do if vaporizer has been tipped? Why?
Liquid from the vaporizing chamber may get into the bypass or outlet. A high concentration of agent will be delivered when the vaporizer is used again. If tipping occurs (CALL BIOMED - DO NOT USE!)
Mechanical Thermocompensation=
Mechanical Thermocompensation = Splitting ratio is altered so that the % carrier gas entering the vaporizing chamber is increase or decrease -Bimetallic strip
Methods of vaporization
Method of vaporization: Flow-over Bubble through Injection
What are the Methods of regulating output concentration?
Methods of regulating output concentration: -Concentration calibrated -Measured flow
pumping effect most likely to be seen when: ??
Most likely to be seen when -Carrier gas is low. -Agent level in vaporizing chamber is low. -Dial setting is low. -Pressure fluctuations are high and frequent
Partial Pressure=
Partial Pressure The part of the total pressure that is due to any one gas (in a mixture of gases) is the partial pressure of that gas
Pumping effect
Pumping Effect - INCREASES vaporizer output due to intermittent back pressure. -Agent re-enters the vaporizer & gets into both the bypass channel and the vaporizing chamber. -Fresh gas continues to enter. -Gases compressed in both areas but vaporizing chamber is larger. **More fresh gas is compressed in the vaporizing chamber and picks up vapor -When back pressure is released the compressed gas expands in all directions **Some vapor gets into bypass channel
Why do we need vaporizers?
The SVP of most inhalation agents is much higher than is required to produce anesthesia. Need to dilute this vapor with carrier gas to deliver a controlled amount to the patient
The changes in vaporizer output caused by pumping effect is usually __?__ than that caused by the pressurizing effect.
The changes in vaporizer output caused by pumping effect is usually greater than that caused by the pressurizing effect.
overfilling of vaporizer causes what?
The filling systems are designed to prevent overfilling but there are ways to override these safety mechanisms. Overfilled vaporizers will result in overdosing.
The lower the liquid temperature, the __?__the gradient and the __?__ the flow of heat from the surroundings
The lower the liquid temperature, the greater the gradient and the greater the flow of heat from the surroundings
The material that the vaporizer is made of should be of a high ___?__
The material that the vaporizer is made of should be of a high specific heat.
Thermal Conductivity =
Thermal Conductivity - A measure of the speed with which heat flows through a substance.
What happens if the Tec 6 vaporizer is tipped?
Tilting not an issue because no bypass chamber
Method of temp compensation - pic
Two metals of different thermal expansion coefficients are fixed together
Increase heat =
increase VP
