Week 7 Vaporizers Flashcards

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1
Q

Warmer temperatures cause the temperature compensator cone to move downward, __________ bypass flow, and diverting _______ gas to the vaporizing chamber.

A

increasing; less

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2
Q

Name 5 factors that can impact vaporizer output.

A
  • 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
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3
Q

Describe components/features of the injection type vaporizer.

A
  • 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
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4
Q

If you place a higher VP volatile agent (Desflurane) in a lower VP vaporizer (Iso or Sevo), will you have an overdose or underdose?

A

Overdose

This could actually cause a hypoxic mixture as well!

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5
Q

Isoflurane vapor pressure @ 20 C.

A

240 mmHg

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6
Q

Sevoflurane vapor pressue @ 20 C.

A

157 mmHg

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7
Q

Enflurane vapor pressure @ 20 C.

A

172 mmHg

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8
Q

Name 3 common features of a Tec 6. Why is this Tec 6 necessary for desflurane?

A
  • 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
  • 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
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9
Q

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.

A

restricts; more

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10
Q

What is the latent heat of vaporization? An how does it affect the liquid anesthetic in the vaporizer?

A
  • 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**
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11
Q

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.

A

directly; indirectly

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12
Q

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.

A

39 F; 1500 mmHg (per slides), 1300 mmHg (per book)

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13
Q

INFO CARD

Example of vapor pressure.

A

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*

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14
Q

List the safety mechanisms of cassette vaporizers.

A
  • 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)
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15
Q

Define thermal conductivity. How does thermal conductivity play a role in vaporizer design?

A
  • 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)
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16
Q

How do changes in altitude effect vaporization within the variable bypass vaporizer?

A
  • 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.

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17
Q

The ratio of gas through the vaporizing chamber and the bypass chamber is dependent on what two factors?

A
  • temperature compensator
  • concentration control dial
18
Q

Desflurane vapor pressure @ 20 C.

A

669 mmHg

19
Q

Describe how the labyrinth compensates for pressure fluctuations.

A

The labyrinth compensates for pressure fluctuations within the vaporizer from the gas supply side and the breathing circuit side to stabilize vaporizer output; it is not present to compensate for changes in atmospheric pressure

20
Q

Though modern vaporizers have many safety features to minimize risks, name 6 potential vaporizer hazards.

A
  • wrong agent in the wrong vaporizer
  • contamination
  • tipping (overdose)
    • Excessive tipping can allow the liquid agent to enter the bypass chamber and cause an extremely high output
  • overfilling (overdose)
    • If overfilled, liquid anesthetic may enter the bypass chamber, and a harmful dose of vapor could be delivered to the common gas outlet
  • leaks (underdose)
    • Loose filler caps, filler plugs, and drain valves are probably the most common sources of leaks
  • pumping effect (overdose)​

**pay attention to “gas analyzer in line”**

21
Q

What is the most important function of the temperature compensating devices?

A

To correct for the effect of evaporative cooling on the liquid anesthetic.

22
Q

The expansion/contraction element of the temperature compensator in the variable bypass vaporizer operates between ____ °F - ____ °F

A

50 - 104 F

23
Q

Vaporizers are built with materials with _____ specific heat and ____ thermal conductivity.

A

high; high

  • high specific heat helps to resist evaporative cooling
  • high thermal conductivity helps maintain consistent temp within vaporizer
24
Q

Name two potential causes of intermittent back pressure or “the pumping effect.”

A
  • positive pressure ventilation
  • the use of the oxygen flush valve

“The pumping effect is caused by retrograde transmission of pressure from the patient circuit to the vaporizer during the inspiratory phase of positive-pressure ventilation or use of the oxygen flush function”

25
Q

Describe two factors that impact Tec 6 vaporizer output and explain each.

A
  • 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%
26
Q

Volume percent(v/v%) =

A

partial pressure of gas / total pressure * 100%

27
Q

Describe general components/features of a variable bypass vaporizer.

A
  • vaporizing chamber and bypass chamber
  • agent specific
    • calibrated to the specific vapor pressure of that agent
  • pressure compensated
    • think “labyrinth”
  • temperature compensated
28
Q

A gas in a container (vaporizer) is “saturated” with volatile agent when the rate of ________ = rate of __________.

A

Rate of evaporation = rate of return to liquid phase

**The anesthetic molecules in the gas phase create a partial pressure known as the saturated vapor pressure, or simply vapor pressure**

29
Q

Halothane vapor pressure @ 20 C.

A

244 mmHg

30
Q

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.

A
  • 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.”

31
Q

If you place a lower VP volatile agent in a higher VP vaporizer, will you get an overdose or underdose?

A

Underdose

32
Q

How does temperature affect vapor pressure?

A
  • increased temperature → increased vapor pressure
  • decreased temperature → decreased vapor pressure
33
Q

Vaporization in a vaporizer is dependent on which 3 factors?

A
  • Vapor Pressure of the anesthetic
  • Temperature
  • Amount of carrier gas used
    • Think of 2 L of FGF vs 10 L FGF - dilutional effect
34
Q

T or F: Vapor pressure is a chemical property of volatile agents.

A

False; vapor pressure is a physical property

35
Q

What unique safety mechanism is in place to prevent retrograde flow of anesthetic vapor into the bypass chamber in the Aladin Casette vaporizing system?

Failure of this mechanism can result in what?

A

the one - way check valve

Failure of the check valve to close can result in anesthetic overdose due to retrograde flow into the bypass chamber.

36
Q

Boiling point occurs when….

A

vapor pressure = barometric pressure

**the liquid then undergoes rapid vaporization**

37
Q

Define specific heat. Why is this concept important to consider in vaporizer design?

A
  • number of calories required to increase the temperature of one gram of a substance by one degree C
  • Specific heat of a volatile agent tells us how much heat must be added to maintain a constant temperature to compensate for the latent heat of vaporization.
  • **vaporizers are built with materials known to have a high specific heat (helps to resist evaporative cooling)
38
Q

Liquids with a _______ tendency to evaporate and generate _______ vapor pressures are described as “more volatile.”

A

greater; higher

39
Q

Describe the Aladin Casette Vaporizers: features, functionality, etc.

A
  • “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
40
Q

When anesthesia providers selct the “dose” of volatile agent that the vaporizer will deliver: we select a _________.

A

volume %

41
Q

How does the desflurane cassette design differ from that of the other Aladin Cassettes?

A
  • when room temperature > boiling point (22.8 C/73 F), desflurane boils and the vaporizing chamber pressure > ambient pressure
  • when vaporizing chamber pressure > bypass chamber, the one way check valve closes and prevents carrier gas from entering the cassette
    • the CPU adjusts the flow control valve to meter in the appropriate flow of pure desflurane vapor needed to achieve the desired final concentration
    • the vaporizer then begins functioning as an injector, as opposed to resembling a variable bypass unit