General Anesthetics: Pharmacology and Clinical Use Part 1 Flashcards
why do we use inhalants? (5)
- use of lungs for delivery and elimination
- do not depend on hepatic or renal function
- rapid and precise adjustment of anesthetic depth
-can measure how much giving at any point during anesthetic event - rapid and complete recovery
-recovery may take longer because have to breathe off all inhalant from body
-but not as smooth recovery as injectables - control/maintain anesthesia induced by injectables
can you use inhalants alone?
yes but not recommended! not super safe to animal or those around and will have an excitatory phase; requires bulky and expensive equipment
how are all inhalants administered?
as vapor! some is liquid and some is gas
a gaseous state with a critical temperature greater than room temp
critical temp is the temp above which only gas exists
what is vapor pressure?
pressure exerted by a vapor when it exists in equilibrium with its liquid state
measured in mmHg in US
-dalton’s law of partial pressure:
Ptotal= P1+P2+P3…
describe vapor pressure of inhalants
relationship between vapor pressure and maximal pressure
vapor pressure/barometric pressure X 100 = vol %
if change altitude, will not affect vapor pressure, but volume % will change! but doesn’t really matter as long as the partial pressure of the brain is the same
describe solubility
solids or gases dissolved in liquid; solute + solvent = solution
describe solubility of inhalants
- inherent property of the vapor/gas
-temperature dependent
-defined as total amount of vapor dissolved within a solute at equilibrium (no net movement) - vapors/gases move from areas of higher partial pressure to lower partial pressure (gradient)
describe partition coefficients
- ratio of the vol% of vapor in one phase compared to the vol% in another phase
- blood/gas partition coefficient: relative affinity of an anesthetic vapor for blood compared to alveolar gas
- partial pressure is the same in both phases!!
describe MAC/potency
the alveolar concentration of an inhalant anesthetic required to prevent movement in 50% of a given patient population when exposed to a noxious stimulus
how is MAC measured?
use only the inhalant with the species with the face mask; test
if move, go up, it no move no more, go down, find sweet spot
do again with another animal
what does MAC tell us? how is it used?
the lower the MAC, the more potent the anesthetic
used to compare anesthetics:
-rule of thumb (MAC x 1.5) because don’t want to be sitting right at MAC or patient may move in surgery
-ED50 vs ED95
-MAC is additive among multiple inhalants (if using 2 inhalants, can use less of each; not done though)
what is the average MAC for isofluorane and sevofluorane across species?
iso: 1.3
sevo: 2.3
what factors increase MAC (make you need more inhalant?)
- hyperthermia
- hypernatremia
- increased levels of excitatory neurotransmitters
-cocaine, amphetamines
what factors decrease MAC? (make you need less inhalant)
- increased age
-exception: neonates need less inhalant in first week of life then increase as grow up then decrease when old - hypothermia
- hyponatremia
- severe hypotension
- severe hypercarbia
- severe hypoxemia
- metabolic acidosis
- other anesthetics
- pregnancy
what factors do NOT affect MAC?
- duration of anesthesia
- gender
- alkalosis
- hypertension
- anemia
- changes in most electrolytes
what characteristic of anesthetic in brain produces anesthesia? describe
partial pressure! NOT concentration
brain partial pressure equilibrates quickly with alveolar partial pressure and brain partial pressure always moves TOWARDS alveolar partial pressure
describe the alveolar bucket concept
- the alveoli are like a bucket that are being filled, but have a leak (uptake)
- you can increase delivery by:
-increasing inhaled partial pressure
-increasing alveolar ventilation: delivers anesthetic from the breathing circuit to alveoli
–if animal has apnea following propofol, breathe for them!!- hook them up!! - you can decrease the leak/uptake factors by
-decreasing solubility
-decreasing cardiac output
-decreasing the alveolar venous partial pressure difference
how do you increase the inhaled anesthetic partial pressure? (3) (to increase delivery)
- turn up the vaporizer
- increase the fresh gas flow
- minimize the volume of the breathing circuit
describe decreasing solubility to decrease the leak to achieve faster equilibration between alveoli and brain
the higher the solubility, the slower the partial pressure rise is going to be, because now has to go through a sponge (dissolve)
describe decreasing cardiac output to achieve faster equilibration between alveoli and brain
using a facemask technique, you will induce general anesthesia faster in a dog with a LOW cardiac output
the bigger the pool, the more dilute the inhalant, the smaller the pool, the more concentrated the inhalant
-think food dye in a cup of water versus food dye in a bathtub
describe decreasing the alveolar-venous partial pressure difference
if partial pressure difference is smaller, will get to equilibrium (FA/FI=1) very quickly
lots of movement of anesthetic from alveoli at beginning of anesthetic event but as time goes on, gas will stop moving away from alveoli as reach 1 and settle into equilibrium
describe elimination of inhalant anesthetics
now want to increase partial pressure difference between brain and alveoli to speed up recovery!
- reverse of uptake
-can’t use over pressure: vapor and oxygen up which increases PP in alveoli very high but can’t deliver less than zero percent??
-can use high fresh gas flows to flush machine so that inhalant wants to flow out
- effect of duration: use context sensitive decrement (flushing out the machine using fresh O2)
-the longer the anesthetic event, the longer it takes to recover from anesthesia (breathe off inhalant)
-after long enough you reached equilibrium, which will take longer to get rid of
describe isoflurane general info
- most commonly used inhalant
- color code is purple
- no preservative needed
describe isoflurane systemic effects
respiratory: dose dependent depression
cardiovascular:
-dose related hypotension
-primarily vasodilation
GI: hepatic metabolism approx 0.2% (not clinically significant)
renal: none
muscular: excellent relaxation
uterus/fetus: crosses placenta, rapidly inhaled
NO ANALEGSIA
describe isoflurane clinical use
MAC: 1.3% (more potent than sevo)
vaporizer settings:
induction: 3-5%
early anesthesia: 2-3%
maintenance: 1-2.5%
other considerations: depth changes slowest among modern inhalants
describe sevoflurane general info
- very popular
- color code: yellow
- no preservative needed
describe systemic effects of sevoflurane
respiratory: dose dependent depressiom
cardiovascular:
-dose related hypotension
-vasodilation
GI: hepatic metabolism approx 5% (not clinically significant)
renal: compound A
muscular: excellent relaxation
uterus/fetus: crosses placenta, rapidly exhaled
NO ANALGESIA
describe sevoflurane clinical use
MAC 2.3% (less potent than iso)
vaporizer settings:
-induction: 5-7%
-early anesthesia: 2-5%
-maintenance: 1.5-4%
other considerations: depth changes quickly
describe desflurane
- least soluble inhalant available
- color code: blue
- requires a special vaporizer: heated, pressurized, electronic control
- may irritate airway
describe systemic effects of desflurane
respiratory: dose dependent depression
cardiovascular:
-dose related hypotension
-vasodilation + myocardial depression
GI: not metabolized in anyway!
renal: none
muscular: excellent relaxation
uterus/fetus: crosses placenta, rapidly exhaled
NO ANALGESIA
describe desflurane clinical use
MAC: 8% (low low potency)
vaporizer settings:
-induction: 15-18%
-early anesthesia: 12-15%
-maintenance: 8-12%
other considerations: depth changes VERY quickly
