What makes inhalant anesthetics unique from other drugs?
- Administered & eliminated through the lungs
- onset & offset does not depend on liver or kidney function
- predictable & rapid titration of anesthesia
- requires a specialized device for delivery
What is the anesthesia triad?
Anti-nociception > Unconsciousness > Muscle relaxation
When and what were the first inhalant anesthetics used?
1840; Chloroform, diethyl ether, nitrous oxide
How many inhalant agents are used in vet med and what are they?
4; Isoflurane, Desoflurane, Sevoflurane, and Nitrous oxide
Why are physiochemical characteristics important?
- determines actions & safety
- determines how they are supplied & delivered
- determines how they interact with other substances within the anesthetic machine
- determines equipment used for delivery
- determines uptake, distribution, & elimination
What are some organic compounds for inhalant agents?
Modern inhalants: isoflurane, sevoflurane, desflurane
What's an example of an inorganic compound in inhalant agents?
Modern inhalants are ______ and have an _______
Halogenated; ether bond
What is halogenation?
Addition of Cl, Br, or F
What are the effects of halogenation?
-decreases reactivity, increases potency, makes inhalants non-flammable, toxicity still possible (esp. with F)
What is the effect of the ether bond in inhalants?
Makes them less arrhythmogenic
What are the properties determining how inhalants are administered?
-vapor pressure -boiling point -liquid density/specific gravity
What are the properties determining how inhalants travel around the body?
- blood:gas partition co-efficient
What does the change in the state of matter depend upon?
Molecular motion and degree of intramolecular attraction
an agent that exists in gaseous form at room temperature and atmospheric pressure
Example: Nitrous oxide
Gaseous state of an agent that exists as a liquid at room temperature and atmospheric pressure -example: isoflurane, sevoflurane, desflurane
Inhalants existing as a ____ still have the same physical properties as a _____ when it exists in its ____ form
What is partial pressure?
The pressure an individual gas exerts on the walls of a closed container; it is an absolute value
What is Dalton's Law of Partial Pressure?
The total pressure of a mixture of gases is equal to the sum of the partial pressure of all the gaseous substances present
What are 3 ways to quantify inhalants?
- pressure (mmHg)
- concentration (%) - most often
- mass (grams or milligrams)
Inhalant concentration changes relative to the concentration of what?
The whole gas mixture
- vol% changes relative to atmospheric pressure
- vol% may be different in various body compartments while partial pressure is the same if the inhalant agent is in equilibrium b/t body compartments
How would you calculate the percent of an agent to be used in a vaporizer, given the total pressure and the alveolar pressure of that agent?
(Alveolar pressure (Pa)/total pressure)x100 = % of that gas
Total pressure = 760 mmHg
Pa of ISO = 15 mmHg
15mmHg/760mmHg x 100 = 2%
Define vapor pressure
The pressure vapor molecules exert when the liquid and vapor phases are in equilibrium
Define saturated vapor pressure
Maximum concentration of molecules in the vapor state that exists for a given liquid at a given temperature
What is the practical difference between a gas and a vapor?
Gas: administer from 0-100% concentration
Vapor: maximum delivered concentration depends on its saturated vapor pressure
What happens to vapor pressure as temperature decreases and why?
It decreases; evaporation > cooling > decreased vapor pressure > decreased delivered vapor concentration
What is the vapor pressure of isoflurane?
What is the vapor pressure of sevoflurane?
What is the vapor pressure of Desflurane?
What happens if someone pours Isoflurane into a Sevoflurane vaporizer and delivers 3% of the agent?
Increased output from the vaporizer > potential anesthetic OVERDOSE (deliver more Iso using Sevo vaporizer) Vapor pressure of Iso (238mmHg) > Vapor pressure of Sevo (157mmHg)
Define gas solubility
The total number of gas molecules dissolved in a solvent; expressed as a partition coefficient
The amount of gas dissolved into a solvent (its solubility) depends on _____
- partial pressure gradient b/t gas and solvent -chemical nature of the gas (molecular weight) -chemical nature of the solvent
Each inhalant has a different solubility in blood, True or False?
Why is gas solubility important?
Blood:gas partition coefficient can help predict SPEED of anesthetic induction, recovery, and change of depth - lower blood:gas partition coefficient = FASTER onset & recovery - the more soluble a gas is = the SLOWER the onset & recovery
Why do drugs with lower solubility have faster onset?
Because agents with low solubility quickly saturated the blood, so additional anesthetic molecules are then readily transferred to the brain
You are performing isoflurane anesthesia on aortic foxes in the Alaskan wilderness in December. Your team is having a difficult time keeping the foxes anesthetized. What is the problem?
The vapor pressure of isoflurane is decreased because of the low ambient temperature -Lower temp > lower saturated vapor pressure > less inhalant will be available for anesthetic
What are the principal sites of action of inhalant anesthetics and what are the effects at each site?
-Brain (amnesia) -Spinal cord (immobility) - partial pressure of anesthetic in the brain/spinal cord produces anesthesia
What is the Minimum Alveolar Concentration (MAC)?
- Minimum alveolar concentration of an anesthetic that prevents gross, purposeful movement in 50% of patients exposed to a noxious stimulus
- describes the dose of anesthetic being delivered
- MA = ED50
- ED95 = 20-40% MAC (humans)
- Potency = 1/MAC
Which gas has the highest MAC?
MAC range of Sevoflurane
MAC range of Desflurane
Why can't we use Nitrous Oxide as a sole anesthetic?
- It has a very high MAC
- has half the potency in non-human species
- usually given concurrently to lower MAC of other agents
What factors increase MAC?
- Drugs causing CNS stimulation
- Increased levels of excitatory NTs
What factors decrease MAC?
- Other anesthetics
- Hypotension (MAP<50mmHg)
- Hypothermia -PaO2 below 40mmHg
- PaCO2 above 90mmHg
- Pregnancy -Inc age
Factors that don't affect MAC