Inhalational Agents Flashcards
(44 cards)
Uptake of a volatile anesthetic into the blood = ____ x _____ x _____
Solubility x CO x A-v
Solubility: how anesthetic partitions between blood/reservoir and gas/alveoli (use partition coefficients aka “Oswaldt Solubility Coefficients”)
A-v is difference in partial pressures in alveolar and venous blood
At normal body temp (37C), list the blood/gas solubility coefficients (Des, N2O, Sevo, Iso, En, Halo)
Des 0.42 (poorly soluble) N2O 0.47 Sevo 0.69 Iso 1.4 En 1.8 Halo 2.4
The lower the solubility (Des), the ____ (faster/slower) induction and emergence
FASTER
Solubility determines how fast the concentration in the alveoli (FA) reaches the concentration inspired (FI), the faster these values, the faster uptake to the brain and induction (PA = Pa = Pbr), also the fastest rate of rise on FA/FI curves
It is faster bc very little anesthetic must dissolve in the blood before partial pressures equilibrate
An anemic patient will have a ____ (lower/higher) coefficient
Lower
Less soluble due to fewer binding sites for anesthetic in the blood, faster uptake of inhalational agent
What does FA and FI represent? What are they determined by?
FA: partial pressure of anesthetic going to the brain. Determined by inhaled partial pressure, alveolar ventilation, breathing circuit (uptake into plastics/rubber), FRC.
FI: inspired %, can be controlled by flow rate and vaporizer setting
Change in FA (alveolar concentration) is ____ (faster/slower) with increase RR and decreased TV
FASTER
Except N2O bc it already has such a low solubility, inc in ventilation will not shift the curve significantly, especially compared to halothane
What is uptake influence on FA/FI?
Uptake opposes the effect of ventilation (which increases FA), there is uptake at tubing, tissues, it all takes away from alveolar concentration
If uptake removes 2/3 of the anesthetic, FA would be 1/3 of FI
(Review slide 23)
What is overpressure?
Use of high delivered concentrations
For example, halothane B/G of 2.4 would result in a slow induction, but using overpressure, increased concentration will speed up the induction
By increasing the PI above that required for maintenance of anesthesia, the high Pa (and slow induction) can be offset to some extent
Increased CO (seen in shock for example), will ____ (inc/dec) speed of induction
DECREASE
Increase CO will increase solubility, the higher solubility, the slower the induction
For lean tissue (brain), coefficients are between __ and __
For fat, coefficients (except for N2O) are between ___ and ___
Lean tissue coeffiecients between 1-2
Fat coefficients are high, from 27-67, except N2O is 2.3
Vessel rich group is __% of body mass and ___ % of CO
Vessel poor group is __% of body mass and ___% of CO
VRG 9% of body mass and 75% of CO
VPG 22% of body mass and 0% of CO
Muscle group is __% of body mass and ___ % of CO
Fat group is __% of body mass and ___ % of CO
MG 50% body mass and 18% CO
FG 19% body mass and 7% CO
In the FA/FI curve, what does the initial steep rise represent? First knee? Second rise? Second knee?
Initial rise: quick rise if low solubility
First knee: uptake by VRG
Second rise: uptake by MR group, uptake by VRG slows
second knee: uptake by MRG slows
____ influences knee height
____ influences tail
B-G solubility influences knee height
Tissue-Gas solubility influences tail
What is MAC
Minimum alveolar concentration, ED50 (effective dose in 50%)
The concentration that will produce absence of movement in 50% of patients to noxious stimuli
To keep FA constant, how should FI be adjusted?
High at first, then decrease as VRG is equilibrated, decrease further when MRG is equilibrated.
The greater than alveolar ventilation/FRC ratio, the ___ (faster/slower) the induction
FASTER
Neonates have a very fast induction for example
Inc ventilation and inc CO has what effect on FA/FI?
In theory no change, but slightly increases FA/FI
2x the ventilation and 2x the CO for example would double both input and removal
What is the concentration effect?
Results when a large volume of gas in absorbed. The remaining residual gas is concentrated as the volume decreases and the inspired ventilation increases which adds more anesthetic (neg pressure created by uptake draws more gas into the lung), just think: alveoli shrink and agents become concentrated
Book definition: The higher the PI, the more rapidly the PA approaches the PI, speeds the rate PA increases
What is the second gas effect?
When N2O is used with another gas, a reduction in volume and replacement of N2O will increase concentration and amount of any gas given with N2O, which was absorbed in large volume
The FA of a gas increases more rapidly when N2O is given as a second gas than when given alone
As the duration of anesthesia lengthens, washout will take ___ (longer/shorter)
Longer, more anesthetic is in the reservoirs
Least soluble washout first
The higher the B/G solubility coefficient, the longer emergence will take
In general, emergence depends on length and depth of anesthesia, solubility of the agent, and MAC awake
Which is more soluble, N2O or nitrogen? What is the implication (which surgeries are N2O avoided in)?
N2O is 34x more soluble.
Nitrogen=0.014, N2O=0.47
The bottom line with this difference is that when using N2O, increased pressure can build up in a closed space (bowel, pneumo, tympanic graft), and in these instances, N2O should be avoided
What is diffusion hypoxia? How is it avoided?
Diffusion Hypoxia: when rapid transfer from blood and tissue to alveoli decreases arterial tension of oxygen
Avoided by washing out with oxygen instead of air for an adequate amount of time
What is the Meyer-Overton Theory? (Critical volume theory)
Correlation between lipid solubility of inhaled anesthetics (oil:gas partition coefficient) and anesthetic potency shows that inhaled anesthetics act by disrupting the structure or dynamic properties of the lipid portions of nerve membranes.
Absorption of anesthetic molecules expands hydrophobic region of lipid bilayer which alters membrane function
