Week 7 Vaporizers Flashcards
Warmer temperatures cause the temperature compensator cone to move downward, __________ bypass flow, and diverting _______ gas to the vaporizing chamber.
increasing; less
Name 5 factors that can impact vaporizer output.
-
Extreme flow rates
- <250 ml/min and >15 L/min output will be slightly less than dialed in.
-
Temperatures at boiling points of volatile agents ~50 C
- total loss of control
-
Pumping effect
- intermittent back pressure or back flow
- labyrinth meant to overcome that risk
- vapor could potentially back flow, and then reenter through the bypass chamber and cause overdose
- Although intermittent backpressure can result in transient rises in anesthetic concentration at the common gas outlet, the effects are mitigated by dilution within the much larger anesthetic breathing circuit
-
Carrier gas composition
- relates to nitrous oxide
- usually not clinically significant
- Barometric pressure
Describe components/features of the injection type vaporizer.
- electronically controlled, out of circuit vaporizer
- agent specific
- unique: no concentration control dial
- vaporizer output set by anesthesia provider via electronic interface on the anesthesia machine
- gas from the anesthesia machine enters through the drive gas inlet
- the gas pressure provides the force to drive liquid through the injector
- liquid anesthetic is injected into a heated vaporizng chamber in pulses determined by a microprocessor.
- injection continues in small increments until the desired anesthetic volume is reached (based on anesthetic concentration and the fresh gas flow settings)
- a dedicated gas analysis line downstream from the vaporizer monitors the output
- an optical sensor in the vaporizer monitors the integrity of the anesthetic injections
If you place a higher VP volatile agent (Desflurane) in a lower VP vaporizer (Iso or Sevo), will you have an overdose or underdose?
Overdose
This could actually cause a hypoxic mixture as well!
Isoflurane vapor pressure @ 20 C.
240 mmHg
Sevoflurane vapor pressue @ 20 C.
157 mmHg
Enflurane vapor pressure @ 20 C.
172 mmHg
Name 3 common features of a Tec 6. Why is this Tec 6 necessary for desflurane?
- Electrically heated
- Pressurized
- Gas - vapor “dual gas” blender
- two independ gas circuits arranged in parallel
- Desflurance VP = 669 mmHg at 20 C
- Excessive bypass chamver flow rates would be required to dilute the vaporizing chamber output to clinical concentrations > 70 ml/min (we normally use 1 - 10 ml/min during a case)
- would make patient cold
- dry out patient airways
- cost prohibited
- Excessive bypass chamver flow rates would be required to dilute the vaporizing chamber output to clinical concentrations > 70 ml/min (we normally use 1 - 10 ml/min during a case)
- Requires significant evaporation because of a relatively low potency = excessive cooling
- MAC = 6.6% (much higher than other agents, sevo ~2%, iso ~1.2%)
-
Boiling point = 73 F at 1 atm
- loss of control of output in variable bypass with boiling
As liquid anesthetic agent cools, the temperature-compensating cone moves upward, _________ bypass flow, and diverts ______ gas to the vaporizing chamber, thereby maintaining relatively stable vaporizer output.
restricts; more
What is the latent heat of vaporization? An how does it affect the liquid anesthetic in the vaporizer?
- amount of energy absorbed by a liquid secondary to evaporation
- the energy for vaporization comes from the liquid itself (or from an outside source)
- with no outside heat source, the liquid will become cooler as more agent vaporizes
- **this can reduce subsequent vaporization**
- with no outside heat source, the liquid will become cooler as more agent vaporizes
The ideal gas law provides an important framework for understanding the behavior of anesthetic gases within vaporizers, anesthesia delivery equipment, and the pulmonary alveolus (Miller). When sealed in a container, gas molecules collide with the walls and exert a force or pressure. This pressure is ________ proportional to the # of molecules/moles(n) of gas present within the container and to the Temperature (T) in Kelvin, and _________ proportional to the volume (V) that confines the gas.
directly; indirectly
In the Tec 6, the sump shut off valve remains closed until the desflurane is heated to about ____ F with a vapor pressure about _____ mmHg, and the concentration control valve is turned to the “on” position.
39 F; 1500 mmHg (per slides), 1300 mmHg (per book)
INFO CARD
Example of vapor pressure.
In figure B, the gases have equilibrated. The amount of isolflurane coming out of the gas and into the liquid, and the amount of isoflurane in liquid phase coming out to the gas phase is equal.
*This proportioning occurs based off of Isoflurane’s vapor pressure*
List the safety mechanisms of cassette vaporizers.
- electronic control of the carrier gas ratio reduces risk of hypoxic gas mixture
- safety relief valve
- opens when cassette pressure > 2.5 atm (1899 mmHg)
- when cassette removed from the workstation, valves close prevent loss of fresh gas
- another valve prevents liquid anesthetic from entering the fresh gas line
- overfilling protection mechanism
- no restrictions on orientation during handling or storage
- (tipping OK)
Define thermal conductivity. How does thermal conductivity play a role in vaporizer design?
- a measure of speed with which heat flows through a substance
- the higher the thermal conductivity, the better the substance conducts heat
- Vaporizers are produced with materials with high thermal conductivity (these metals help maintain consistent temp within the vaporizer)
How do changes in altitude effect vaporization within the variable bypass vaporizer?
- Typically not clinically significant in that it does not typically require any adjustment
- At atmospheric pressures < 760 mmHg, a variable bypass vaporizer will have an output concentration that is greater than that set on the dial in volume percent, but potency (partial pressure) in creases by a lesser amount
i.e. With a constant dial setting of 0.89%, at 1 atm, a well-calibrated isoflurane variable bypass vaporizer would deliver 0.89 v/v% isoflurane, and the partial pressure of isoflurane output would be 6.8 mm Hg. Assume that we maintain the same dial setting and lower the atmospheric pressure to 0.66 atm or 502 mm Hg (roughly equivalent to an elevation of 10,000 feet. This results result in an increase in the isoflurane concentration output to 1.75% (a 97% increase), but the partial pressure increases to only 8.8 mm Hg (a 29% increase). A similar change in output partial pressure at sea level, in terms of volume percent, would correspond to an isoflurane concentration increase of only 0.2%. So while the anesthetic concentration (v/v%) changes significantly in this example, it is the partial pressure of volatile agent in the brain that is ultimately responsible for anesthetic depth, and that change is minimal.
Describe two factors that impact Tec 6 vaporizer output and explain each.
-
Altitude
- effect is that you would have to deliver HIGHER dialed concentration of desflurane at higher altitude/lower barometric pressure
- i.e delivering 6.6% desflurance @ 760 mmHg = 50.16 mmHg
- delivering 6.6% desflurane @ 500 mmHg = 33 mmHg
- Required dial setting = Normal dial setting (%) x 760 mmHg Ambient Pressure (mmHg)
-
Carrier Gas
- vaporizer calibrated for 100% oxygen
- if carrier gas <100 % O2 used = reduced vaporizer output
- i.e. with low flows and nitrous oxide used as the carrier gas, desflurane output can be reduced by 20%
- vaporizer calibrated for 100% oxygen
Describe general components/features of a variable bypass vaporizer.
- vaporizing chamber and bypass chamber
- agent specific
- calibrated to the specific vapor pressure of that agent
- pressure compensated
- think “labyrinth”
- temperature compensated
In the Aladin Cassette, a flow control valve, modulated by a central processing unit (CPU), precisely meters the amount of gas flow through the vaporizing chamber, which then rejoins the bypass flow. Vaporizer concentration output is ultimately determined by a multitude data input received by the CPU.
List the data input received by the CPU.
- concentration control dial (set by anesthesia provider)
- pressure and temperature sensors inside the vaporizing chamber
- flow sensors in the bypass and vaporizing chambers
- also receives input from flowmeters regarding the carrier gas composition
“Using these data, the CPU precisely regulates fresh gas flow through the vaporizing champer to obtain the desired vapor concentration output.”
How does temperature affect vapor pressure?
- increased temperature → increased vapor pressure
- decreased temperature → decreased vapor pressure
Describe the Aladin Casette Vaporizers: features, functionality, etc.
- “think of it like an electronically/computer controlled variable bypass vaporizer”
- one electronically controlled vaporizer can deliver several different inhaled anesthetic agents
- permanent internal control unit within the anesthesia machine with interchangable casettes
- cassettes filled with anesthetic liquid and serve as vaporizing chambers
- filled using agent specific fillers and color - coded
- magnetically coded to allow workstation to identify which cassette has been inserted
- some include a fan that warms the air within the casette to offset cooling with high gas flows and/or high dial settings
When anesthesia providers selct the “dose” of volatile agent that the vaporizer will deliver: we select a _________.
volume %
