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What determines phases of matter?
What is vapor pressure?
Measure of collisions of molecules with the container
What is saturated vapor pressure?
- Pressure of a vapor in equilibrium with evaporation of liquid within the container
- There is equal number of molecules leaving liquid phase as are leaving gas phase
What is vapor pressure dependent on? How?
- As temperature increases, VP increases
What is volatility?
Tendency of a liquid to readily change from liquid to vapor state
How is volatility related to vapor pressure?
More volatile liquids evaporate more readily and have higher vapor pressure than less volatile liquids
What is boiling point?
- The TEMPERATURE at which VAPOR PRESSURE of a liquid equals atmospheric PRESSURE
- Another way of saying this is - temperature at which VP = 1 atmosphere
What is boiling point dependent on?
- BP rises as ambient pressure rises and vise versa
How are BP and VP related for water?
- BP of water at normal atmospheric pressure (760mmHg) is 100 degrees C
- So VP of water at normal atmospheric pressure is 760mmHg
How does high altitude affect BP and VP?
- Higher altitude has lower atmospheric pressure
- Boiling point is lower because PRESSURE is lower
- Vapor pressure is lower because TEMPERATURE is lower
Why are anesthetic agents called volatile?
- They have a natural property to change from liquid to vapor state at STP
- The more volatile the agent, the stronger the tendency to enter vapor phase, the higher the vapor pressure
What is STP?
- Standard temperature is 0 degrees C
- Standard pressure is 760mmHg
What does vaporization of agent depend on?
- 1) vapor pressure of the agent
- 2) ambient temperature
- 3) amount of carrier gas flow
What properties of vaporizers prevent cooling of agent as liquid enters the vapor phase?
- 1) high thermal conductivity - environmental heat transferred to liquid agent
- 2) high thermal capacity - thermal reservoir to stabilize the liquid agent temperature
What is the saturated vapor pressure and boiling point at STP for Halothane?
What is the saturated vapor pressure and boiling point at STP for isoflurane?
What is the saturated vapor pressure and boiling point at STP for sevoflurane?
What is the saturated vapor pressure and boiling point at STP for desflurane?
- 22.8 degrees C
- Most volatile agent with strongest tendency to become vapor
What does saturated vapor pressure of agents depend on?
- 1) which agent it is
- 2) ambient temperature
What is Dalton's law of partial pressure?
In a mixture of gases, the pressure exerted by each gas is the same pressure it would exert if it alone occupied the container.
Calculate the % of exhaled CO2 with ETCO2 of 38mmHg.
38/760 = 5%
What do anesthetic vaporizers do?
- Change liquid anesthetic to vapor phase
- Add controlled amount of this vapor to the FGF
What is the difference between Tec 4, Tec 5, and Tec 6 vaporizers?
- Tec 4 and 5 are variable bypass with a split carrier gas; automatic temperature compensation
- Tec 6 (desflurane) is not; carrier gas is not split; electrically heated
What happens to vapor pressure when carrier gas is allowed to flow over liquid agent?
- VP decreases because vapor molecules are removed causing a loss of heat
- This is why vaporizers are designed to maintain constant temperature
What is latent heat of vaporization?
Amount of heat or energy required to convert 1ml of liquid into a vapor
What agent property are vaporizers temperature compensated and designed for?
The agents latent heat of vaporization
What is the latent heat of vaporization for halothane?
About 20 degrees C
What is the latent heat of vaporization for isoflurane?
About 25 degrees C
What is evaporation?
- Vaporization of liquid to gas
- Occurs well below boiling point
What is the name given to the loss of heat caused by evaporation?
What is the rate of agent cooling proportional to?
- Vapor pressure
- Latent heat
What is characteristic of high VP substances during evaporation?
There is rapid evaporation and therefore rapid cooling
What would happen with desflurane in variable bypass vaporizer?
It would boil in chamber and have nearly 100% output creating a danger of hypoxic gas mixture
Why is desflurane in a heated vaporizer?
To keep the liquid from rapidly cooling and to maintain a constant vapor pressure
What is desflurane's latent heat of vaporization?
- 39 degrees C at 2 atm (1520mmHg)
- Liquid agent is in reservoir tank
What are some unique characteristics about the Tec 6 desflurane vaporizer?
- 1) electronically heated
- 2) electronic controls inject pure vapor into FGF stream from flow meters - no carrier gas flow needed
- 3) shut off in power failure and has alarms
- 4) may be filled during use
What happens if wrong agent is placed in vaporizer?
- HLH - high low high (halothane in sevoflurane vaporizer)
- Agent with high vapor pressure in vaporizer of agent with low vapor pressure will deliver higher concentration than dialed
- LHL - low high low (sevoflurane in halothane vaporizer)
- Agent with low vapor pressure in vaporizer of agent with high vapor pressure will deliver lower concentration than dialed
What is the function of vaporizers?
Decrease the concentration of the anesthetic agent to a clinically useful concentration
What is the % concentration within a vaporizer?
- VP/atmospheric pressure x 100
- Isoflurane 238mmHg VP at 20 degrees C
- 31% concentration in vaporizer - saturated vapor pressure
How many ml of gaseous isoflurane are derived from 1ml of liquid?
How many ml of gaseous halothane are derived from 1 ml of liquid?
If you have 10cc liquid isoflurane in a vaporizer and are giving 1% concentration at a flow of 1lpm, how long will it last?
- 10cc liquid x 206 = 2060cc gas
- 1% of 1000cc/min = 10cc/min gas delivery
- 2060/10 = 206mins of agent left at current FGF and vapor concentration
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