Inhalational Anaesthetic Agents

Question 1

Define MAC?

Answer 1

The MAC is that concentration required to prevent 50% of patients moving when subjected to a standard midline incision.

Note MAC value is calibrated for 40yo so needs adjusting for age

Question 2

What is important for the potency of volatile anaesthetic the concentration or the partial pressure?

Answer 2

The partial pressure

Note atmospheric pressure at sea level is 101kPa

Therefore 2% sevoflurane is the equivalent of 2kPa at sea level

Question 3

What is the MAC value and O:G partition coefficient for sevoflurane?

Answer 3

O:G partition coefficient: 80
MAC 2%

Question 4

What is the MAC value and O:G partition coefficient for Isoflurane?

Answer 4

O:G partition coefficient: 98
MAC 1.2%

Question 5

Which factors would make you consider using a higher MAC?

Answer 5

Stimulant use

Chronic alcohol dependence

Exogenous catecholamine use

Hyperthyroidism

Hyperthermia

Young age

Anxiety and stress

Question 6

Which factors would make you consider using a lower MAC?

Answer 6

Nitrous oxide

Alpha2 adrenergic agents

Benzodiazepines

Intravenous anaesthetics used by infusion

Opioid analgesics

Acute alcohol intoxication

Reduced GCS

Hypothermia

Hypothyroidism

Increasing age

Question 7

What is the significance of the oil:gas partition coefficient?

Answer 7

The oil:gas partition coefficient determines the lipid solubility of gas.

This is important as only lipid soluble molecules will cross the blood brain barrier.

Therefore the higher the O:G partition coefficient the more lipid soluble and the more potent the anaesthetic gases.

Question 8

What is the MAC value and O:G partition coefficient for Nitrous oxide?

Answer 8

O:G partition coefficient: 1.4

MAC 103%

Question 9

Draw a graph with the ratio: Fa (alveolar concentration)/Fi (inspired concentration) on the Y axis and time on the left. Draw curves for N2O, isoflurane and sevoflurane.

Answer 9

All eventually plateau.

N20 has a sharp ascent and has the fastest onset followed by: isoflurane and sevoflurane.

Question 10

What is the blood gas partition coefficient?

Answer 10

The blood–gas partition coefficient is a ratio of the concentration of volatile anaesthetic in blood compared to alveolar gas once the partial pressure has equilibrated.

A high blood gas partition coefficient means that there is a higher concentration of volatile anaesthetic in the blood.

Question 11

How does the blood gas partition coefficient effect onset time of the volatile anaesthetic agents?

Answer 11

The lower the blood gas partition coefficient the quicker the onset time.

MAC (mean alveolar concentration) works on the assumption that it reaches equilibrium with the arterial blood and that this reaches equilibrium with the brain.

If a gas is highly soluble then more is dissolved in the bloodstream, this in turn reduces the alveolar concentration and it takes longer for equilibrium to be reached.

Therefore induction would take longer.

Question 12

How does minute ventilation affect the speed of onset of anaesthesia?

Answer 12

The higher the minute ventilation the more volatile gas which reaches the alveoli therefore speeding up the onset of anaesthesia.

Question 13

How does increased cardiac output effect the onset of anaesthesia?

Answer 13

Increased cardiac output slows onset of volatile anaesthesia.

As CO increases blood passes more quickly past the alveoli and therefore there is less time for the anaesthetic to diffuse into the bloodstream.

Question 14

What is the blood gas partition coefficient for the following: Nitrous oxide, sevoflurane and isoflurane

Answer 14

Nitrous oxide: 0.47
Sevoflurane: 0.6
Isoflurance 1.4

Question 15

What is the concentration effect?

Answer 15

Nitrous Oxide is delivered at much higher concentrations than other anaesthetic gases (50-70%)

As a result the alveolar concentration rises more rapidly and reaches equilibrium with blood, faster than would be expected from the blood gas partition coefficient alone.

This is called the concentration effect.

Question 16

What is the second gas effect?

Answer 16

When using Nitrous oxide alongside another volatile agent then the alveolar concentration of the second agent will rise more rapidly than if it was used by itself.

As a result induction will be more rapid.

Question 17

What is diffusion hypoxia?

Answer 17

If Nitrous oxide has been used for an anaesthetic.

At the end of the anaesthetic nitrous oxide will diffuse more rapidly back into the alveolus than Nitrogen (from inspired air) will diffuse into the bloodstream. As a result there will be a dilutional drop in O2 in the alveolus.

This is referred to as dilutional hypoxia. It is important that whenever Nitrous oxide is used that supplementary O2 is administered (this is a safety feature in most anaesthetic machines)

Question 18

What are the effects of the volatile anaesthetics on respiratory system?

Answer 18

All the volatile anaesthetic agents cause a dose related reduction in tidal volume.

Nitrous oxide has little effect on minute ventilation.

Isoflurane is an irritant gas which causes coughing and may cause laryngospasm.

Sevoflurane is more potent than isoflurane in reduction of minute ventilation and may cause apnoea.

Question 19

What are the effects of the volatile anaesthetics on the cardiovascular system?

Answer 19

All the volatile anaesthetics cause a dose dependent reduction in SVR and a slight reduction in contractility.

Isoflurane causes a greater drop in SVR and is accompanied by a reflex tachycardia. It has also been associated with coronary steal.

Nitrous oxide cause a slight reduction in contractility which is usually offset by an increase in sympathetic activity.

Question 20

What is coronary steal?

Answer 20

Coronary steal is ischaemic pain associated with dilation of the coronary aa’s in someone with established coronary disease.

In chronically stenosed coronary vessels they receive less blood flow than normal vessels, as a compensatory mechanism there is vasodilation of the vessel distal to the stenosis.

When isoflurane is used and there is vasodilation of the non stenosed coronary vessels this diverts blood flow away from the stenosed vessel causing ischaemia.

Question 21

What are chemical group are the volatile anaesthetics?

Answer 21

Halogenated Ethers

Ethers are organic compounds in which there is an Oxygen bound to 2 aryl or akyl groups e.g. R-O-R.

Halogenated refers to one or more of the Hydrogens being replaced with a halogen.

Question 22

How are the volatile anaesthetics metabolised?

Answer 22

They are metabolised in the liver by CYP2E1, one of the cytochrome P450 family of enzymes.

Sevoflurane is metabolised more than isoflurane, when metabolised it produces lots of Fluoride ions.

Fluoride ions can cause renal impairment, despite this sevoflurane does not cause renal impairment even in prolonged exposure.

Note: Nitrous Oxide is not metabolised.

Question 23

What percentage of sevoflurane is metabolised?

Answer 23

6%

Question 24

What percentage of isoflurane is metabolised?

Answer 24

0.2%

Question 25

Draw the chemical structure for isoflurane?

Answer 25

Google

Question 26

Draw the chemical structure for sevoflurane?

Answer 26

Google