Systems Lecture 4: Vaporizers and Agents

What is a vaporizer?

a delivery device for anesthetic agents that adds a controlled amount of vapor to breathing system FGF

1mL of liquid anesthetic agent can be converted into how much anesthetic vapor?

~200mL

Anesthetic Gases work in what division of the nervous system?

the CNS

Potency of general anesthetics correlate with their...? Indicating the importance of?

solubility in oil; interaction with hydrophobic targets

Anesthetics enhance and suppress which types of signals

enhance inhibitory signals

suppress excitatory signals

Yellow vaporizer is which drug?

Sevoflurane (Ultane)

Purple vaporizer is which drug?

Isoflurane (Forane)

Blue Vaporizer is which drug?

Desflurane (suprane)

How many vaporizers can be used simultaneously?

only one can be used at a time

Myer Overton Rule

Higher lipid solubility = Higher potency

What was morton's vaporizer made of?

sponge soaked in ether

A drug that is slow to induction will be fast or slow during emergence?

slow induction drug = slow emergence

higher lipid solubility = _____ potency

higher potency

MAC stands for

minimum alveolar concentration

What is MAC?

Alveolar concentration that prevents movement in 50% of patients in response to noxious stimuli

Gas with a lower MAC correlates with...

higher potency

What can impact MAC values? (3)

age

temperature

acute intoxication

FGF is determined by... (2)

vaporizer and flowmeter settings

Fi stands for

inspired gas concentration

What 3 things affect Fi?

- FGF

- Breathing circuit volume

- Circuit absorption

FGF + induction/recovery Time

higher FGF, faster induction/recovery time

Circuit Absorption + induction/recovery Time

Lower Circuit Absorption, faster induction/recovery times

Breathing system volume+ induction/recovery Time

Smaller Breathing system, faster induction and recovery time

FA stands for

alveolar gas concentration

What affects FA?

Uptake

Ventilation

Concentration Effect

Second Gas Effect

Fa stands for

arterial gas concentration

What affects Fa?

v/q mismatching

What can FA/Fi represent?

How fast you go to sleep and wake up

What is uptake?

gas molecules taken up from alveoli into pulmonary circulation

Uptake of gas is dependent on (3)

- solubility of gas

- cardiac output

- Alveolar-Venous partial pressure difference

Circulatory uptake and FA

greater circulatory uptake = slower rise in FA

cardiac output and induction speed

lower cardiac output, higher induction speed

what does the partition coefficient express?

solubility in blood, air, and tissues

Partition coefficient meaning

Ration of concentrations of gas in each of 2 phases at steady state

Blood Gas Partition coefficient of N2O

0.47

N2O's blood gas partition coefficient of .47 indicates

at steady state, 1mL of blood contains .47 as much N2O as does 1mL of alveolar gas

Low Solubility + Flow into Blood

Slower flow into blood, faster increase in alveolar concentration

High Solubility + Flow into blood

higher solubility, alveolar partial pressure rises more slowly, and thus induction prolonged

Gases take the longest time to get into what type of tissue?

fat

Low solubility and speed into the blood

Fastest into and out of the blood

Blood/gas partition coefficient: Desflurane

0.42

Blood/gas partition coefficient: Sevoflurane

0.65

Blood/gas partition coefficient: Isoflurane

1.4

MAC of Desflurane

6%

MAC of Sevo

2%

MAC of iso

1.15%

MAC of N2O

105%

Higher solubility and alveolar partial pressure

Alveolar partial pressure rises more slowly

High Cardiac Output and Anesthetic Uptake

as cardiac output increases, anesthetic uptake increases, rise in alveolar pressure will be slow and induction is delayed

Lower Cardiac Output and Anesthetic Uptake

as cardiac output decreases, anesthetic uptake decreases -> rise in alveolar pressure will be faster and so will induction

High blood flow and FA/Fi ratio

High blood flow through lungs, more blood removing anesthetic, lower FA/Fi ratio

Transfer of anesthetic blood to tissues is dependent on which (3) factors

- tissue solubility

- tissue blood flow

- difference in partial pressure between arterial blood and tissue itself

Which types of organs are the first to take up a significant amount of anesthetic?

vessel rich organs

Vessel rich organ examples

Brain, heart, kidney, liver, endocrine

Which types of organs are next in uptake? Why?

Muscles; not as well perfused

What is next in rate of uptake? why?

fat; very soluble

What is slowest in rate of uptake?

vessel poor (ligaments, bones, teeth, hair, cartilage), insignificant uptake

What 3 things affect uptake?

Solubility of gas, cardiac output, alveolar-venous partial pressure difference

The greater the anesthesia uptake, the _____ the rise in FA and the _____ induction

slower; slower

Do you increase or decrease ventilation in order to speed up induction? why?

increase; constantly replacing anesthetic taken up by the bloodstream, better maintaining the alveolar concentration

increasing ventilation to speed up ventilation is much more profound in...

soluble agents

Concentration Effect

Increasing concentration of gas increases rate of rise and alveolar concentration itself

Second Gas Effect

large volume uptake of one gas increases the concentration of the second gas

What is boiling point?

temp where liquids vapor pressure is equal to barometric pressure

Lower Barometric Pressure = ______ BP

lower boiling point

What is vapor pressure?

partial pressure of agent in vapor phase

Temperature and Its Relationship to vapor pressure

as temperature increases, so does vapor pressure

vapor pressure of isoflurane

238 torr

vapor pressure of desflurane

669 torr

vapor pressure of sevoflurane

157 torr

Mac of Halothane

0.75

Mac of Isoflurane

Fractional concentration of agent at outlet of vaporizer formula

(QV x PA) / ((PB - PA) x QT)

Specific Heat

heat (calories) required to raise temp of 1 gram of agent by 1 degree celsius

Specific heat determines what in reference to vaporizers?

the material that the vaporizer is made out of

What materials are vaporizers typically made out of?

copper, bronze bc of high thermal conductivity

What is thermal conductivity?

measure of speed at which heat flows through a substance

Heat of vapor

amount of heat (calories) required to convert 1 gram of liquid agent into a vapor

As you begin to heat up liquid agent into a vapor, what will happen to the remaining liquid?

temperature of remaining liquid will drop, lowering vapor pressure

What color is Halothane

red

What color is enflurane

orange

What kind of filling error would result in higher concentration delivered?

putting an agent with higher vapor pressure into a vaporizer intended for a lower vapor pressure

What kind of filling error would result in a lower concentration delivered?

putting an agent with a lower vapor pressure into vaporizer intended for higher vapor pressure

What is variable bypass?

As concentration is increased, more of the gas flow is diverted into vaporizing chamber. This gas then re-meets with FGF downstream

What is the splitting ratio?

ratio of bypass gas to gas going through vaporizing chamber

Why do vaporizers have to do Temperature Compensation?

◦As liquid is vaporized, energy in form of heat is lost

◦As temp of liquid decreases, so does its vapor pressure

◦Must maintain a constant vapor output during times of fluctuation in liquid anesthetic temp

Mechanical thermocompensation

◦Alters splitting ratio - increase or decrease flow of carrier gas that is directed to vaporizing chamber

Mechanical thermocompensation: If temperature decreases: what happens to flow of gas?

◦If temp decreases, more gas is allowed to pass thru vaporizing chamber

Mechanical thermocompensation: If temperature increases, what happens to flow of gas?

◦If temp increases, restricts more gas to pass thru vaporizing chamber

How many flow streams in the Tec-4 vaporizer?

2

How many wicks line the outlet in a ten-4 vaporizer? why?

2; to keep gas in contact with liquid agent

Tec-6 Vaporizers always contains

DES

Desflurane Vaporizer: Unique qualities

- plugs in

- will turn off if tilted past 10 degrees

- maintains agent vapor and fresh gas at same pressure when mixed

Which vaporizers can you fill in the middle of case?

You CAN refill DES

You CANNOT refill Sevo/Iso

What is unique about the aladin cassette?

can be flipped upside down, sideways, etc. without any problems

Why does an aladin cassette have no problem with tilting or gas escaping during handling?

No bypass flow channels

How many cassettes does the machine accept at a time?

1

What helps adjust the amount of FGF into the vaporizer in an aladin cassette?

Flow restricter valve

Best practice when filling cassette vaporizers

Push both inlet and outlet valves at the same time to depressurize before filling to the line

What allows gas to flow into the cassette vaporizer?

Pins in the back