Week 1- Inhalational agents Flashcards
What is an inhalational anaesthetic agent?
- Volatile anaesthetic drug administered by inhalation
- Vapour or gas
What is the indication for using an inhalational anaesthetic agent?
Induction and maintenance of anaesthesia
What is saturated vapour pressure?
Pressure exerted by the vapour on its surroundings (liquid) in a closed container at equilibrium at
certain temperature
Max concentration of molecules in the vapour state that exist for a given liquid for a given
temperature, at equilibrium
Measure the ability to evaporate
* ↑ SVP → ↑ [inhalant] delivered to paƟent * Isoflurane > Sevoflurane
What is solubility measured as?
partition coefficient
capacity of a solvent to dissolve anaesthetic gas
How does the blood/ gas partition coefficienct help us?
Low blood solubility → > rapid equilibraƟon: > rapid inducƟon, change of
anaesthetic depth & elimination
What is the minimal alveolar concentration?
Minimum alveolar concentration of anaesthetic agent at which 50 % of patients fails to respond (by purposeful movement) to a standard supramaximal noxious stimulus (i.e., skin incision)
What factors may increase the MAC?
- Body size
- Hypernatraemia
- Hyperthermia
What factors may decrease the MAC?
- Pregnancy
- CNS depressant drugs
- Hypothermia
- Severe hypotension
What is the function of the vapouriser?
- Converts liquid anaesthetic into its vapour form
- controls the concentration of anaesthetic delivered into the patient
How do pharmacokinetics work?
Inhalational agents move
down a pressure gradient
(from high to low) until
equilibrium
* Depth of anaesthesia
depends on Partial
Pressure of anaesthetic
drugs in the brain (Pbrain)
* Alveolar partial pressure
of anaesthetic agents
important to control
What effects the elimination and recovery of pharmacokinetics?
Depends on rate of decrease of Pbrain : return of consciousness
* Exhalation
* Metabolism (liver primarily, Cyt P450 enzymes):
Minimal for modern inhalational agents (Isoflurane 0.2%, Sevoflurane 2-5%, Nitrous oxide 0.004%)* Prolonged general anaesthesia → inhalant accumulaƟon in fat → slow recovery
* Inhalant may be lost from breathing circuit (leaks) & patient (open cavities) * Adsorption or degradation by CO2 absorber
What is the second gas effect?
Ability of one gas (1st gas, soluble in plasma, i.e. nitrous oxide ) to accelerate the rise of
alveolar concentration of a 2nd gas (volatile anaesthetic, O2) when administered together
* “first gas” that is soluble in plasma, moves rapidly from the lungs to plasma. *
→↑ alveolar concentration and hence rate of uptake into plasma of the “second gas”
* To speed anaesthetic induction
What is the third gas effect?
Occurs during recovery
* Nitrous oxide is discontinued
* diffuses back from blood into alveoli
* Dilution of inspired O2 causes hypoxia
* Dilution of inspired Co2 causes a decrease in respiratory drive
How might you solve diffusion hypoxia?
Administer 100% oxygen on recovery
What are some potential issues with the vaapouriser?
- vapouriser filling
- leaks from around the patients airway
- patient exhalation
