Inhalational Anaesthetics

Question 1

What is the Meyer-Overton hypothesis?

Answer 1

Potency of an inhalational agent (MAC) correlates with its lipid solubility (oil:gas coefficient). Higher lipid solubility → lower MAC.

Question 2

Which receptors are primary targets of inhalational agents?

Answer 2

GABA_A, glycine receptors, and two-pore-domain potassium channels. Some agents also affect NMDA/HCN channels.

Question 3

Define MAC. What is the MAC of sevoflurane?

Answer 3

Minimum Alveolar Concentration preventing movement in 50% of patients. Sevoflurane MAC = 1.8%.

Question 4

Name three factors that DECREASE MAC.

Answer 4

Age >40, opioids, hypothermia, pregnancy, chronic alcohol use.

Question 5

How does a low blood:gas coefficient affect induction?

Answer 5

Low coefficient (e.g., desflurane 0.45) → rapid onset/offset due to faster equilibration between alveoli and brain.

Question 6

Why is N₂O contraindicated in pneumothorax?

Answer 6

N₂O diffuses into air-filled spaces, increasing volume (risk of tension pneumothorax).

Question 7

Compare halothane and isoflurane’s effect on cerebral blood flow.

Answer 7

Halothane ↑↑↑ CBF; isoflurane ↑ CBF minimally (Table 4).

Question 8

What is halothane hepatitis? Risk factors?

Answer 8

Fulminant hepatic necrosis. Risks: obesity, multiple exposures, female sex, pre-existing liver disease.

Question 9

Why does desflurane require a Tec 6 vaporizer?

Answer 9

Low boiling point (23.5°C) → requires heating to maintain consistent vapor pressure.

Question 10

What is the second gas effect?

Answer 10

High-concentration N₂O ↑ uptake of co-administered volatile agent (e.g., sevoflurane), speeding induction.

Question 11

Which agent is metabolized to nephrotoxic fluoride ions?

Answer 11

Enflurane (2% metabolism → fluoride). Avoid in renal impairment.

Question 12

Which agent is associated with Compound A formation?

Answer 12

Sevoflurane reacts with soda lime → Compound A (nephrotoxic in animals; safe in humans at low flows).

Question 13

Compare cardiac effects of halothane vs. desflurane.

Answer 13

Halothane: ↓↓ BP, bradycardia. Desflurane: ↑↑ HR, minimal BP change (Table 4).

Question 14

What toxicity is unique to N₂O?

Answer 14

Inactivates vitamin B₁₂ → megaloblastic anemia, neurological damage (prolonged exposure).

Question 15

Why is sevoflurane preferred for inhalational induction?

Answer 15

Non-pungent odor, rapid onset (blood:gas 0.70), bronchodilation.

Question 16

What is coronary steal? Which agent is implicated?

Answer 16

Diversion of blood from stenotic coronary arteries. Debated with isoflurane.

Question 17

Which agent is explosive? Key precaution?

Answer 17

Ether. Avoid sparks/diathermy; scavenge vapor. Safe without O₂ in resource-limited settings.

Question 18

What is xenon’s key advantage? Limitation?

Answer 18

Rapid onset/offset (blood:gas 0.14), neuroprotective. Limited by high cost.

Question 19

Which agent has the highest metabolism rate? Metabolites?

Answer 19

Halothane (20% metabolized → trifluoroacetic acid, Br⁻, Cl⁻).

Question 20

Compare respiratory effects of enflurane and desflurane.

Answer 20

Enflurane: ↓↓ tidal volume, ↑↑ PaCO₂. Desflurane: ↑ PaCO₂ but less than enflurane (Table 4).

Question 21

Why avoid enflurane in renal failure?

Answer 21

Metabolism releases fluoride ions (threshold >40 µmol/L → nephrotoxicity).

Question 22

What effect does halothane have on catecholamines?

Answer 22

Sensitizes myocardium → arrhythmias (limit adrenaline dose to <1 µg/kg).

Question 23

Which agent increases ICP the most?

Answer 23

Halothane (↑↑↑ CBF/ICP). Avoid in neuroanaesthesia.

Question 24

Which agent is safest in porphyria?

Answer 24

Isoflurane, desflurane, sevoflurane. Avoid halothane (hepatic enzyme induction).

Question 25

Compare MAC values: N₂O vs. desflurane.

Answer 25

N₂O MAC = 105%, desflurane MAC = 6.6% (N₂O is a weak agent used adjunctively).

Question 26

Which agent is achiral?

Answer 26

Sevoflurane (unlike isoflurane/enflurane, which are structural isomers).

Question 27

What is diffusion hypoxia? How to prevent?

Answer 27

N₂O exit dilutes alveolar O₂ at end of anaesthesia. Prevent with 100% O₂ post-op.

Question 28

Which agent is metabolized least?

Answer 28

Desflurane (0.02% metabolism). Followed by isoflurane (0.2%).

Question 29

What are the three phases for achieving satisfactory brain levels of an inhaled anaesthetic agent?

Answer 29

Delivery phase, Pulmonary phase, and Circulatory phase.

Question 30

Define Minimum Alveolar Concentration (MAC) and its clinical significance.

Answer 30

MAC is the alveolar concentration of anaesthetic at equilibrium that prevents a reflex response to skin incision in 50% of subjects; it serves as an ED50 measure and guides dose requirements.

Question 31

List the factors that influence the pulmonary uptake of volatile anaesthetic agents.

Answer 31

Inhaled concentration, alveolar ventilation, diffusion, blood/gas partition coefficient, partial pressure in the pulmonary artery, pulmonary blood flow, ventilation/perfusion distribution, concentration effect, and second gas effect.

Question 32

How does the blood/gas partition coefficient affect the rate of anaesthetic equilibration?

Answer 32

A low blood/gas partition coefficient indicates low solubility in blood, so equilibrium is reached rapidly; a high coefficient indicates high solubility and slower equilibration.

Question 33

Compare the wash-in characteristics of nitrous oxide and halothane.

Answer 33

Nitrous oxide has a low blood/gas solubility and rapidly approximates inspired levels, whereas halothane is highly soluble in blood and takes considerably longer to reach equilibrium.

Question 34

What is the primary route of elimination for volatile anaesthetic agents, and how does metabolism vary among agents?

Answer 34

Volatile agents are predominantly eliminated via the lungs; metabolism varies—for example, desflurane is metabolised only 0.02% in the liver, while halothane is about 20% metabolised.

Question 35

Describe the cardiovascular effects of volatile agents as a group.

Answer 35

They generally reduce myocardial contractility, preload, and afterload, leading to lowered blood pressure; however, specific agents differ in their effects on heart rate and contractility.

Question 36

What respiratory effects are associated with desflurane?

Answer 36

Desflurane is highly irritant to the respiratory tract, increases bronchial and salivary secretions, and at concentrations above 6% may cause coughing, breath-holding, and laryngospasm—especially in children under 12.

Question 37

How do sevoflurane and isoflurane differ in terms of blood/gas and oil/gas solubilities, and what is the clinical impact?

Answer 37

Sevoflurane has lower blood/gas and oil/gas solubilities than isoflurane, leading to more rapid changes in alveolar concentration, faster induction, and quicker recovery.

Question 38

Explain the second gas effect and its significance in inhalational anaesthesia.

Answer 38

The second gas effect occurs when a rapidly absorbed gas (typically nitrous oxide) is administered at high concentration along with a volatile agent of lower solubility, thereby increasing the alveolar concentration of the latter and promoting its uptake.

Question 39

What factors influence the circulatory phase of inhaled anaesthetic uptake?

Answer 39

Cardiac output, cerebral blood flow, and distribution to other tissues determine how the dissolved agent is transported to the brain.

Question 40

How do volatile agents affect cerebral blood flow (CBF) and intracranial pressure (ICP)?

Answer 40

All volatile agents increase CBF and ICP, but halothane has the greatest effect, followed by desflurane, while isoflurane and sevoflurane produce lower increases.

Question 41

Compare the MAC values of desflurane, halothane, isoflurane, and sevoflurane and explain what they indicate about potency.

Answer 41

MAC values: Desflurane ~6.35%, Halothane ~0.75%, Isoflurane ~1.15%, Sevoflurane ~2.0% (v/v). Lower MAC indicates higher potency; thus, halothane is most potent and desflurane is least potent.

Question 42

What is the clinical significance of the minimal metabolism of desflurane?

Answer 42

With only 0.02% metabolism, desflurane produces minimal toxic metabolites, leading to low toxicity and supporting rapid recovery via lung elimination.

Question 43

What toxic metabolites are produced by halothane metabolism, and why is this clinically important?

Answer 43

Halothane is metabolised to trifluoroacetic acid, chloride, and bromide ions; higher metabolism increases toxic metabolite production, contributing to risks such as halothane hepatitis and fluoride toxicity, especially in morbidly obese patients.

Question 44

How does nitrous oxide differ from volatile agents in its pharmacokinetic properties?

Answer 44

Nitrous oxide is a vapor with a very high saturated vapor pressure and low potency (MAC ~3.2%), is primarily eliminated unchanged via the lungs, and equilibrates rapidly; its low potency necessitates use in combination with other agents.

Question 45

List the contraindications and potential adverse effects of nitrous oxide.

Answer 45

Nitrous oxide can expand air-filled spaces, interfere with vitamin B12 metabolism leading to megaloblastic changes and neurological damage in chronic exposure, and is contraindicated in conditions with air-filled cavities.

Question 46

Describe the main properties of xenon as an anaesthetic agent.

Answer 46

Xenon is a noble gas with very low blood/gas solubility, providing rapid induction and emergence; it is highly cardiostable with no myocardial sensitisation, non-irritant, but has a high MAC (low potency) and is expensive.

Question 47

What is Entonox and what is its clinical application?

Answer 47

Entonox is a 50% nitrous oxide in oxygen mixture, primarily used as an inhaled analgesic (e.g., in labour) with properties similar to nitrous oxide.

Question 48

Define the characteristics of an ideal volatile anaesthetic agent as per the text.

Answer 48

An ideal volatile agent is 1. liquid at room temperature, 2. has low latent heat of vaporisation and specific heat capacity, 3. high saturated vapor pressure, 4. is stable, 5. non-flammable, 6. inexpensive, 7. environmentally safe, 8. has a pleasant smell, 9. low blood/gas solubility, 10. low MAC, 11. high oil/water solubility, 12. provides analgesia, is 13. non-epileptogenic, 14. lacks cardiac/respiratory depression, 15. is non-irritant, 16. facilitates muscle relaxation, 17. does not increase ICP, 18. is unaffected by renal/hepatic failure, and 19. undergoes minimal metabolism.

Question 49

How does the concentration effect influence the uptake of volatile agents?

Answer 49

High inspired concentrations of an agent result in a smaller proportional loss during each respiratory cycle, thereby maintaining higher alveolar concentrations compared to lower inspired concentrations.

Question 50

How does ventilation/perfusion mismatch affect the uptake of volatile anaesthetics?

Answer 50

A mismatch reduces perfusion of well-ventilated alveoli and increases perfusion of alveoli with lower anesthetic concentrations, potentially slowing overall uptake.

Question 51

Compare the respiratory irritation profiles of desflurane, isoflurane, halothane, and sevoflurane.

Answer 51

According to the grading (Table 29.6), desflurane has the highest respiratory irritation, isoflurane and halothane cause moderate irritation, while sevoflurane is the least irritating.

Question 52

What effects do volatile agents have on neuromuscular function?

Answer 52

Volatile agents produce a dose-dependent depression of neuromuscular function and potentiate both depolarising and non-depolarising neuromuscular blocking agents, likely via effects at the neuromuscular junction and direct reduction in contractility.

Question 53

How do volatile agents affect basal metabolic rate?

Answer 53

Inhalation of 2 MAC of a volatile agent reduces basal metabolic rate of oxygen consumption (BMRO2) by approximately 30%, thereby lowering oxygen consumption and CO2 production.

Question 54

What impact do volatile agents have on immune function?

Answer 54

They reduce the killing function of polymorphonuclear cells by interfering with calcium flux and superoxide generation.

Question 55

What adverse reaction can occur with the use of dry soda lime in the presence of desflurane or isoflurane?

Answer 55

Dry (used) soda lime can lead to the formation of carbon monoxide when in contact with desflurane or isoflurane.

Question 56

What is Compound A in relation to sevoflurane, and what factors influence its formation?

Answer 56

Compound A is a degradation product of sevoflurane formed by its reaction with CO2 absorbers; its production is higher with baralyme (due to higher temperatures) and is reduced by higher water content in the absorbent.

Question 57

Compare the cardiovascular depression effects of halothane, isoflurane, desflurane, and sevoflurane.

Answer 57

Halothane produces the greatest cardiovascular depression, isoflurane and sevoflurane cause moderate depression, while desflurane shows the least cardiovascular depression, partly due to sympathetic stimulation.

Question 58

Which volatile agent is associated with the greatest cerebral vasodilatation and intracranial pressure elevation?

Answer 58

Halothane is associated with the highest increase in cerebral blood flow and intracranial pressure compared to desflurane, isoflurane, and sevoflurane.