Physical properties
1. Non-flammable, non-explosive at room temperature
2. Stable in light.
3. Liquid and vaporisable at room temperature i.e. low latent heat of vaporisation.
4. Stable at room temperature, with a long shelf life
5. Stable with soda lime, as well as plastics and metals
6. Environmentally friendly - no ozone depletion
7. Cheap and easy to manufacture
Biological properties
1. Pleasant to inhale, non-irritant, induces bronchodilatation
2. Low blood:gas solubility - i.e. fast onset
3. High oil:water solubility - i.e. high potency
4. Minimal effects on other systems - e.g. cardiovascular, respiratory,
hepatic, renal or endocrine
5. No biotransformation - should be excreted ideally via the lungs, unchanged
6. Non-toxic to operating theatre personnel
MAC: minimum alveolar concentration
The minimum alveolar concentration of inhalational anaesthetic agent is the concentration that prevents movement in response to skin incision in 50% of (unpremedicated animals) subjects studied at sea level (1 atmosphere), in 100% oxygen. Hence, it is inversely related to potency.
The rationale for this measure of anaesthetic potency is:
 Alveolar concentration can be easily measured
Near equilibrium, alveolar and brain tensions are virtually equal
The high cerebral blood flow produces rapid equilibration
Factors which support the use of this measure are:
MAC is invariant with a variety of noxious stimuli
Individual variability is small
Sex, height, weight & anaesthetic duration do not alter MAC
Doses of anaesthetics in MAC's are additive
Factors Which Affect MAC
Increase MAC
Hyperthermia
Hypernatraemia
Sympathoadrenal stimulation
Chronic alcohol abuse
? Chronic opioid abuse
Increases in ambient pressure
Hypercapnia
Decreasing age
Thyrotoxicosis
Decrease MAC
Nitrous oxide
Hypothyroid/myxoedema
Hypocapnia
Hypothermia-decrease is roughly linear
Hyponatraemia
Increasing age - (MAC Hal < 3 months ~ 1.1 % MAC Hal > 60 years ~ 0.64 %)
Hypoxaemia
Hypotension
Anaemia
Pregnancy
CNS depressant drugs - opioids, benzodiazepines, major tranquilizers, TCA's
Other drugs - lithium, lidocaine, magnesium, methyl dopa, clonidine
Acute alcohol abuse
No change in MAC
Sex
Weight
Duration of anaesthesia
Hypo/hyperkalaemia
PO2 > 40 mmHg
MAP > 40 mmHg
Partition coefficient
This is the ratio of the amount of substance in one phase to the amount in another phase at a stated temperature, with the two phases being of equal volume and at equilibrium with each other. The blood/gas coefficient is the ratio of the amount of anaesthetic in blood and gas when the two phases are of equal volume and pressure and in equilibrium at 37 degrees C. It is the partial pressure of the agent in the blood and hence the brain that gives rise to anaesthesia. Therefore, agents with a low blood:gas coefficient exert a high partial pressure and therefore a more rapid onset/offset of action. The oil:gas coefficient is an index of potency and is inversely related to MAC. The action of anaesthetic agents is suggested to be related to the lipid solubility (Meyer-Overton theory).
Summary of physical properties
|
Halothane |
Isoflurane |
Enflurane |
Desflurane |
Sevoflurane |
Molecular
weight |
197 |
184 |
184 |
168 |
200 |
Boiling
point (degrees C) |
50.2 |
48.5 |
56.5 |
22.8 |
58.5 |
| SVP at 20 degrees C |
243 |
238 |
175 |
669 |
157 |
| MAC in 100% O2 |
0.75 |
1.15 |
1.8 |
6 |
2.05 |
| MAC in 70% N2O |
0.29 |
0.56 |
0.57 |
2.5 |
0.66 |
% Biotrans
formation |
20 |
0.2 |
2 |
<0.1 |
3 - 5 |
| Blood / gas |
2.2 |
1.36 |
1.91 |
0.45 |
0.6 |
| Oil / gas |
224 |
98 |
98.5 |
28 |
47 |
References
[i] A brief history of the origin of minimum alveolar concentration (MAC).
Eger EI 2nd
Anesthesiology 2002; 96(1): 238-9
[ii] Myocardial protection by anesthetic agents against ischemia-reperfusion injury: an update for anesthesiologists.
Kato R, Foex P
Can J Anaesth 2002; 49(8): 777-91
Related examination questions
1. List the properties of ideal anaesthetic agent under the following headings:
- Physical properties
- Biological properties
ArticleDate:20041124
SiteSection: Article
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