Week 2 - Vaporizers Flashcards
What is saturated vapor pressure?
The pressure exerted by the vapor molecules within the container
Closed container and room temp anesthetic agent is in liquid form with some molecules escaping the surface of the liquid to form a gas or vapor - equilibrium is established between molecules in vapor phase and those in liquid phase (gas phase above the liquid is said to be saturated when it contains all the anesthetic vapor it can hold at a given temp)
How does temperature affect saturated vapor pressure?
Equilibrium with be shifted so more molecules enter the vapor phase and the vapor pressure will increase
vapor pressure depends only on the liquid and the temperature
Define expressing the concentration of gas as a partial pressure
a mixture of gases in close container exerts a pressure on the walls – the part of the total pressure due to any one gas in the mixture is the partial pressure
depends only on the temp of that agent and is unaffected by the total pressure above the liquid
highest PP that can be exerted by a gas at a given temp is its vapor pressure
Expressed as an absolute pressure
Define expressing the concentration of gas as a volume percent
number of units of volume of a gas in relation to a total 100 units of volume for the total gas mixture
in a mixture of gases, each component gas exerts the same proportion of the total pressure as its volume is of the total volume
Expresses the relative ratio of gas molecules in a mixture (Vol% = Partial Pressure / Total Pressure)
What is heat of vaporization?
- Energy (heat) is required to promote the vaporization process
- Liquid temp decreases as vaporization proceeds because heat is drawn from the liquid itself as well as the container — As vaporization proceeds, the cooling of the liquid anesthetic and container slows down the vaporization process
How do modern vaporizers compensate for the heat of vaporization?
They have temperature compensatory systems to ensure that heat loss due to vaporization is compensated and therefore doesn’t reduce output
- most compensate for temp changes by altering the splitting ratio so the percentage of carrier gas directed through the vaporizing chamber is increase or decreased
- an electric heater can be used to supply heat to a vaporizer and maintain it at a constant temp
What is specific heat?
A substance’s specific heat is the quantity of heat required to raise the temp of 1mL of the substance by 1*C
The higher the specific heat the more heat is required to raise the temp of a given quantity of that substance
Why is specific heat important to consider in vaporizer construction and function?
it 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 for choosing the material to construct a vaporizer (temp changes more gradually for materials with a high specific heat than those with a low specific heat
Are vaporizers made of material with high or low thermal conductivity?
High Thermal Conductivity (Copper/Bronze) – minimizes temp changes when the vaporizer is in use
the higher the thermal conductivity, the better the substance conducts heat (using materials with high thermal conductivity achieves thermostabilization)
Describe the design of temperature compensated-calibrated variable bypass vaporizers
- Fresh gas enters the vaporizer, where its flow is split between a larger bypass flow and a small flow to the vaporizing chamber or sump
- In the sump is the agent at its saturated vapor concentration
- Saturated vapor mixes with the bypass flow, which dilutes it to the concentration dial setting
What is a splitting ratio?
Ratio of bypass gas to gas going to the vaporizing chamber
(Vaporizing Chamber Flow Rate : Bypass Flow Rate)
-depends on the resistances in the two pathways (controlled by the concentration dial) as well as the saturated vapor pressures
Why can’t there be one vaporizer for all volatile anesthetics?
They have different saturated vapor pressures
if an agent of low potency/volatility is placed in a vaporizer of a higher potency/volatility, the effect will be an output of less than the dial reading – high potency agent in low potency vaporizer = higher output of agent than the dial reading
Describe Flow-Over Vaporizer
A stream of carrier gas passes over the surface of the liquid
-increasing the area of the carrier gas-liquid interface enhances vaporization efficiency (done by using baffles or spiral tracks to lengthen gas pathway over the liquid or employing wicks that have their bases in the liquid)
Describe Injection Vaporizer
control the vapor concentration by injecting a known amount of liquid anesthetic into a know volume of gas
Why is Desflurane not administered by a flow-over vaporizer?
Due to its very high saturated vapor pressure it would require really high flows to dilute it down to an acceptable concentration
Desflurane vaporizer is heated to above ambient temp and is pressurized, as the control dial is turned, appropriate amounts of saturated vapor are added to the fresh gas flow from the bypass (fresh or diluent gas is entirely separate from the vaporizing chamber)
