Lecture 8 9/10/24 Flashcards
Which halogenated ethers are most commonly used in vet med?
-isoflurane
-sevoflurane
What are the characteristics of an ideal anesthetic?
-chemically stable
-non-flammable
-environmentally friendly
-inexpensive
-easy delivery
-potent
-low solubility
-minimal metabolism
-minimal toxicity
-minimal side effects
-analgesia
Which state are modern inhalants in?
vapors
What are the physiochemical properties of solubility?
-dissolved concentration in liquid
-net movement of gas into liquid until equilibrium
-temperature dependent
What are the physiochemical properties of partial pressure?
-pressure the gas exerts to escape from solution
-each gas exerts a pressure
-mixture of gases = partial pressure the same for individual gases
How do anesthetics travel?
via partial pressure gradient
What is the gas partition coefficient?
-concentration ratio of anesthetic in gas vs liquid or between two tissues
-describes capacity of particular solvents/tissues to dissolve anesthetic gas
Why is the gas partition coefficient important?
it provides means of predicting speed of induction, recovery, and change in depth
How does rate of rise of alveolar partial pressure relate to solubility?
inversely proportional
Why do highly soluble anesthetics take a longer time to rise in alveolar partial pressure?
because the anesthetic is rapidly taken up by the blood and held there
What does P-alveolar represent?
P-CNS
Why do poorly soluble anesthetics have a quick rise in alveolar partial pressure?
the gas passes through the blood and right into the brain
Why is a low blood:gas partition coefficient desirable?
it leads to faster induction and recovery
What are the characteristics of the oil:gas partition coefficient?
-ratio of anesthetic in oil compared to gas at equilibrium
-more lipid soluble = more rapidly taken up by brain
How does oil:gas partition coefficient relate to potency?
direct correlation
What is the primary goal of inhalant anesthetic delivery?
ensure adequate partial pressure of anesthetic in the CNS to produce CNS depression and general anesthesia
What is important about the pathway in the body that anesthetic inhalants follow?
they do not go to the liver or kidneys; there is no hepatic or renal metabolism
What are the characteristics of inhalant uptake?
-PA rapidly equilibrates with arterial partial pressure (Pa)
-Pa rapidly equilibrates with Pbrain
-PA useful indicator of brain partial pressure
Which factors affect inhalant uptake?
-delivery to the alveoli
-uptake by the blood
What aspects of inspired inhalant concentration impact delivery to the alveoli?
-vaporizer setting
-fresh gas flow
-volume of the anesthesia circuit
-type of anesthetic circuit
-rubber and plastic in the circuit
How can the inspired concentration be increased?
-increase vaporizer setting
-increase fresh gas flow
-decrease volume of anesthesia circuit
-changing plastic vs. rubber content
What is the overpressure technique?
using high O2 setting and high vaporizer setting to start anesthesia in order to achieve a quick rise in alveolar concentration
What are the characteristics of alveolar ventilation?
-improving alveolar ventilation leads to more rapid induction
-want to decrease dead space ventilation
Which factors impact blood uptake of inhalant anesthetics?
-solubility
-cardiac output
-alveolar to venous anesthetic partial pressure gradient
What are the characteristics of solubility?
-more soluble = slower rise in anesthetic conc.
-inversely related to potency
How does low cardiac output affect uptake?
-more rapid rise in alveolar conc.
-less uptake from alveoli to oppose input
-blood becomes concentrated with drug quickly
How does high cardiac output affect uptake?
-slower rise in alveolar conc.
-increased capacity of blood to hold anesthetic
What is the driving force of inhalant uptake and elimination?
-partial pressure gradient
-larger gradient = more uptake and longer time to achieve desired PA
What are the characteristics of potency?
-amount of drug needed to achieve general anesthesia
-inversely related to MAC
What is MAC?
alveolar concentration that prevents gross, purposeful movement in 50% of patients in response to noxious stim.
What are the MAC values for isoflurane and sevoflurane?
iso: 1.3
sevo: 2.3
How is MAC determined?
-using Palveolar expired conc.
-without other drugs
-in healthy animals
Which animals have a variation from the standard MAC values?
cats - MAC values for iso and sevo are about 0.3 higher
Why is MAC important?
-anesthetic requirement
-compare agents
-evaluate pharmacologic effects
-want to set vaporizer around MAC value
What are the theories behind how inhalant anesthetics work?
-modulation of ion channels**
-decreased neuronal excitability
-inhibition of action potential
-enhanced inhibitory synaptic transmission
What are the sites of action for inhalant anesthetics?
-brain
-spinal cord
What are the advantages of using inhalants?
-tailored to individual
-rapid changes in depth
-minimal metabolism
What are the primary goals of using inhalants?
achieve muscle relaxation and hypnosis
What are the disadvantages of using inhalants?
-necessary equipment
-airway management
-waste gases
-side effects
What are the effects of inhalants?
-dose dependent hypotension
-decreased ventilatory response to CO2
-dose dependent hypoventilation
-unconsciousness
-decreased cerebral metabolic rate of O2
-increased cerebral blood flow
-increased intracranial pressure
-decreases in renal blood flow and glomerular filtration rate
Why is it important to pre-medicate when using inhalants?
side effects are dose dependent, so we can reduce the dose of inhalants when pre-medicating
What are the characteristics of malignant hyperthermia?
-genetic mutation of ryanodine receptor
-use of inhalants can result in hyperthermia, muscle rigidity, increased EtCO2, hypocalcemia, and hyperkalemia
-reversed with dantrolene
-very poor prognosis; nearly always fatal
When is trace gas exposure most likely to occur?
-vaporizer filling
-leaks
-recovery
