Vaporizers Flashcards
Physical Principles of Vaporizers
Vapor Pressure Latent Heat of Vaporization Specific Heat Thermal Conductivity Ambient Pressure Effects
Vapor Pressure
what temp to they exist in liquid
Contemporary volatile anesthetics exist in liquid state at temperatures at 20 C.
When a volatile anesthetic is in a closed container molecules escape from liquid to vapor phase until the number in vapor phase is constant
These molecules bombard the side of the container and create a saturated vapor pressure
Vapor Pressure Facts
Vapor pressure (VP) increases with temperature but is independent of atmospheric pressure
VP depends only on the physical characteristics of the liquid and its temperature
atmospheric pressure does not effect the vapor pressure
Boiling Point
Boiling point of a liquid is defined as that temperature at which the vapor pressure equals atmospheric pressure
Boiling points for contemporary agents: Desflurane-22.8 C* (note that des requires a special device due to its low boiling point) Isoflurane-48.5 C Halothane-50.2 C Sevoflurane-58.5 C
Latent Heat of Vaporization
When a molecule is converted from a liquid to the gaseous phase energy is consumed
The amount of energy consumed for a given liquid is the latent heat of vaporization
Energy for vaporization must come from either the liquid itself or an outside source
So it is the release of heat
Latent Heat of Vaporization Facts
Heat of vaporization is the amount of calories required to convert 1 g of liquid to vapor without temperature change in the remaining fluid
Thus temperature of the remaining liquid will drop as vaporization proceeds, greatly decreasing subsequent vaporization
As well, VP be lowered unless temperature drop is prevented
(It is very important to maintain the temp of the vaporizer)
Specific Heat
A substance’s specific heat (SH) is the quantity of heat required to raise the temperature of one gram (1 ml) of a substance by 1degree C.
SH is important when considering the amount of heat that must be supplied to a liquid anesthetic to maintain a stable temp when heat is lost during vaporization
Also manufacturers select materials with high SH to build vaporizers (e.g., copper)
Properties of Gases
Standard Vapor pressures, MAC
Iso 239/ 1.15
Sevo 157/ 2.0
Des 672/ 6.4 (des has a low boiling point)
Thermal Conductivity
Thermal conductivity is a measure of the speed with which heat flows through a substance
The higher the TC the better the substance conducts heat
This is referencing what the vaporizer is actually held in, so copper and bronze move heat well
Thick metal casing help with this conductivity
Anesthetic Vaporizers
bypass circuit- it what goes over the chamber with the agent
Gas as it passes volatilezes the agent
THe amount of gas detemines the concentration
Variable Bypass Vaporizers
Vapor pressures of most anesthetics are much greater than the partial pressure to produce anesthesia
To produce clinically useful concentrations a vaporizer dilutes saturated vapor
Fresh gas flow from flow meter enters the inlet of vaporizer
Concentration control dial splits stream into bypass gas (does not enter chamber it is what is used to dilute the volatile agent) and carrier gas (chamber flow or what gas actually picks up the agent)
Operating Principles of Variable Bypass Vaporizers
so the concentration dial is what actually determines the % of gas going to the patient
Total FGF enters and splits into carrier gas (much less than 20%) and bypass gas (80%)
Two gas flows rejoin at vaporizer outlet
The “splitting ratio” of the two flows depends on the ratio of resistances to their flow (determined by concentration dial) and automatic temperature concentration valve
Variable Bypass Vaporizers and “Flow Over
Carrier gas flows over the surface of the liquid volatile agent in the vaporizing chamber
Increasing contact area between gas and liquid increases efficiency of vaporization
Done by baffles/spiral tracks or wicks
Factors Affecting Vaporizer Output
Flow through the vaporizing chamber Temperature Efficiency of Vaporization Carrier Gas Composition Fresh gas flow Ambient pressure Intermittent back pressure from ventilator
Temperature
As temperature increases, output increases
Output remains the same if some form of temperature compensation occurs
Almost all vaporizers have temperature compensation
- Providing heat to the liquid to minimize the temperature drop
- Increase the flow of fresh gas into the vaporizing chamber to compensate for the reduced vaporization efficiency of the cold fluid
Temperature Compensated
Equipped with automatic devices that ensure steady vaporizer output over a wide range of ambient temperatures
Mechanical thermocompensation compensates by altering carrier gas flow (more chamber flow) and less bypass flow
Some vaporizers supply heat to a vaporizer to maintain constant temperature
Automatic Temperature Compensating Valve
Influences how much flow occurs into the vaporizing chamber
Splitting of gas between the bypass pathway and the vaporizing chamber is controlled by two valves:
- The dial you set (splitting valve)
- The temperature compensating valve which operates automatically according to the liquid temperature
What are the diffrent compensating valves
Bi metallic
Metal rod
Bellows
Efficiency of Vaporization
modern bypass vaporizers, use wicks within the chamber of the gas to increase surface area so that more molecules are picked up by the gas
Carrier Gas Composition
N2O will pick up less gas than O2
Fresh Gas Flow Rate to high or to low
The bypass gas dilutes the gas coming out of the chamber
At high flow rates (> 8 liters/min) the gas leaving the vaporization chamber is less saturated
At low flow rates (< 500 ml/min) insufficient
turbulence is generated to push the molecules out of the vaporizer
These variations only occur at extremes, usually inspired changes occur as a result of more rapid equilibration within the circle system
some machines can calibrate to this and it wont effect them
Ambient Pressure
SVP is solely a function of temperature
If pressure is decreased (e.g., higher altitudes) output increases by Dalton’s Law
Agents with low boiling points are more affected by pressure changes (e.g., desflurane)
For example, a halothane vaporizer calibrated at sea level and set to deliver 2% will produce about 2.7% halothane if used in Denver, Colorado
Altitude will effect your vaporizers due to pressure changes
How is Des delivered to the patient
Note that it is NOT delivered by variable bypass like iso and sevo, note that iso and sevo will have a by pass pathway and a carrier gas
“Pumping” Effect
Describes intermittently fluctuating pressure in the breathing system
More pronounced at low FGF rates
Modern vaporizers negate this by incorporating a smaller vaporizing chamber, larger inflow channel
Electronic Vaporizers
The volume of carrier gas necessary to produce the desired agent concentration may be determined by a computer
The computer calculates the carrier gas flow that needs to pass through the vaporizing chamber to produce the desired agent concentration
Another type of electronic vaporizer withdraws a calculated amount of liquid agent and injects the liquid into the FGF
Special Considerations with Desflurane Vaporizer
NOT a variable-bypass vaporizer
Desflurane SVP is near atmospheric (699 mmHg) so vaporizer is pressurized
Sensitive to temperature so that constant vaporizer pressure must be maintained (So it must be heated)
Dual circuit blender, the pressure in vapor circuit is equal to fresh gas circuit
Affected by atmospheric pressure (altitude) more than variable-bypass vaporizers
Tec 6/Desflurane Vaporizer
Carrier gas restricted by orifice (O) proportional to carrier gas flow
This pressure is sensed by pressure transducer (P) which adjusts a resistor (R1) so that flow is proportional to carrier gas flow
The control dial adjusts second resistor which controls output of desflurane
These are comlicated
Greatest accuracy is with FGF < 5 L/m, dial < 3%
Prone to increases from pumping effect (excess positive pressure)
Easy to overfill by tilting
If center vaporizer is removed in series of three can turn two on at once
Tec 7 Vaporizers (Newer)
Essentially the same as Tec 5 except less affected by pumping and FGF
Sevoflurane output is lowered by use of N2O
Changes in barometric pressure is compensated for automatically
Vapor 2000 (Variable Bypass)
“Tippable” vaporizer (must be on “T”)
Air and N2O lower output
Vaporizer should not be left in the ON position as vapor may accumulate
Must depress “O” button on handwheel to adjust
Aladin Vaporizer (Variable Bypass)
Consists of two parts
Electronic control mechanism is in anesthesia machine
Agent is in a portable cassette
Cassette holds 250 ml and may be “tipped”
Inside vaporizing chamber metal plates improve temp stability and vaporization
It is a pain cause you have to pull it out to use a second gas
Vaporizers and Standards
ASTM established standards (1988)
Must be able to take 15 L/min FGF
Extent to which temp and flow affect concentration must be stated
Vaporizer cannot be overfilled in normal position
Vaporizers must be isolated
Counterclockwise to increase concentration
All vapors must be “concentration calibrated”
3 x FGF (L/m) x volume%=ml liquid used/hr
Typically, 1 ml liquid yields 200 ml vapor
http://www.slideshare.net/drunnikrishnanz/vaporizers-basics
Hazards of Vaporizers
Fill with wrong agent
Contamination/leak into FGF
Tipping (liquid agent enters the vaporization chamber) (will deliver a high dose)
Overfilling (liquid agent enters the vaporizing chamber)
Leaks (loose filler cap, O-ring junction between vaporizer and manifold)
No vapor output (99% of time from empty)
Safety Features of Vaporizers
Keyed fillers Low filling port Secured vaporizers (less tipping) Interlocks Concentration dial increases output in all when rotated clockwise
Filling and Draining Vaporizers
If wrong agent is used, discard contents of vaporizer and run at 5 liters oxygen flow until no agent detected (about 20 minutes)
Key Points
Modern vaporizers are accurate over a wide range of temperatures and fresh gas flows
SVP is a measure of the volatility of the liquid anesthetic in the carrier gas
Heat of vaporization is the amount of calories required to convert 1 g of liquid to vapor without temperature change in the remaining fluid
A substance’s specific heat (SH) is the quantity of heat required to raise the temperature of one gram (1 ml) of a substance by 1degree C.
Factors potentially affecting vaporizer output include FGF, temperature, carrier gas composition, and ambient pressure
Increasing contact area between gas and liquid in a vaporizer increases efficiency of vaporization (baffles, spiral tracks or wicks)
Three characteristics of modern vaporizers include agent-specificity (keyed filling port required), temperature compensation and flow-over
Vaporizer interlock is a safety feature that only allows one vaporizer on at a time
The basic operating premise of variable-bypass vaporizers states that FGF enters the vaporizer and splits into carrier gas (much less than 20%) and bypass gas (80%)
The “splitting ratio” of the two flows depends on the ratio of resistances to their flow (determined by concentration dial)
Output is relatively constant over a wide range of flows (250 ml/min to 15 L/min)
“Pumping Effect” describes the affect on vaporizer output from intermittently fluctuating pressure in the breathing system
ASTM established standards (1988) that included FGF limits (15 L/m), unidirectional rotation for concentration increase, concentration calibration of vaporizers, and temp/pressure effects
Hazards associated with vaporizers include using the wrong agent, tipping, overfilling and empty vaporizer
Safety features of vaporizers include agent-specific keyed fillers, secured vaporizers, interlocks and counterclockwise rotation of dial for increase in %
Compared to newer vaporizers (Tec 6, 7 etc.), Tec 5 vaporizers require FGF close to 5 L/m, and are more affected by pumping
Desflurane is heated to 39 C. (above boiling point) as large amounts are vaporized, and overpressurized to 1300 mmHg
Electronic control mechanism in the aladin cassette is in the anesthesia machine
Aladin cassettes may be tipped
Vapor 2000 vaporizers may be tipped as long as in the “T” setting
If wrong agent is used, discard contents of vaporizer and run at 5 liters oxygen flow until no agent detected (about 20 minutes)
