Anesthesia Vaporizers Flashcards
Vapor Pressure (VP)
Created by molecules in the vapor phase bombarding the walls of a container
Saturated Vapor Pressure (SVP)
Gas phase above the liquid usually at 20C
Increase Temp –> Increase SVP
VP is independent of
atmospheric pressure if the temperature remains constant
Boiling Point
The temperature at which the vapor pressure equals atmospheric pressure
Units of Vapor Concentration
Absolute pressure (mmHg) Volumes percent
Volumes Percent is volume of vapor per 100 volumes of
total gas the partial pressure due to vapor/total ambient pressure X 100%
Dalton’s Law of Partial Pressures
The total pressure exerted by a gas mixture is the sum of the individual pressures of its constituents
Latent Heat of Vaporization
The # of cal required to change 1 g of liquid into vapor without a temp change.
Specific Heat
The # of cal required to increase the temp of 1 g of a substance by 1 deg C
Dalton’s Law of partial pressure summary
In a mixture of gasses the pressure exerted by each gas is the same as that which it would exert if it alone occupied the container.
Diffusion Can take place
- thru membrane
- thru gas/liquid interface
- between gasses
- between liquids
Application of Fick’s Law
Oxygen diffuses in
CO2 diffuses out
Diffusion respiration - pre-oxygenation and emergencies
Henry’s Law
At a gas/liquid interface, some of the gas will dissolve in the liquid.
Relative amount dissolved in solvent depends on the
chemical nature of the gas and solvent
Molecules of the gas within the liquid will exert
the same amount of pressure as the molecules overlying the liquid (at equilibrium)
PP will the same, but the number of molecules needed to exert the pressure depends on
the gas and solvent
Effect of pressure
As pressure of gas overlying liquid increases, pressure of gas in liquid and number of molecules of the gas increase proportionally
Temperature must be named in anesthesia,
37 degrees is used
blood:
gas most important
gas:
oil is next most important
Solubility Coefficient
Ostwald - the volume of gas which dissolves in one unit volume of the liquid at the temperature concerned
Useful in anesthesia
Does the Oswald solubility coefficient dependent on pressure
Independent of pressure
Vaporizers: Measured Flow
Copper Kettle, Vernitrol
Vaporizers: Variable Bypass
Ohmeda Tec 4 & 5, Drager 19.1, 19.3
Vaporizers: Blender
Ohmeda Tec 6 (Desflurane)
What happens to SVP with increased temperature?
More in the vapor phase
Difference MEASURED FLOW (COMM)
Carrier gas (CG) bubbles through agent Manual Temp. conversion Multi-agent Operator determines CG split No longer manufactured (but still present on field machines)
Different Variable Bypass
CH - AAA
CG flows over agent Auto adjust for Temp & Pressure Agent specific Auto CG split Higher safety standards
Decrease Temperature–> Decrease Vapor Pressure
Sluggish/not hitting the sides of the vaporizer
More flow must go through the vaporizing chamber so that the patient receives the same concentration of agent
As liquid is vaporized energy is lost in the form of
Heat
Increase Temperature–> Increase Vapor Pressure
Hitting the sides of the vaporizer rapidly
Less flow must go through the vaporizing chamber so that the patient receives the same concentration of agent
Increase temp will______vapor pressure
Increase
TEC 6
No Output Alarm Agent < 20 ml (1 bar) Tilt > 10 degrees Power Failure Internal malfunction Low Agent Illuminates with 50cc of agent Warm-Up 10 minutes to reach operating temp of 39C
Why a different vaporizer for Desflurane?
Requires an external heat source
- 4-9 times more Des is required than other IA
- Absolute amount of Des vaporized is much higher than other IA which would cause excessive cooling
- Traditional vaporizers could not compensate adequately
Traditional vaporizer would cause an
anesthetic overdose Des = 735 ml/min opposed to Iso = 47 ml/min
VP of Desflurane
VP of 669 mmHg – near 1 atm
Working pressure is affected by
FGF and viscosity of the carrier gases
Increase in FGF increase
vapor flow as well
Factors That InfluenceTec 6 Vaporizer Output
Varied Altitudes
Carrier Gas Composition
Tec 6 – Effects of Varied Altitudes
Requires manual adjustments of the concentration dial with changes in barometric pressure
- It is a pressurized vaporizer
- –It works at an absolute pressure
- —Partial pressure of Des changes with changes in ambient pressure
Required setting =
Normal setting (%) X 760 mmHg /ambient pressure (mm Hg)
Tec 6 – Carrier Gas Composition
- Decreased concentration output can occur with low flows of gases other than oxygen due to changes in viscosity
Vaporizer output is 20% less with nitrous than with oxygen - Nitrous oxide is less viscous than oxygen resulting in a lower working pressure at the differential pressure transducer
- Take home – May have to increase dialed concentration with low flow anesthesia
- Careful with low FIO2
Variable Bypass Vaporizers -
Varied Altitudes and the effects on agent output
***HypoBaric take home
The high resistance pathway through the vaporizing chamber offers less resistance thereby increasing vaporizer output.
Decrease Barometric Pressure –> INCREASED Vapor administered = Give more vapor than you have dialed on the vaporizer
Opposite of changes in temperature
***HYPERBARIC TAKE Home
Changes in the density of vapors/gases cause more resistance to flow through the vaporizing chamber and a decrease in vaporizer output
INCREASE Barometric Pressure –>DECREASED Vapor administered = Give less vapor than you have dialed on the vaporizer
HAZARDS (FOISTIP)
Incorrect agent Simultaneous agent administration Free Standing Overfilling Tipping Intermittent Back Pressure “Pumping Effect”
Safety Mechanisms (VPICK)
Push/Turn Interlock Color Codes “Keyed” filler system Vigilance+Vigilance+Vigilance+…
Never fill the vaporizer with the
dial in the ON position
Be careful to keep the amount of liquid in the vaporizer
between the black lines.
Tipping the Vaporizer during Transport
Agent enters the bypass chamber
Causes High Output Concentrations
OVERDOSAGE
Less of a problem with vaporizers that have extensive baffles (Tec 4)
If Tipping occurs:
- In hardened facilities turn in to maintenance
- Flush for 20-30 minutes with high FGF and set the vaporizer concentration low
- End-tidal agent monitoring
Intermittent Back PressurePumping Effect
- Associated with positive pressure ventilation or oxygen flushing
Oxygen flush is at __ psig-transmitted back
Occurs during the inspiratory phase - PPV - Reverse flow through the vaporizing chamber back into the bypass chamber (Bypass has diluted vapor and vaporizing chamber is also saturated with vapor)
- Causes higher than expected vapor concentrations
Intermittent Back PressurePumping Effect
- Associated with positive pressure ventilation or oxygen flushing
Oxygen flush is at (50) psig-transmitted back
Occurs during the inspiratory phase - PPV - Reverse flow through the vaporizing chamber back into the bypass chamber (Bypass has diluted vapor and vaporizing chamber is also saturated with vapor)
- Causes higher than expected vapor concentrations
Pumping Effect is more Pronounced with
Low flow rates Low dial settings Low levels of agent in the chamber Rapid respiratory rates High peak inspired pressures Rapid drops in pressure during expiration
Vigilance Steps
- Make sure the vaporizer is in the OFF position after the machine check
- Make sure that all filler ports are in place and secured tightly
- Minimize possible leaks
- Don’t overfill the vaporizer
- Make sure the vaporizer has at least liquid up to the bottom line to minimize the pumping effect
- Keep extra agent bottles in the drawer
Calculate % of oxygen or nitrous oxide given a total FGF
Ex: 1 L/min of O2 , 1 L/min of N2O
1/1+1 = ½ = 50% O2 % 50% N2O
How to determine flow %
Flow of particular gas/Total gas flow
Commonalities – Modern Vaporizers
BALT- C
“Button” must be pressed Counterclockwise Agent specific T&P compensated Lock-out mechanism
Less soluble
WIll dissolve quicker
The temperature at Mount everest is about
220 mmHg so HIGHER gas will be delivered then set
Boiling point of each VA, and MAC of each And Coefficients
Volatile
Only Electronic vaporizer
TEC 6
Higher you go the LOWER
ATMOSPHERIC PRESSURE
TEC 6 has to reach
39 degrees C
WHy do we need so much DESFLURANE
because the solubility is so low
INhale maybe 5.5, exhale could be 5.4
Meaning it’s not BEING ABSORBED
The colder the patient
Their solubility is affected, slower metabolism
WIll take LONGER TO wake, NMB reversal more slowly
Higher atmospheric pressure _______output
LOWER
Lower atmospheric pressure _______output
HIGHER
Running sevo with less oxygen like (0.5L) can lead to
Compound A