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You are in Home >> Resources >> Pharmacology >> Inhalational agents

The ideal anaesthetic agent

Created: 24/11/2004

 
 

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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