Inhaled Anesthetics Flashcards
What is minimum alveolar concentration?
The minimum alveolar concentration of the inhaled anesthetic that prevents movement in 50% of patients in response to an abdominal incision.
What factors decrease MAC?
- Acidosis
- Acute alcohol use
- Advanced age
- Anemia; hypoxia
- Drugs: benzos, opiates, IV anesthetics
- Higher altitude
- Hypotension (severe)
- Hypothermia
- Pregnancy
When using two inhaled anesthetics, are their MAC’s additive?
Yes, roughly additive; e.g. 0.5 MAC of N20 plus 0.5 MAC of sevo ~ 1.0 MAC
What factors increase MAC?
- Chronic alcohol use
- Cocaine
- Very young age (greatest at 1 year)
- Increased temperature
- Decreased altitude
- Drugs: MAOIs, TCAs, amphetamines
How does MAC change with age?
MAC is greatest at 1 year and decreases by 6% per decade of life.
What is the MAC of desflurane?
6%
What is the MAC of nitrous oxide?
104%
Therefore, it cannot be used as a solo agent
What is the MAC of sevoflurane?
2.05%
What is the MAC of isoflurane?
1.15%
Why should you administer 100% O2 for 5-10 minutes after discontinuation of N2O?
- High concentrations of N2O diffuse out of the blood and into the alveoli, displacing and reducing alveolar concentrations of O2 and CO2.
- Dilution of alveolar O2 can lead to hypoxia
- Dilution of alveolar CO2 can decrease ventilators drive and worsen hypoxia
Which inhaled anesthetic(s) can trigger malignant hyperthermia?
All of the volatile anesthetics
Of the four commonly used inhaled anesthetics, which are flammable?
None, but N2O does support combustion so it should be avoided in cases where lasers or cautery are used in air spaces that may contain the drug.
In what form are the inhaled anesthetics stored?
The volatile anesthetics are stored as liquids. Each has a specific vaporizer which can transform it into a much larger volume of gas.
Nitrous oxide is stored as a compressed gas (in a blue tank) and thus does not require a vaporizer.
How is accumulation of CO2 in the breathing circuit prevented?
The anesthesia machine in includes a canister of CO2 absorbent (e.g. soda lime) to prevent its accumulation.
What are the disadvantages of CO2 absorbent?
The volatile agents are somewhat unstable when exposed to soda lime and form small amounts of toxic substances:
- Desflurane > isoflurane form carbon monoxide
- Sevo forms a nephrotoxic vinyl compound
Why is blood:gas solubility important?
The lower the blood:gas solubility of an anesthetic, or the more insoluble it is, the more rapid its onset of action, meaning a more rapid induction.
What is the order of blood:gas solubility of the four major inhaled anesthetics from lowest to highest?
- Desflurane: 0.45 (most insoluble; most rapid induction)
- N2O: 0.47
- Sevoflurane: 0.65
- Isoflurane: 1.4 (most soluble; slowest induction)
Why is fat:blood solubility important?
Fat:blood solubility is a major factor in determining rate of emergence from anesthesia and a minor factor in determining onset of action.
Increased blood:fat solubility prolongs emergence and may slightly increase onset of action (lipid solubility favorable for crossing the blood:brain barrier)
What is the order of blood:gas solubility of the four major inhaled anesthetics from lowest to highest?
- N2O: 2.3 (least soluble; rapid emergence)
- Desflurane: 27
- Isoflurane: 45
- Sevoflurane: 48
Which inhaled anesthetics do NOT cause airway irritation? Why is this favorable?
Sevo and N2O; this is favorable because they can be used for a pleasant induction.
Isoflurane and desflurane are airway irritants and can cause coughing and even laryngeal spasm. Because of this, desflurane is only used for maintenance of anesthesia.
Which inhaled anesthetics cause bronchodilation?
Sevo and Iso
N2O has no effect
Desflurane causes bronchoconstriction in smokers but no effect in non-smokers
Which inhaled anesthetics cause respiratory depression?
All inhaled anesthetics cause respiratory depression in the form of rapid rate, reduced tidal volume, and loss of the awake respiratory variability.
Which inhaled anesthetics increase arterial carbon dioxide?
The volatile anesthetics
Which inhaled anesthetics depress the ventilators responses to hypercarbia and hypoxia?
All of them.
*The ventilators response to hypoxia may be depressed even at sub-anesthetic concentrations.
Which inhaled anesthetics potentiate neuromuscular blockade?
The volatile anesthetics all help relax skeletal muscles, with desflurane and sevoflurane > isoflurane
Low hepatic metabolism is favorable for an inhaled anesthetic because its half-life will be longer. What is the order from lowest to highest?
- N2O: negligible
- Desflurane and Isoflurane: 0.2%
- Sevo: 4% (most of any IA still used*)
*Halothane’s is 20%
What are the potential organ toxicities of the four major IAs?
- Hepatic: desflurane and isoflurane
- Renal: sevoflurane
- Bone marrow: N2O
What is recommended if a pt has a hx of anesthetic-induced hepatitis?
Avoid all volatile IAs
Which IAs have an analgesic effect?
Only N2O
*This effect is blocked with narcan (naloxone)
Which IAs increase cerebral blood flow causing increased ICP? Which impair autoregulation of cerebral vascular tone?
All of them do both.
- Halothane increases CBF and ICP the most
- Sevo has the least effect on autoregulation
Which IAs decrease cerebral metabolic rate?
The volatile agents.
*N2O may increase cerebral metabolic rate
Which IAs can cause seizures?
Sevo at high doses has been associated with epileptiform EEG.
*Desflurane and isoflurane at < 1 MAC can suppress status epilepticus
Which IAs decrease cardiac contractility?
All of them
Which IAs increase HR?
Desflurane, isoflurane and N2O
Which IAs decrease SVR?
Desflurane, iso and sevo decrease SVR.
N2O and halothane have no effect on SVR
Which IAs increase cardiac output?
N2O can either increase or have no effect on CO.
Iso has no effect. The others decrease CO
Why does halothane have the highest potential for hepatic toxicity?
Halothane decreases portal vein flow and causes hepatic artery vasoconstriction, increasing the potential for hypoxic hepatic injury.
The other agents decrease portal vein flow but INCREASE hepatic artery flow to compensate.
In which patients/surgeries should N2O be avoided?
N2O rapidly infuses into and expands air-filled cavities. It should be avoided in:
- Air embolism
- Pneumothorax
- Bowel obstruction
- Pneumocephalus
- Middle ear and retinal procedures
What are the advantages of isoflurane?
- Highest potency (lowest MAC: 1.15%)
- Low hepatic metabolism (0.2%)
- Bronchodilator
- Inexpensive
What are the disadvantages of isoflurane?
- Slowest induction (highest B:G sol.)
- Slow emergence (low F:B sol.)
- Not used for induction (pungent odor, airway irritant)
- Hypotension (strong vasodilator)
- Carbon monoxide forms in CO2 absorbent reaction
- Trigger for malignant hyperthermia
What are the advantages of desflurane?
- Fastest induction (lowest B:G sol.)
- Fast emergence (low F:B sol.)
- Low hepatic metabolism
What are the disadvantages of desflurane?
- Not used for induction b/c of pungent odor and airway irritation (laryngospasm)
- Bronchoconstriction in smokers
- Hypotension (strong vasodilator)
- CO formed in CO2 absorbent
- Requires a special heated vaporizer
- Trigger for malignant hyperthermia
- Sympathetic stimulation
What are the advantages of sevoflurane?
- Fast induction and emergence
- Bronchodilator
- Pleasant odor
- No airway irritation
- Suitable for mask induction (esp. for children)
- Less tachycardia than iso or des (b/c it doesn’t sensitize the myocardium to catecholamines)
What are the advantages of halothane?
- Low pungency
- Inexpensive
- Increases CBF more than other volatiles
- Potent bronchodilator
What are the disadvantages of halothane?
- Hepatic ischemic injury (all decrease portal vein flow but halothane also decreases hepatic artery flow)
- Rare but fulminant postoperative autoimmune hepatitis (why it is rarely used)
- Greatest decrease of cardiac output and contractility
- Myocardial sensitization to catecholamines (higher risk of ventricular dysrhythmias)
What are the disadvantages of sevoflurane?
- High hepatic metabolism (4%)
- High inorganic fluoride ion levels
- Compound A formed in CO2 absorbent -> renal toxicity
- Most soluble in fat and muscle
- Hypotension (vasodilator)
- Trigger for malignant hyperthermia
What are the advantages of N2O?
- Analgesia
- Fastest induction (low B:G sol.)
- Fastest emergence (low F:B sol.)
- Can be used for induction (not pungent or an airway irritant)
- Less hypotension
- Less cardiac depression
- Less respiratory depression
What are the disadvantages of N2O?
- Nausea and vomiting
- Cannot be used as a solo agent (MAC 104%)
- Bone marrow toxicity
- Expands in close spaces/cavities
- Supports combustion
What is Heliox? What are its benefits?
Heliox is a non-anesthetic gas mixture, commonly of 70-79% helium and 21-30% oxygen.
Being a lower density gas promotes laminar flow and reduces turbulence in upper airway obstruction, asthma and COPD.
It helps decrease pressure needed to ventilate pts with small-diameter ETTs. It also decreases work of spontaneous breathing.
