Module B-1 (gases) Flashcards
Cerebral vasculature response to CO2
Hypocapnia-vasoconstrict
Hypercarbia-vasodilate
Sevoflurane
-MAC
-B/G coefficient
-oil/gas coefficient
2%
0.6
50
Isoflurane
-MAC
-B/G coefficient
-oil/gas coefficient
1.15%
1.4
99
Nitrous oxide
-MAC
-B/G coefficient
-oil/gas coefficient
105%
0.47
1.4
Desflurane
-MAC
-B/G coefficient
-oil/gas coefficient
5.8%
0.42
18.7
Factors that increase MAC
Hyperthermia
Hypernatremia
Increase in CNS activity (drug induced)
Chronic alcohol abuse
Factors that decrease MAC
Hypothermia
Increased age
Preoperative sedatives
Alpha-2 agonists
Pregnancy
Acute alcohol
Hyponatremia
Hypotension
MAC
Minimum Alveolar Concentration
Analogous to ED50- given in volume% needed to produce lack of movement with surgical stimulation
Blood/Gas solubility coefficient
Higher the solubility the higher the coefficient
-this means more gas is bound in blood and not being utilized at effect site
Isoflurane is 1.4 and nitrous is 0.47, which has faster induction??
Pros/Cons of manipulating minute ventilation
Pro: faster Ve (minute ventilation)= faster induction
Con: faster Ve= drop in PaCO2= cerebral perfusion and slower drug delivery to brain
Overpressuring
Similar to a loading dose
Initially administering a higher concentration of gas than would be needed to maintain anesthesia
Second Gas Effect
A high volume first gas (N20) accelerates the rate of rise of a second gas (Isoflurane)
N2O changes the concentration of gases in the alveoli-facilitating diffusion into blood
Cardiac output and induction speed
High CO slows rise in alveolar concentration because it rapidly removes drug
PA=Pbrain
Drug is rapidly distributed to the tissues and more slowly to the brain
Oil/Gas solubility coefficient
Lipid solubility of an anesthetic gas
-ability to access CNS
-indicator of potency
Hypothermia
Solubility and potency both increase- negate each other
Slowed recovery- decreased perfusion and increased tissue capacity or anesthetics
Diffusion hypoxia
With abrupt discontinuation of nitrous, it floods the alveoli-displacing normal respiratory gases during a rapid exit from the body
1-5 minutes post discontinuation
-admin 100% 02 to negate this effect
Obesity considerations with inhaled anesthetics
Higher ratio of fat to lean mass leads to a larger reservoir for anesthetics to accumulate. Using anesthetic with low tissue:blood coefficient like desflurane can mitigate
-rate of induction should not be impacted
Fractional mass
Ratio for mass of a substance to the total mass of the mixture
O2 is 21% or 0.21 of normal air
Daltons Law
Partial pressures of a gases in a mixture are independent from one another
Ptotal=P1+P2+P3…
Henry’s Law
Concentration of gas= Partial Pressure x solubility coefficient
Think: Mixture of gas and liquid- the pressure is the same in both phases but the concentration changes based on the gas solubility
How do we know the level of anesthetic in the brain??
At equilibrium:
Brain =Arterial =Alveolar partial pressure
Easily measured via end tidal sampling
Factors that influence Anesthetic Transfer
Machine
drug
Ventilation
Tissue
Where can anesthetic be lost in the “breathing system”
Absorption in plastic components
CO2 absorbents theoretically degrade anesthetic
Factors that lead to faster induction/higher concentration in alveolus- machine factors
Higher concentration (set on machine)
Lower circuit absorption
Lower circuit volume
Higher Fresh Gas Flow
Why is nitrous so high on the Fa/Fi curve?
It has a low blood/gas coefficient but it is also given in much higher concentrations than our other anesthetics
Fastest to slowest induction based on Blood/gas coefficient
Desflurane 0.42
Nitrous oxide 0.47
Sevoflurane 0.6
Isoflurane 1.4
Ether is 14! Holy fuck
Prolonged emergence in hypothermic patients
Gas solubility in liquids is inversely proportional to temperature
-think boiling water
-cold humans=more soluble gases
Why not nitrous?
It’s propensity to fill airspaces in the body and equipment via diffusion
-pneumos will worsen
Functional Residual Capacity
Volume of gas present in the lungs at end expiration during normal tidal breathing
Impact of FRC on anesthesia onset
Large FRC dilutes concentration of anesthetic- slower onset time
Left to right shunt
Blood recirculates through the lungs, less impact on IA concentration
Right to left shunt
Returning Venous blood bypasses the lungs-
Soluble anesthetics can partially compensate because they are taken into the blood more easily
Problematic for insoluble gases
Oil:gas & MAC relationship
The higher the coefficient the lower the MAC because it is a more potent drug (requiring less)
% hepatic biotransformation of IA’s in order
No Nitric Oxide 0.004
Dick Desflurane 0.02
Is Isoflurane 0.2
Salacious Sevoflurane 2-5
