Volatile Anaesthetics

Question 1

Mean Alveolar Concentration

Answer 1

The alveolar concentration of a gaseous agent required to ensure that 50% of a test population at sea level does not respond to a standard surgical skin incision

Question 2

Dose-response curve for volatile agents

Answer 2

20% below MAC - almost all patients move in response to surgical stimulus
20% above MAC - less than 5% of patients move

Question 3

Blood:Gas Coefficient

Answer 3

If a substance has a high blood:gas coefficient, it easily dissolves into the blood resulting in a low partial pressure. This results in a slower onset and offset of anaesthesia (paradoxically)

A measure of how quickly Fa/Fi approaches 1 - no units!

Question 4

Oil:Gas Coefficient

Answer 4

If the Oil:Gas coefficient is high, the substance is more lipid soluble and therefore tends to pass readily into lipid rich organs e.g. the brain. Potency is proportional to O:G (Meyer-Overton)

Question 5

Factors Decreasing MAC

Answer 5

Pregnancy, neonates, old age
Chronic amphetamine use
Acute alcohol intoxication
Acute opioid use
Lithium, benzodiazepines and lidocaine

Question 6

Factors Increasing MAC

Answer 6

Infancy
Acute amphetamine use
Hyperthermia, hyperthyroidsim, hypernatraemia
Chronic alcoholism
Chronic opioid use
Catecholamines and sympathomimetics
Stress response

Question 7

Enflurane: MAC

Answer 7

MAC 1.7

Question 8

Enflurane: Oil:Gas coefficient

Answer 8

O:G 98

Question 9

Enflurane: Blood:Gas coefficient

Answer 9

B:G 1.9

Question 10

Isoflurane: MAC

Answer 10

MAC 1.2

Question 11

Isoflurane: Oil:Gas coefficient

Answer 11

O:G 98

Question 12

Isoflurane: Blood:Gas coefficient

Answer 12

B:G 1.4

Question 13

Sevoflurane: MAC

Answer 13

MAC 1.8

Question 14

Sevoflurane: Oil:Gas coefficient

Answer 14

O:G 80

Question 15

Sevoflurane: Blood:Gas coefficient

Answer 15

B:G 0.7

Question 16

Desflurane: MAC

Answer 16

MAC 6.6

Question 17

Desflurane: Oil:Gas coefficient

Answer 17

O:G 29

Question 18

Desflurane: Blood:Gas coefficient

Answer 18

B:G 0.4

Question 19

Halothane: MAC

Answer 19

MAC 0.8

Question 20

Halothane: Oil:Gas coefficient

Answer 20

O:G 224

Question 21

Halothane; Blood Gas coefficient

Answer 21

B:G 2.2

Question 22

Carbon dioxide absorbants

Answer 22

UK: sodium hydroxide
USA: potassium hydroxide (baralyme)
Requires water
H20 + CO2 -> H2CO3 (carbonic acid)
H2CO3 + 2NaOH -> Na2CO3 +2H2O
Na2CO3 + Ca(OH)2 -> CaCO3 + 2NaOH
Sodium carbonate + calcium hydroxide -> Calcium carbonate + sodium hydroxide

Question 23

Enflurane: structure

Answer 23

Molecular weight: 184

Structural isomer of isoflurane

Question 24

Isoflurane: structure

Answer 24

Molecular weight: 184

Structural isomer of enflurane

Question 25

Sevoflurane: structure

Answer 25

Molecular weight: 200 | Sevoflurane is achiral

Question 26

Halothane: structure

Answer 26

Molecular weight: 197

Question 27

Desflurane: structure

Answer 27

Molecular weight: 168

Question 28

Desflurane: boiling point

Answer 28

23.5°C (requires specialized Tec 6 vaporizer)

Question 29

Desflurane: SVP at 20°C

Answer 29

89.2 kPa

Question 30

Enflurane: boiling point

Answer 30

56.5°C

Question 31

Enflurane: SVP at 20°C

Answer 31

23.3 kPa

Question 32

Halothane: boiling point

Answer 32

50.2°C

Question 33

Halothane: SVP at 20°C

Answer 33

32.3 kPa

Question 34

Isoflurane: boiling point

Answer 34

48.5°C

Question 35

Isoflurane: SVP at 20°C

Answer 35

33.2 kPa

Question 36

Sevoflurane: boiling point

Answer 36

58.5°C

Question 37

Sevoflurane: SVP at 20°C

Answer 37

22.7 kPa

Question 38

Desflurane: cardiovascular effects (contractility, heart rate, systemic vascular resistance, blood pressure, coronary steal, splanchic blood flow, catecholemine sensitization)

Answer 38

``` Contractility: minimal Heart rate: increased (increased ++ at MAC > 1.5). Can induce tachycardia and hypertension at MAC > 1 (classic MCQ question) Systemic vascular resistance: -- Blood pressure: -- Coronary steal: no Splanchic blood flow: unchanged Catecholemine sensitization: nil ```

Question 39

Enflurane: cardiovascular effects (contractility, heart rate, systemic vascular resistance, blood pressure, coronary steal, splanchic blood flow, catecholemine sensitization)

Answer 39

``` Contractility: -- Heart rate: + Systemic vascular resistance: - Blood pressure: -- Coronary steal: No Splanchic blood flow: - Catecholamine sensitization: + ```

Question 40

Halothane: cardiovascular effects (contractility, heart rate, systemic vascular resistance, blood pressure, coronary steal, splanchic blood flow, catecholemine sensitization)

Answer 40

``` Contractility: --- Heart rate: -- Systemic vascular resistance: - Blood pressure: --- Coronary steal: No Splanchic blood flow: - Catecholamine sensitization: +++ (causes arrhythmias) ```

Question 41

Isoflurane: cardiovascular effects (contractility, heart rate, systemic vascular resistance, blood pressure, coronary steal, splanchic blood flow, catecholemine sensitization)

Answer 41

``` Contractility: - Heart rate: ++ (this reflex tachycardia suggests that the carotid sinus reflex is preserved) Systemic vascular resistance: -- Blood pressure: -- Coronary steal: Possibly Splanchic blood flow: Unchanged Catecholamine sensitization: Nil ```

Question 42

Sevoflurane: cardiovascular effects (contractility, heart rate, systemic vascular resistance, blood pressure, coronary steal, splanchic blood flow, catecholemine sensitization)

Answer 42

``` Contractility: - Heart rate: nil effect Systemic vascular resistance: - Blood pressure: - Coronary steal: No Splanchic blood flow: Unchanged Catecholamine sensitization: Nil ```

Question 43

The volatile most likely to cause arrhythmia secondary to catecholamine sensitization

Answer 43

Halothane

Question 44

The volatile most likely to cause coronary steal syndrome

Answer 44

Isoflurane

Question 45

Desflurane: CNS effects (cerebral blood flow, cerebral O2 requirements, EEG, potentiation of muscle relaxants, analgesia)

Answer 45

``` Cerebral blood flow: + Cerebral O2 requirements: - EEG: burst supression Potentiation of muscle relaxants: significant Analgesia: some ```

Question 46

Isoflurane: CNS effects (cerebral blood flow, cerebral O2 requirements, EEG, potentiation of muscle relaxants, analgesia)

Answer 46

Cerebral blood flow: + (nil if MAC<1) Cerebral O2 requirements: autoregulation preserved EEG: burst supression Potentiation of muscle relaxants: some relaxation Analgesia: some

Question 47

Sevoflurane: CNS effects (cerebral blood flow, cerebral O2 requirements, EEG, potentiation of muscle relaxants, analgesia)

Answer 47

``` Cerebral blood flow: + Cerebral O2 requirements: - EEG: burst supression Potentiation of muscle relaxants: some relaxation Analgesia: some ```

Question 48

Enflurane: CNS effects (cerebral blood flow, cerebral O2 requirements, EEG, potentiation of muscle relaxants, analgesia)

Answer 48

Cerebral blood flow: + Cerebral O2 requirements: - EEG: Epileptiform activity (3Hz spike and wave) Potentiation of muscle relaxants: significant Analgesia: some

Question 49

Halothane: CNS effects (cerebral blood flow, cerebral O2 requirements, EEG, potentiation of muscle relaxants, analgesia)

Answer 49

``` Cerebral blood flow: +++ Cerebral O2 requirements: - EEG: burst supression Potentiation of muscle relaxants: some relaxation Analgesia: none ```

Question 50

Desflurane: metabolism

Answer 50

0.02%

Question 51

Enflurane: metabolism

Answer 51

2%

Question 52

Halothane: metabolism

Answer 52

20% Can cause Type I (benign, self limiting) or Type II (severe) hepatotoxicity via T-cell mediated inflammation Metabolism to trifluoroacetic acid under oxidative conditions - implicated in Type II hepatotoxicity Reductive metabolism to F-C-Br bonds are more easily metabolised than C-F bonds

Question 53

Isoflurane: metabolism

Answer 53

0.2% - non toxic NB due to the -CHF2 group, it may react with dry soda lime producing carbon monoxide e.g. in circle system that has been left with dry gas circulating over the weekend

Question 54

Sevoflurane: metabolism

Answer 54

3.5% Undergoes hepatic metabolism by cytochrome P450 (isoform 2E1) to produce hexafluoroisopropanol and inorganic fluoride ions (known to cause renal toxicity)

Question 55

Compound A

Answer 55

Created when sevoflurane is used in the rpesence of carbon dioxide absorbents More readily produced with (dry) potassium hydroxide Human nephrotoxic threshold of 150-200 ppm. At flow rates of 0.25l/min for 5 hours, compound A level < 20ppm (therefore unlikely to cause harm in anaesthesia)

Question 56

Sevoflurane: manufacture

Answer 56

One pot method - all the ingredients are added together to produce sevoflurane and then water is added to 300ppm Chloro-fluoro method - basic molecular architecture is manufactured but with chlorine attached. This is then substituted with fluorine

Question 57

Sevoflurane: respiratory effects

Answer 57

Pleasant odor Depresses ventilation with reduction in minute volume Inhibits pulmonary vasoconstriction

Question 58

Sevoflurane: storage

Answer 58

Can produce hydrofluoric acid if stored in glass or with concentration of added water <100ppm (attack by Lewis acids) - hydorfluoric acid corrodes glass Therefore formulated with 300ppm water and stored in polyethylene napthalate bottles (alternatively in aluminium bottle with resin laqcuer - can be formulated with <130ppm water)

Question 59

Halothane: respiratory effects

Answer 59

Sweet, non-irritant odour Bronchodilatory High concentrations significantly reduce ventilation