Inhalants Flashcards
Define saturated vapor pressure
When the pressure exerted by the gaseous molecules on the walls of the container is equal to the pressure of the liquid phase (when they are at equilibrium)
Saturated vapor pressure depends on what two things?
The temperature and the agent; it is NOT affected by barometric pressure
Define minimum alveolar concentration (MAC)
It is the measure of the anesthetic potency; it is at which 50% of patients don’t respond with a movement to surgical stimulation
Surgical MAC and MAC*BAR are what MAC multiple?
Surgical: 1.2-1.4x MAC MAC (blunt autonomic response): >2x MAC
Rank the inhalants by their MACs in dogs and cats
Lowest: Halothane < Iso < Sevo << Des <<< N2O
What factors or conditions reduce the MAC?
Analgesic drugs, Ace, N2O, age, hypothermia, hypothyroidism, pregnancy
Inhaled anesthetic concentration in the brain depends on what?
Alveolar concentration; thus if alveolar concentration rises fast, the anesthetic effect will be fast too
If uptake of the anesthetic agent by circulation increases, the alveolar concentration ____, and the anesthetic effect ______
Drops; decreases
After saturation of highly perfused tissues, alveolar concentration ___ arterial concentration ____ concentration in CNS
=, =, =
Uptake is determined by what factor?
The solubility of the anesthetic agent (partition coefficient)
The blood/gas partition coefficient tells you what?
The speed of partial pressure change; thus a low B/G coefficient = a fast change
The olive oil/gas partition coefficient tells you what?
The potency of an inhalant anesthetic
How does solubility relate to uptake?
Less soluble = less uptake = faster rise in alveolar concentration
Rank the anesthetic agents by their rate of uptake (B:G)
Fastest: Des > N2O > Sevo > ISO > Hal
At equilibrium, the _______ are the same, but _____ are usually different
Partial pressures; concentrations
Why is it difficult to induce an excited patient with an inhalant?
B/c high cardiac output removes the anesthetic agent so quickly from the alveoli so the partial pressure in the alveoli does not have time to increase and neither does that in the blood
In order to obtain a fast change in anesthetic plane, we need a fast rise in alveolar partial pressure. What 3 things do we need to achieve this?
High alveolar ventilation, low partition coefficient, low cardiac output
Describe the difference between a gas and a vapor
- Gas
- exist in gaseous form at room temp and sea level
- can be given between 0-100%
- Vapor
- gaseous state of a substance that is a liquid at room temp and sea level
- needs a vape
- max concentration depends on saturated vapor pressure
Describe nitrous oxide
- Gas at room temp/sea level
- very high MAC - not hypnotic to animals
- reduces MAC when administered with O2 and halogenates
- acts as an NMDA antagonist when used as an adjuvant analgesic
- Increases ICP, symp stim, myocardial depression
Describe halothane
- Formulated with thymol to increase stability
- causes dose dependent resp depression, arrhythmias, and bradycardia
- can trigger malignant hyperthermia and produce hepatic damage
- potent (low MAC) and high B/G ratio (slow)
Describe isoflurane
- Does not require preservative
- faster than halothane and less potent
- Marked dose dependent drop in BP and SVR, respiratory depression
- Little myocardial depression
- Does not sensitize myocardium to epinephrine-induced tachyarrhythmias (like hal)
- low rate hepatic metabolism
- can trigger malignant hyperthermia
Describe sevoflurane
- Non-irritating for resp tract and non-pungent odor
- faster induction and recovery than Iso (lower partition coefficient)
- causes dose-dependent CV and resp depression
- 5% hepatic metabolism —> free floride
- Reacts with soda lime to form Compound A (nephrotoxic in rats)
Describe desflurane
- due to low solubility —> very fast induction and recovery
- can be blunted by co-administration of over drugs
- causes dose dependent CV and resp depression
- Sudden arousal when moving animals from OR to recovery
- Pungent odor —> airway irritation, breath holding, salivation (not ideal for mask induction)
- Very expensive
- Decomposes to carbon monoxide when using dry soda lime
Which is the cheapest: Iso, Sevo, or Desflurane?
Iso < Sevo <<< Des
Which drug has the lowest partition coefficient?
Des < Sevo << Iso
What are some examples of scenarios when using a less soluble agent might be preferable?
- Horses having colic surgery
- Septic patients
- C-sections
T or F: all volatile inhalation anesthetics cause dose dependent and drug specific changes in cardiovascular performance
True; the mechanisms often include direct myocardial depression and a decrease in sympathoadrenal activity (reduction of SVR) —-> decr MAP
Which inhalant is the most depressing to cardiac output?
halothane
Describe the respiratory effects caused by inhalant anesthetics
- Dose-dependent respiratory depression
- Reduction in total volume (shallow breath) followed by reduction of resp rate
- Depression of hypercapnia and hypoxemia respiratory drive
- Bronchodilatory effects
Describe the CNS effects caused by inhalants
- Dose dependent depression
- Decrease cerebral metabolic rate
- Increase cerebral blood flow and ICP
Define cerebral perfusion pressure
- the driving pressure for brain perfusion
- = difference between mean arterial blood pressure (MAP) and intracranial pressure
Halogenates can do what to cerebral auto regulation?
Blunt it (especially >1 MAC)
