Principles of Inhalational Anesthesia Flashcards
Hypotension during anesthesia is most likely due to which class of anesthetic drug?
Inhalants
History
- 1800s: chloroform, ether, NO
- 1951: halothane
- 1958: methoxyflurane
- 1981: isoflurane
- 1992: desoflurane
- 1994: sevoflurane
What 2 inhalant drugs are no longer available in the US?
Halothane and methoxyflurane
Inhalants
- powerful drugs w/ low margin of safety
- induce/maintain anesthesia
- non-flammable
- vaporizer, machine, breathing system
- aversive to humnas and animals
How inhalant anesthetic is delivered
Delivered from an anesthesia machine to the patient’s lungs
- inhaled anesthetic drugs enter the blood and produces general anesthesia in the CNS
How to we measure inhalational anesthesia?
Measure levels in the lungs
- not feasible to measure inhalant anesthetic levels in the spinal cord/brain
MAC
Minimum alveolar concentration
- MAC = potency = ED50 = dose/response
- low MAC = high potency
MAC definition
Inhalant percent in alveoli that prevents movement in response to noxious stimulation in 50% of animals tested
Median ED50 response to supramaximal stimulation is used to define _____
Various endpoints
- MAC awake and MAC bar
How is MAC used?
To determine where to dial vaporizer setting
- surgical anesthesia in unpremedicated patients is approximately 1.3 times MAC
MAC is determined in the ______ of any other drugs
Absence
- drugs modify the MAC value, usually making it less than what it really is
***MAC of common agents
- isoflurane: 1.3-1.6%
- sevoflurane: 2.4-2.6%
- desflurane: 7.2-10.3%
_____ MAC = higher potency
Lower
Factors decreasing MAC
- hypothermia
- pre-meds!!
- pregnancy
- old age
- hypotension/hypoxemia
- CNS depressants
Factors increasing MAC
- hyperthermia
- CNS stimulants
Unchanged MAC
- gender
- duration of anesthesia
- potassium abnormalities
- thyroid disorders (either hypo or hyper)
What is a vapor
Gaseous phase of volatile liquid
- volatile liquid: liquid at ambient temp
- evaporates readily
Name one familiar volatile liquid
Water, gasoline
Volatile liquids
Evaporate in gas (oxygen) for delivery to patient
Vaporization
Process of changing from liquid to gas
Vapor pressure
Atm pressure when liquid and gas are in equilibrium
Vapor pressures of modern inhalants are ______ to be used safely without vaporizer
Too high
Vapor pressures (Pv/Pb)
- sevo: 22% (170 mmHg)
- iso: 31% (240 mmHg)
- des: 88% (669 mmHg)
If you measured the amount of liquid vaporizing at the liquid/gas interface on an open bottle, it is a _______
Lethal amount
Vapor pressure vs MAC
- vapor pressure tells how much inhalant can be evaporated
- MAC tells how much vapor can be safely given to the patient
Absorption of inhalants
Diffuse down a partial pressure gradient from machine through breathing system to lungs
Absorption
Diffuse across alveolar membranes to blood
- similar to oxygen, but all is dissolved in plasma
Distribution of inhalants
Travel to spinal cord/brain to produce anesthesia
- amount of inhalant needed to produce unconsciousness is <50% of that needed to produce immobility
Factors affecting absorption and distribution
- machine: vaporizer setting, O2 flow rate (time constant)
- breathing system: volume, ventilation status
- patient: gas exchange, CO, volume of distribution
- anesthetic agent: inspired conc, solubility!
Solubility
How well inhalant dissolves in blood
- measured as blood:gas coefficient
Distribution - solubility
Low B:G = insoluble in blood!
- insoluble = rapid effect!
- rapid unconsciousness and rapid recovery
You want an anesthetic agent that has _______
Low BG:solubilty
B:G Solubility
- desflurane: 0.42 (<1 min)
- sevoflurane: 0.6 (1-2 min)
- isoflurane: 1.4 (1-3 min)
The more soluble the inhalant is in blood, the ______ it takes to produce anesthesia and to recover from anesthesia
Longer!
Metabolism
- early inhalants: too much
- modern inhalants: minimal
- iso: 0.1%
- sevo: 3-5%
More soluble =
More metabolism in the liver
Elimination
Dissolved in blood, inhalant travels back to lungs
- exhaled: breathing system (scavenge system or re-breathed)
Anesthesia
GABA agonists, voltage-gated ion channels + restricted syntaxin 1A moblity
- bind to proteins, receptors
- unconsciousness, relaxation, blunted autonomic responses
Inhalant safety margins
Therapeutic index –> low!
- dose-dependent depression of CV and respiratory function
Cardiovascular effects
Vasodilation = decreased blood pressure
- heart rate increases, offsets decrease in blood pressure unless higher concentrations are used
- dose-dependent decrease in contractility
Respiratory effects
Dose-dependent depression
- hypoventilation > increased CO levels
- apnea at high doses
- animals can still have a high respiratory rate and be hypoventilated (function of CO2 being retained)
CNS effects
- increase cerebral blood flow
- increase intracranial pressure
- iso may be better than other agents
Renal effects
- dose-dependent
- decrease renal blood flow
- decrease GFR
- decrease urine output
- sevoflurane
Hepatic effects
Decreased hepatic blood flow at high concentrations
Nitrous oxide
Only anesthetic associated with adverse health effects!
- not potent enough to produce anesthesia in animals (MAC > 200%)
- large volumes used
- can displace oxygen (hypoxia)
- distends gaseous spaces inside the animal
Wash-in exponential function
y = yoo (1-e^-kt)
- yoo: limiting value of y
- e: base of natural logarithms
- k: constant defining rate of build-up, reciprocal of the time constant
- used for tissue blood flow, exchange of gases in lungs and other enclosed spaces
Time constant
t = 1/k
- calculated as volume divided by inflow rate
- increase inflow rate to make the time constant go faster (reduces time constant)
- if you want something to happen faster with the anesthetic machine, then increase the vapor setting flow rate
