Inhaled Anesthetics Flashcards
What is a MAC?
minimum alveolar concentration = the concentration at which 50% of patients will not move in response to surgical stimuli
MAC is (proportionally/inversely) related to potency.
inversely
increase MAC, decrease potency
MAC is (proportionally/inversely) related to lipid solubility.
inversely
increase MAC, decrease lipid solubility
Give the MAC values for the following:
halothane
isoflurane
enflurane
sevoflurane
desflurane
N2O
halothane=0.7
isoflurane=1.2
enflurane=1.7
sevoflurane=2.0
desflurane=6
N2O=104
What is the oil:gas coefficient?
reflects the tissue affinity for an inhaled anesthetic
measure of anesthetic potency
How are MAC and oil:gas coefficient related?
inversely
increase O:G coefficient, decrease MAC
How are oil:gas coefficient and potency related?
proportional
(O:G coefficient is a measure of potency)
What factors increase the MAC?
(increase anesthetic needs)
increased CNS catecholamines
hyperthermia
chronic ETOH use
hypernatremia
hypokalemia
young age (esp if <1yr old)
What factors decrease MAC?
(decrease anesthetic needs)
CNS depressants
hypothermia
advanced age
anemia
hypotension
hypoxia
hypercarbia
hyponatremia
elevated Mg+, Ca++, or K+
pregnancy
acute ETOH intoxication
Define MAC-awake
50% of subjects respond to a command (usually about 1/3 of MAC)
=0.4 MAC
Define MAC-BAR
BAR=block adrenergic response; no change in HR in response to surgical stimulation
=1.1-1.5 MAC
How does MAC change with age?
6% decrease in MAC per decade of age
What are the four determinants of inhaled anesthetic uptake?
inspired concentration
partial pressure (alveolar gradient)
solubility
blood flow
How are solubility and alveolar partial pressure related?
lower solubility promotes a more rapid increase in alveolar partial pressure toward the inspired concentration (faster induction)
How are alveolar partial pressure and gas concentration related?
concentration of gas is directly proportional to alveolar partial pressure
What is Henry’s Law?
the solubility of gas increases as the partial pressure of the gas above a solution increases
Describe the blood-gas coefficient:
aka Ostwald coefficient
the ratio of the concentration in blood to the concentration in gas that is in contact with that blood, when the partial pressure in both compartments is equal
reflects the solubility of a gas in the blood (affects speed of induction)
low B:G coefficient = low solubility = fast induction/recovery
high B:G coefficient = high solubility = slow induction/recovery
How does cardiac output affect induction time?
high CO = blood rushing by quickly, pulls gas out of alveoli and reduces PP = slower induction time
low CO = blood rushing by slowly, does not pull as much out of alveoli so PP can build = faster induction time
Compare the molecular weights of the common inhaled anesthetics
`Sevo - 200.05g
Iso - 184.5g
Des - 168g
Nitrous - 44g
Halothane - 197.4g
Enflurane - 184.5g
Compare the MAC of the common inhaled anesthetics. What does this mean?
Sevo - 2%
Iso - 1.2%
Des - 6.6%
Nitrous - 104%
Halothane - 0.75%
Enflurane - 1.68%
Inverse relationship between MAC and potency;
Lower MAC = higher potency
Iso is most potent, Nitrous is least potent
Compare the O:G coefficients of common inhaled anesthetics. What does this mean?
Sevo - 47-55
Iso - 91
Des - 19
Nitrous - 1.4
Halothane - 224
The oil:gas coefficient is a measure of lipid/tissue solubility and therefore of potency. A substance that is highly lipid soluble will readily cross the lipid bilayer. The higher the O:G coefficient, the more potent the drug. There is an inverse relationship between O:G and MAC.
Iso is most potent, Nitrous is least potent - Halothane is not frequently used anymore but was most potent back in the day
Compare the B:G coefficients of common inhaled anesthetics. What does this mean?
Sevo - 0.63
Iso - 1.46
Des - 0.42
Nitrous - 0.46
Halothane - 2.4
Enflurane - 1.8
blood:gas coefficient is a measure of blood solubility and therefore speed of induction. Substances that readily dissolve in blood and want to stay there have a higher B:G coefficient and therefore a slower speed of induction. Nitrous has the fastest induction time (d/t concentrating effect), followed by Des, Sevo, then Iso
Compare the vapor pressures of common inhaled anesthetics. What does this mean?
Sevo - 157
Iso - 238
Des - 667
Nitrous - 37,770
Halothane - 243
Enflurane - 172
Vapor pressure is a measure of volatility/substance’s desire to be in the gaseous state
Nitrous is highly volatile, followed by Des –> Iso/Halothane –> Sevo/Enflurane
Compare the boiling points of common inhaled anesthetics. What does this mean?
Sevo - 58.6
Iso - 48.5
Des - 23.5
Nitrous - -88
Halothane - 50.2
Enflurane - 56.5
Volatile liquids have low boiling points
Nitrous has the lowest boiling point at -88, making it the most volatile
Des is next most volatile, then Iso, Halothane, and Sevo/Enflurane
Describe the chemical structure of sevoflurane
sevo = 7 Fs
Describe the chemical structure of isoflurane
5 Fs and a Cl; Cl sits on a carbon directly adjacent to oxygen
Describe the chemical structure of desflurane
6 Fs
Describe the chemical structure of nitrous oxide
O double bond N triple bond N
Describe the chemical structure of halothane
No oxygen, 3 Fs, a Cl, and a Br
Which is the most frequently used inhalation agent?
Nitrous oxide
Which inhalation agent does not trigger malignant hyperthermia?
Nitrous oxide
Which inhalation agent supports combustion?
Nitrous oxide
What is the mechanism of action of nitrous?
NMDA antagonism (moderate) - euphoric, analgesic effect
minimal GABA-A potentiation - poor sedation/anxiolytic
Define the additive nature of N2O
0.5 MAC of N2O + 0.5 MAC of other volatile has the same effect at the brain as either drug alone at 1 MAC
What are some common concentrations of N2O used in practice?
Analgesia - 20%
Sedation - 30-70%
Induction - 60-70%
Maintenance - 50-70%
What is the concentrating effect?
When N2O is introduced into the lung, the volume of N2O going from alveolus –> blood is much higher than the amount of N2 leaving from blood –> alveolus. Result: alveoli shrink causing a relative increase in concentration of N2O in the alveoli. Diffusion gradient remains high and results in more rapid uptake of N2O by the blood.
What is the second gas effect?
When N2O and another inhalation agent are simultaneously delivered, the alveoli shrink and the concentration of BOTH drugs remains elevated in the alveoli resulting in increased rate of diffusion of both drugs. AKA the uptake of a second gas is increased when administered simultaneously with N2O.
What is diffusion hypoxia? How is it mitigated?
When N2O is turned off, N2O diffuses from blood to alveoli so rapidly that PAO2 and PACO2 are diluted
Administration of 100% O2 for 3-5 mins after N2O is discontinued will mitigate the dilution of alveolar O2.
What is the clinical relevance of N2O and air-containing spaces?
N2O will diffuse into air-containing spaces more rapidly than N2 is absorbed by the bloodstream. Results in increased volume of compliant spaces (pulmonary blebs, sulfa hexafluoride bubble in eye, bowels, pneumoperitoneum, airway cuffs) and increased pressure of noncompliant spaces (middle ear, brain).
List some contraindications for N2O use
hx of PONV (risk significantly decreases with at least 1 antiemetic)
pulm HTN d/t increased PVR
gas bubble use for retinal detachment
air is somewhere it shouldn’t be
Vit B12 deficiency
Isoflurane is an isomer of which older anesthetic?
Enflurane
Isoflurane is more potent (MAC of 1.2 versus Enflurane has a MAC of 1.6)
Which inhaled anesthetic is most affected by a high cardiac output? Why?
Isoflurane - highest B:G solubility so slowest induction. High CO means the blood is moving fast, removing more anesthetic from the lungs and reducing the alveolar partial pressure
Over pressuring has the most obvious effect on which anesthetic?
Isoflurane - most potent, could be more dangerous
Which anesthetic has the most respiratory depressant effects?
Iso
decrease Vt
increase rate
blunts response to hypoxia, hypercapnia
Compare effects of anesthetics on BP
Sevo - mild reduction
Iso, Des, Hal - mod reduction
N2O - no effect
Compare effects of anesthetics on HR
Iso - mild increase (10-30bpm)
Des - none to mild
Hal - mild decrease
Sevo, N2O - no effect
Compare effects of anesthetics on SVR
Sevo - mild decrease
Iso & Des - mod decrease
N2O - no effect
Compare effects of anesthetics on cardiac output
Sevo - mild decrease
Des - none to mild decrease
Iso & N2O - no effect
**Iso is most affected by high CO
Which anesthetic causes a coronary steal? What does that mean?
Iso - dilates the coronary arteries resulting in decreased blood flow through stenosed vessels
Which anesthetics increase cardiac sensitization to catecholamines?
Iso & Hal
Which anesthetic is used for ECMO and bypass?
Iso
Which gas is considered coronary protective?
Des
Which gas is the strongest myocardial depressant?
Hal
Which gases are respiratory irritants?
Iso & Des
Compare bronchodilating effects of gases
Sevo, Iso, & Hal - potent bronchodilators
N2O - bronchoconstrictor
Des - dilates @ 1MAC, constricts at 2MAC
Which gases have the strongest effect on Vt?
Iso & Hal
What effect to the gases typically have on RR?
mild increase
Hal has moderate increase
Which is the drug of choice for reactive airways?
Sevo
(non-irritating, potent bronchodilator, less foul smelling)
Which gas has the highest risk for CO production?
Des
Which gas should be avoided in patients with pulmonary HTN?
N2O
increases PVR and decreases hypoxic drive
What effect on CBF and ICP do the gases have?
generally a mild increase
Which gases reduce CMRO2?
Sevo, Iso, Des, Hal (mild)
Which gas can increase seizure activity at higher MAC?
Sevo
the rest tend to decrease likelihood of seizures
Which gas can cause emergence delirium in peds?
Sevo
What effect does Des have on CSF production?
dose-dependent increase in CSF production
What effect do the gases have on renal blood flow?
generally mild to mod decrease; Sevo can produce a mild increase
What is Compound A and which drug is it associated with?
Sevo
a degradation product formed when Sevo interacts with sodalime; theoretically can cause nephrotoxicity if rebreathed
How can providers minimize exposure to Compound A?
keep Sevo flow rates at 2L/min
do not exceed 2 MAC-hours with Sevo at <2L/min
In general, how do anesthetics affect hepatic blood flow? Which drug is different?
Generally a mild decrease
Sevo - decrease portal BF + increase arterial = hepatic BF is maintained
How much of each anesthetic is biotransformed?
Sevo - 2%
Iso - 0.2%
Des - 0.02%
N2O - 0.004%
Hal - up to 20%
Which anesthetic gas is associated with hepatitis?
Halothane (halothane hepatitis)
2 types; type II is worse (50-80% mortality)
Which gas may cause a postop ileus?
Iso
Which gas(es) produce the worst PONV?
Des and N2O (although N2O is much better with at least one antiemetic)
Which would be the drug of choice for myasthenia gravis patients? Why?
Sevo - potentiates NMBAs leading to profound muscle relaxation
In general, combining use of gas with benzos, opioids, or propofol will (increase/decrease) the MAC of gas required
decrease
Which gas has the worst/highest GWP100?
Des
(then Iso, N2O, and Sevo)
