Inhaled Agents Apex Flashcards
What is the blood:gas coefficient for Sevoflurane?
0.65
What is the blood:gas coefficient for Desflurane?
0.42
What is the blood:gas coefficient for Isoflurane?
1.46
What is the blood:gas coefficient for N2O?
0.46
What is an anesthetic gas with low blood:gas solubility likely to do?
It is less likely to be taken up into the blood.
What is the result of an anesthetic gas having low blood:gas solubility?
More of the agent is available to exert a partial pressure in the alveoli and brain.
What happens to anesthetic gas with high blood:gas solubility?
It is more likely to be taken up into the blood.
What is the effect of high blood:gas solubility on the availability of anesthetic agent?
Less of the agent is available to exert a partial pressure in the alveoli and brain.
What is the purpose of administering a volatile anesthetic?
To produce a state of anesthesia by building up a partial pressure of anesthetic agent inside the patient’s brain and spinal cord.
What does the concentration of an anesthetic agent inside the alveoli (FA) indicate?
It is proportional to its concentration inside the brain.
Why do we use alveolar partial pressure (FA)?
As a surrogate for the brain partial pressure.
What factors determine anesthetic uptake into the blood?
- Agent solubility
- Partial pressure difference between the alveoli and the blood
- Cardiac output
What does the FA/FI curve represent?
It helps us appreciate the speed at which FA equilibrates with FI (speed of induction).
How does an anesthetic agent’s solubility affect FA/Fl equilibration?
Low solubility leads to faster equilibration of FA/Fl and faster onset, while high solubility leads to slower equilibration and slower onset.
What does increased Fa/FI indicate?
Faster Onset
What happens to the curve with increased Fa/FI?
Curve Pushed Up
What factors contribute to increased wash in?
High fresh gas flow, High alveolar ventilation, Low FRC, Low time constant, Low anatomic dead space
What factors contribute to decrease uptake?
Low solubility, Low cardiac output, Low PA-P difference
What does decreased FA/FI indicate?
Slower onset
Curve pushed down
What factors contribute to decreased wash in?
Low fresh gas flow, low alveolar ventilation, high FRC, high time constant, high anatomic dead space
What factors increase uptake?
High solubility, high cardiac output, high PA-PV difference
What is the Vessel-Rich Group’s body mass percentage and CO percentage?
10% Body Mass, 75% CO
Which organs are included in the Vessel-Rich Group?
Brain, Heart, Kidney, Liver, endocrine glands
What is the Muscle group’s body mass percentage and CO percentage?
50% Body Mass, 20% CO
Which components are included in the Muscle group?
Skeletal muscle, Skin
What is the Fat group’s body mass percentage and CO percentage?
20% Body Mass, 5% CO
What is the Vessel-Poor Group’s CO percentage and body mass percentage?
<1% CO, 20% Body Mass
Which components are included in the Vessel-Poor Group?
Bone, Tendon, Cartilage
What is the primary mechanism of elimination for inhaled anesthetics?
Elimination from the alveoli
What is the secondary mechanism of elimination for inhaled anesthetics?
Hepatic biotransformation
What is the minimal mechanism of elimination for inhaled anesthetics?
Percutaneous loss
Which system is involved in the hepatic biotransformation of halogenated anesthetics?
The P450 system
What is the percentage of metabolism for Sevoflurane?
2% metabolized
What is the percentage of metabolism for Isoflurane?
0.2% metabolized
What is the percentage of metabolism for Desflurane?
0.02% metabolized
What is the percentage of metabolism for Nitrous oxide?
0.004% metabolized
What important metabolite is produced by Desflurane and Isoflurane?
Trifluoroacetic acid
Small risk of immune-mediated hepatic dysfunction.
What does Sevoflurane metabolism produce?
Free fluoride ions
Theoretical risk of high output kidney failure.
What does Sevoflurane generate when exposed to soda lime?
Compound A
Desiccated soda lime accelerates compound A production.
What do Desflurane and Isoflurane produce when exposed to desiccated soda lime?
Carbon monoxide
What is the second gas effect?
The second gas effect states that administering one gas during anesthetic induction (nitrous oxide) will hasten the onset of a second gas (volatile agent).
What is diffusion hypoxia?
Diffusion hypoxia is the movement of nitrous oxide from the tissue back into the alveoli during emergence, which dilutes alveolar oxygen and carbon dioxide.
What can diffusion hypoxia lead to?
It can lead to transient hypoxemia and hypocarbia.
What does classic teaching suggest to mitigate dilution of alveolar oxygen after nitrous oxide?
Classic teaching suggests administering 100% oxygen for 3 - 5 minutes after nitrous oxide has been discontinued.
What does newer evidence suggest regarding FiO2 during emergence?
Newer evidence suggests a lower FiO2 during emergence as a way of reducing absorption atelectasis and improving post-operative gas exchange.
What is the effect of a right-to-left shunt on induction with a volatile agent?
Slower induction with a volatile agent (less soluble agents are affected to a greater extent).
What is the effect of a right-to-left shunt on induction with an IV agent?
Faster induction with an IV agent.
What is the impact of a left-to-right shunt on induction with a volatile agent?
No meaningful impact on induction with a volatile agent.
What is the effect of a left-to-right shunt on induction with an IV agent?
Slower induction with an IV agent.
Examples of Right-to-Left Shunts
- Tetralogy of Fallot
• Foramen ovale
• Eisenmenger’s syndrome - Tricuspid atresia
• Ebstein’s anomaly
How does a right-to-left shunt affect the FA/FI of anesthetic agents?
In the presence of a right-to-left shunt, the Fa/F of an agent with lower solubility is more affected than that of an agent with higher solubility.
**Footnote
It’ll take longer to induce
What happens to more soluble anesthetic agents in a right-to-left shunt?
More soluble agents experience greater uptake by the blood, which partially offsets the dilution effect.
What is the effect on less soluble anesthetic agents in a right-to-left shunt?
Less soluble agents undergo very little uptake by the blood, and the effect of dilution is unchecked.
Which anesthetic agent has the lowest blood:gas partition coefficient?
Desflurane has the lowest blood:gas partition coefficient (0.42), so its FA/F curve is affected the most.
Which anesthetic agent has the highest blood:gas partition coefficient?
Isoflurane has the highest blood:gas partition coefficient (1.46), so its FA/F curve is affected the least.
How does a right-to-left intracardiac shunt affect IV induction?
A right-to-left intracardiac shunt produces a faster IV induction as blood bypasses the lungs and travels to the brain faster.
How much more soluble is nitrous oxide compared to nitrogen?
Nitrous oxide is 34 times more soluble than nitrogen.
What happens to the volume of a compliant airspace when nitrous oxide enters?
Nitrous oxide increases the volume of the space.
What are some examples of fixed airspaces affected by nitrous oxide?
Middle ear, eye during retinal detachment surgery, brain during intracranial surgery.
What anesthesia equipment can nitrous oxide increase the volume and pressure in?
Endotracheal tube cuff, LMA cuff, balloon-tipped pulmonary artery catheter.
Is nitrous oxide flammable?
Nitrous oxide is not flammable, but it does support combustion.
How does nitrous oxide affect vitamin B12?
Nitrous oxide irreversibly inhibits vitamin B12, which inhibits methionine synthase.
What is Minimum Alveolar Concentration (MAC)?
MAC is the concentration of inhalational anesthetic that prevents movement following a painful stimulus in 50% of the population.
What is the MAC for Isoflurane?
1.2%
What is the MAC for Sevoflurane?
2.0%
What is the MAC for Desflurane?
6.6%
What is the MAC for Nitrous Oxide?
104%
What are the effects of general anesthetics?
Amnesia, loss of consciousness, immobility, modulation of autonomic function, and some analgesia.
Is MAC additive?
Yes, MAC is additive.
What is MAC-awake during induction?
Approximately 0.4 - 0.5 MAC.
What is MAC-awake during emergence?
0.15 MAC.
What is MAC-bar?
1.5 MAC.
At what MAC is movement prevented in 95% of the population?
Approximately 1.3 MAC.
At what MAC are awareness and recall prevented?
Approximately 0.4 - 0.5 MAC.
What factors increase MAC?
Chronic alcohol consumption, increased CNS neurotransmitter activity, hypernatremia, infants 1 - 6 months, hyperthermia.
What factors decrease MAC?
Acute alcohol consumption, sedative drugs, hyponatremia, old age (decreased by 6% per decade after age 40), extremes of age, pregnancy.
What factors have no effect on MAC?
Hyper- or hypothyroidism, hyper- or hypokalemia, gender.
General anesthesia is produced by?
General anesthesia is produced by membrane-bound protein interactions in the brain and spinal cord.
How do volatile anesthetics generally affect receptors?
Volatile anesthetics either stimulate inhibitory receptors or inhibit stimulatory receptors.
What is the most important site of action for general anesthesia in the brain?
The most important site of action in the brain is the GABA-A receptor.
What happens when the GABA-A receptor is stimulated?
Stimulation of the GABA-A receptor increases chloride influx and hyperpolarizes neurons.
What are the important sites of action for general anesthesia in the spinal cord?
The important sites of action in the spinal cord are the glycine receptor (stimulation), NMDA receptor (inhibition), and sodium channels (inhibition).
What do gaseous anesthetics like nitrous oxide and xenon target?
Gaseous anesthetics (nitrous oxide and xenon) target the NMDA receptor (antagonism) and the potassium 2P-channel (stimulation).
How do volatile anesthetics produce unconsciousness?
By interacting in the cerebral cortex, thalamus, and reticular activating system.
Where is amnesia produced?
In the amygdala and hippocampus.
What produces autonomic effects?
The pons and medulla.
Where is analgesia produced?
In the spinothalamic tract.
What produces immobility?
The ventral horn of the spinal cord.
What is the effect of halogenated anesthetics on MAP?
They reduce MAP in a dose-dependent fashion. At equivalent doses, there’s little difference between agents.
How do halogenated anesthetics affect contractility?
They decrease contractility in a dose-dependent fashion.
What is the effect of halogenated anesthetics on SVR?
They decrease SVR in a dose-dependent fashion. Sevoflurane reduces SVR the least.
Which halogenated anesthetics increase heart rate?
Isoflurane and desflurane increase heart rate. Sevoflurane does not.
What effect does nitrous oxide have on MAP and SVR?
Nitrous oxide increases MAP and SVR by SNS activation.
What is the current thinking about coronary steal in relation to volatile anesthetics?
Current thinking disputes the theory of coronary steal. In fact, volatile anesthetics precondition the myocardium and protect it against ischemia.
What is the effect of halogenated anesthetics on PaCO2?
They cause hypercapnia through depression of the central chemoreceptor and respiratory muscles.
How do halogenated anesthetics alter respiratory patterns?
They decrease tidal volume (Vt) and increase respiratory rate (RR).
What is the effect of halogenated anesthetics on the apneic threshold?
They increase the apneic threshold.
What happens to upper airway tone with halogenated anesthetics?
There is relaxation of the muscles that maintain upper airway tone, leading to airway obstruction.
What is the effect of halogenated anesthetics on bronchodilation?
They cause bronchodilation, but this effect is minimal in the absence of increased airway resistance.
What are the effects of halogenated anesthetics on PaO2?
They impair peripheral chemoreceptors, increasing the risk of hypoxemia.
At what MAC does impaired response to acute hypoxemia occur?
Impaired response occurs at 0.1 MAC.
Which halogenated anesthetic impairs the hypoxic drive the least?
Desflurane impairs the hypoxic drive the least.
It is the best choice for patients who rely on the hypoxic drive to breathe, such as those with emphysema or sleep apnea.
Do pain and surgical stimulation reverse the depression of the hypoxic ventilatory drive?
No, they do not reverse the depression of the hypoxic ventilatory drive.
What is the effect of halogenated anesthetics on CMRO2?
They cause a dose-dependent reduction in CMRO2.
How do halogenated anesthetics affect cerebral blood flow and ICP?
They cause a dose-dependent increase in cerebral blood flow, cerebral blood volume, and ICP.
What MAC is required to produce an isoelectric state?
1.5 - 2.0 MAC is required to produce an isoelectric state.
What can high concentrations of Sevoflurane produce?
Seizure activity, especially at 2.0 MAC, exacerbated by hypocapnia and more common with pediatric inhalation induction.
What effect does N2O have on CMRO2 and cerebral blood flow?
N2O increases CMRO2 and cerebral blood flow.
Alternative to Cefazoli?
For patients allergic to β-lactams, alternatives like vancomycin or clindamycin are suggested, with additional coverage for gram- negative bacilli if needed.
The usual recommended dose of atropine with edrophonium is
7 mcg/kg
