Inhaled anesthetics 2: Pharmacodynamics Flashcards
What is the blood:gas partition coefficient of nitrogen?
0.014
Nitrous oxide is ______ times more soluble than nitrogen.
~34 times
For every ___ molecule of nitrogen that leaves a closed space, _____ molecules of nitrous oxide enter to take its place.
1; 34
Describe how nitrous oxide works in a compliant airspace vs. a fixed airspace.
compliant airspace- nitrous oxide increases the volume of the space (blebs, bowel, air bubbles in the blood)
fixed airspace- nitrous oxide increases the pressure of the space (middle ear, eye during retinal detachment surgery, brain during intracranial surgery)
Nitrous oxide can increase the volume and pressure in the following anesthesia equipment:
LMA cuff
endotracheal tube cuff (pressure should be checked with a manometer)
balloon-tipped pulmonary artery catheter
Nitrous oxide is not ___________, but it does support ________.
flammable; combustion
Nitrous oxide irreversibly inhibits _________, which inhibits ____________.
vitamin B12; methionine synthase (required for folate metabolism and myelin production)
Nitrous oxide blood:gas partition coefficient is
0.46
The rate of pressure change inside the space is dependent on these four thing.
compliance of the space
the partial pressure of nitrous oxide
the perfusion of the surrounding tissue
time
Describe what happens inside a compliant airspace.
volume increases
pressure unchanged
Describe what happens inside a noncompliant airspace.
pressure increases
volume unchanged
Nitrous oxide quickly increases _____ ear pressure, which can damage
middle ear pressure; tympanic membrane grafts
Discontinuation of nitrous oxide can quickly decrease _____ ear pressure, which can lead to
middle; serous otitis
Why is nitrous oxide avoided during retinal detachment surgery?
a gas bubble (sulfur hexafluoride) is placed over the site of the retinal break; this functions as a “splint” to hold the retina in place while healing occurs
because nitrous oxide can diffuse into the bubble faster than the other gases in the bubble can diffuse out, nitrous oxide can expand the bubble, compromise retinal perfusion, and cause permanent blindness
When to avoid nitrous oxide with a sulfur hexafluoride bubble:
discontinue Nitrous oxide at least 15 minutes before placement
after avoid for 7-10 days
How long should nitrous oxide be avoided after an air bubble is placed?
5 days
How long should nitrous oxide be avoided after perfluoropropane is placed?
30 days
How long should nitrous oxide be avoided after silicone oil is placed?
no contraindication to N2O
The risk of complications from inhibition of vitamin B12 with nitrous oxide is increased by
prolonged exposure and in patients with pre-existing B12 deficiency
Patients who have pre-existing B12 deficiency include
pernicious anemia
alcoholism
strict vegan diet
recreational use of N2O
Side effects of methionine synthase include
decreased DNA synthesis
immunocompromise
Neuropathy
megaloblastic anemia (bone marrow suppresion)
concerns of teratogenicity (lack of clinical data)
possible risk of spontaneous abortion (many clinicians avoid N2O in the first two trimesters)
Order the volatile anesthetics from lowest to highest potency.
Nitrous oxide, desflurane, sevoflurane, isoflurane
What is MAC?
Minimum alveolar concentration is the concentration of inhalational anesthetic that prevents movement following a painful stimulus in 50% of the population
What is the MAC of iso?
1.2%
What is the MAC of sevoflurance?
2%
What is the MAC of desflurane?
6.6%
What is the MAC of nitrous oxide?
104%
General anesthetics produce the following effects:
amnesia
loss of consciousness
immobility
modulation of autonomic function
some analgesia
What is MAC bar?
1.5 MAC
Awareness and recall is prevented at a MAC of
~.4-0.5 MAC
Movement is prevented in 95% of the population at a MAC of
1.3
MAC-awake during emergence is
0.15 MAC
MAC-awake during induction is
0.4-0.5 MAC
The effects of inhalation anesthetics are dose-dependent. Supraspinal effects such as ________ and _____________ occur at lower concentrations, while _____requires a higher concentration.
amnesia and loss of consciousness; immobility
What is MAC-bar mean?
the alveolar concentration required to block the autonomic response following a supramaximal painful stimulus
What is a MAC hour?
one times the alveolar concentration that prevents movement in response to a noxious stimulus in 50% of subjects administered for 1 hour
Volatile anesthetic potency is increased by (select 4):
a. hyponatremia
b. red hair
c. lithium
d. clonidine
e. chronic alcohol consumption
f. old age
g. hypokalemia
h. hypothyroidism
a. hyponatremia
c. lithium
d. clonidine
f. old age
Factors that increase MAC include:
chronic alcohol consumption
increased CNS neurotransmitter activity
hypernatremia
infants 1-6 months
hyperthermia
Factors that decrease MAC include:
acute alcohol consumption
sedative drugs
hyponatremia
old age
extremes of age (prematurity)
pregnancy
hypothermia
Select factors that have no effect on MAC:
hyper or hypothyroidism
hyper or hypokalemia
gender
PaCO2 (15-95 mmHg)
Hypertension
Additional factors that decreases MAC include
hypotension
hypoxia
anemia
cardiopulmonary bypass
metabolic acidosis
hypo-osmolarity
postpartum period (24-72 hours)
PaCO2 >95 mmHg
Examples of increased CNS neurotransmitters that increases MAC include
acute amphetamine intoxication
acute cocaine intoxication
MAOIs
ephedrine
levodopa
The Meyer-Overton rule states:
a. all anesthetics share a similar mechanism of action, but each may work at a different site
b. lipid solubility is directly proportional to potency
c. inhaled anesthetic agents interact with stereoselective receptors
d. decreased body temperature reduces anesthetic requirements
B. lipid solubility is directly proportional to potency
The __________- states that all anesthetics share a similar mechanism of action, but each may work at a different site.
unitary hypothesis
