1
Q
What is vaporization
A
- converting a liquid to a gas
2
Q
What does volatility mean?
A
-the ability of a substance to break it’s intermolecular forces
3
Q
Why do we call our inhaled agents volatile agents?
A
- liquids are converted to vapor to be inhaled by patient and provide general anesthesia
4
Q
What is the goal of the vaporizer
A
1) convert liquid to vapor at a consistent concentration with
2) tight control
so that we can deliver anesthesia safely (no underdosing or overdosing)
5
Q
Vaporization is dependent on what three things?
A
- vapor pressure of each volatile agent
- temperature of the ambient environment (linear)
- amount of carrier gas (FGF) increase in flows increase dilution (linear)
6
Q
What gas laws apply to vaporization
A
- Ideal Gas Law
- Dalton’s Law of Partial Pressure
7
Q
Describe the process of vaporization. How do the components of the Ideal Gas Law relate to vaporization?
A
- in a sealed container gas molecules crash into the walls and exert a pressure
- that pressure is:
- directly proportional to the number of moles of gas molecules present
- directly proportional temperature of the system
- inversely proportional is volume
8
Q
Dalton’s Law of Partial Pressure
A
-total pressure in a given space is calculated by adding the partial pressures of each gas
9
Q
How is vapor pressure calculated from total pressure?
How is concentration calculated from vapor pressure?
A
- (molar fraction)(total pressure) = (vapor pressure)
- vapor pressure/barometric pressure = concentration or molar fraction
10
Q
What is concentration measured in?
A
V/V %
11
Q
Enflurane Vapor Pressure
A
172 mmHg
12
Q
Isoflurane Vapor Pressure
A
240 mmHg
13
Q
Halothane Vapor Pressure
A
244 mmHg
14
Q
Sevoflurane
A
157 mmHg
15
Q
Desflurane Vapor Pressure
A
669 mmHg
16
Q
When anesthesia providers select a dose of volatile agent that the vaporizer will deliver they select what?
A
- v/v %
- partial pressure of gas / total atmospheric pressure x 100
17
Q
What is the partial pressure of oxygen, nitrogen, and other at atmospheric temp?
A
- Oxygen = .21 x 760 = 160
- Nitrogen = .779 x 760 = 592
- Other = .011 x 760 = 8.4
18
Q
What effect does temperature have on the vaporization process?
A
-(direct) as temperature decreases, vapor pressure decreases
19
Q
What is the latent heat heat of vaporization?
A
- the energy (calories or joules) at which a liquid or volatile anesthetic changes from a liquid to a gas without changing its temperature
20
Q
Why is there no change in temperature?
A
There is no temperature change because although you are adding energy to the liquid, energy is escaping in the form of kinetic energy once the molecules at the surface leave in the form of gas molecules
21
Q
When the latent heat of vaporization is reached, what effect happens to the liquid
A
-the remaining liquid will be depleted of its energy, having a cooling effect, decreasing vapor pressure
22
Q
Why is the liquid cooling a problem?
A
Decrease in vapor pressure means decrease in anesthetic concentration means under-dose
23
Q
How do you prevent the cooling of liquid
A
-add a constant heat source
24
Q
Liquid and vapor are _____ at vapor pressure
A
Equilibrium
25
Adding heat source shift equilibrium toward _____
| at this time vapor pressure is ________
(Right) toward vapor
| Increases
26
Adding cooling source shift equilibrium toward _____
| at this time vapor pressure is ________
(Left) liquid
| Decreasing
27
Adding blow by gas shift equilibrium toward _____
| at this time vapor pressure is ________
(Right) vapor
| Decreased or constant
28
Specific heat
- the amount of energy required to increase the temperature of a specific volume of substance by 1 degree centigrade
29
Vaporizers are made with materials with a ______ specific heat. Why?
High
| - resists changes in temperature so resists evaporative cooling and delivers gas at a more constant concentration
30
Thermal conductivity
-measures the speed at which heat flows through something; higher thermal conductivity the better the substance conducts heat
31
Vaporizers are produced with materials with _____ thermal conductivity, why?
High
| -to maintain a consistent temperature within the vaporizer
32
What effect does altitude have on the barometric pressure?
(Inverse)
- as altitude increases, barometric pressure decreases
- as altitude decreases, barometric pressure increases
33
Types of vaporizers
- variable bypass
- tech 6
- cassette
- injection
34
How does a Variable bypass vaporizer work? How is it compensated?
- has a bypass chamber and vaporizing chamber
- portion of gas goes to vaporizing chamber to be saturated; another portion goes through the bypass chamber
- a variable orifice at the inlet controls gas flow
- vapor laden portion meets bypass portion to be diluted and delivered.
- agent specific, temp pressure compensated
-this device uses:
pressure compensation mechanism
temperature compensation mechanism
split ratio to deliver a safe, consistent anesthetic vapor
35
Pressure compensation mechanism
- prevents major overdose
- detects “pumping” or retrograde pressure in the system
- does not allow it to create retrograde flow of gas into the bypass chamber
36
Temperature compensation mechanism
- prevents major underdose
- metal that expands/shrinks in response to evaporative cooling
- proportions the bypass chamber flow to the vaporizing chamber flow
37
What two mechanisms control the concentration of volatile agent through the outlet
1) temperature compensation mechanism
| 2) concentration control dial
38
What temperatures does the temperature compensating mechanism work in?
50-104 degrees F
| 10 - 40
39
Factors that impact vaporizer output
- extreme flow rates = underdose
- temperature at boiling points
- pumping effect = overdose
- carrier gas composition (not clinically relevant)
- barometric pressure = under/over
40
How do extreme flow rates affect concentration of volatile anesthestic
-if you administer really high flow (>15L/min), and really low flow (<125mL/min) your v/v% may be slightly less than what you dialed in
41
How does temperature at boiling point affect volatile anesthetic concentration?
-at the boiling point the volatile agent is moving into a gas too quickly, there is no control over it’s flow (~50 degrees C)
42
How does the pumping effect affect volatile anesthetic concentration?
-this will result in a back flow of anesthetic vapor into the main inflow line increasing the concentration overall, resulting in massive overdose
43
How does carrier gas composition effect the volatile agent concentration?
-if you add nitrous as carrier gas instead of oxygen, it is a little more soluable in liquid so it can increase concentrations
44
How does barometric pressure affect the volatile anesthetic concentration?
At higher altitudes and lower barometric pressure, the partial pressure of the volatile agent will be a greater percentage of the barometric pressure.
45
What are some potential vaporizer hazards?
- wrong agent wrong vaporizer (high VP gas in low VP container = overdose + hypoxemic mixture; low VP gas in high VP container = underdose)
- contamination (can cause bacterial growth)
- tipping (overdose)
- overfilling (overdose)
- leaks (underdose)
- pumping effect (overdose)
46
In high altitudes do you have to make changes to a variable bypass vaporizer? Do you make changes in a tech 6?
- no changes with variable bypass
| - changes with tech 6
47
Why do we need a Tech6 to give Des?
- VP 669 (87%) at 1atm so need HIGH FGF to dilute des (not economic)
- Des not very potent, requires MAC of 6.6% so has very fast evaporative cooling and requires constant heat source
- Des boiling point 73 degrees at 1atm; will quickly boil and lose control of output
48
Tech 6 vaporizer
- electrically heated
- pressurized
- “dual gas” blender two independent gas circuits in parallel
49
What two variables impact tech 6 vaporizer output?
1) Altitude: you have to deliver higher dialed desflurane than at sea level;
- (desired % at sea level)(760mmHg)/ (current barometric pressure)
Carrier gas: vaporizer calibrated to 100% oxygen
- lower O2 concentrations than 100% or nitrous as carrier gas can decrease output by 20%
50
Aladin Cassette Vaporizers
| **used at georgetown**
- electronicallly controlled variable bypass unit in which the bypass circuit is within the anesthesia machine and the vaporizing chamber and circuit are within the cassette
- color coded and magnetically coded for safety
- some have warming fans that warm arm to decrease evaporative cooling effect
51
Desflurane = ____
Blue
52
Isoflurane = _____
Purple
53
Sevoflurane = _____
Yellow
54
What does the check valve do in a cassette vaporizer
-prevents against overdose by preventing retrograde pressure from O2 flush or retrograde flow by tipping etc.
55
From what area does the CPU in the cassette vaporizer gather data
- pressure and temperature sensors in the vaporizing chamber
- flowmeters in the bypass chamber and vaporizing chamber
- control dial set by the provider
56
How does the CPU affect outflow of gas
-CPU will use the flow control valve to determine the amount of flow leaving the vaporizing chamber, dictating the percent of fresh gas saturated with volatile anesthetic at the outflow
57
Safety mechanisms of the cassette vaporizer
- electronic control of the carrier gas ratio
- safety relief valve
- when cassette removed from machine, valves close to prevent FGF loss
- spill valve
58
-Electronic control of carrier gas ratio
-prevents hypoxemic gas mixture
59
Safety relief valve
-detects obstruction in system; cassette pressure up 2.5atm; valve opens
if desfluane boils; valve open up and allows pressure to escape into atm;
60
Closing valves
-allows you to change cassettes during case
61
Spill valve
- prevents retrograde flow from tipping overfilling
62
What happens in the desflurane cassette when desflurane boils?
- when OR temp is higher than 73 degrees and pressure becomes higher in vaporizer chamber than bypass chamber, the one way check valve closes and does not allow any FGF into vaporizing chamber; CPU starts to inject desflurane
63
What types of vaporizers does the Desflurane cassette function as?
- variable bypass until boiling
- switches to injector
- controlled by CPU
64
Injector Vaporizer
- agent specific
- electronically controlled
- vaporizer output set by electronic interface on anesthesia machine
- gas enters via drive gas inlet, pressurized liquid, moving it to injector
- liquid injected into heat vaporizing chamber and monitored by microprocessor
- injections continue in small amounts until desired volume is reached
- gas analyzer downstream measures output
- optical sensor in the chamber monitors integrity of injections
Intro to Anesthesia
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