Pharmacology for Anaesthesia and Intensive Care

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8 General anaesthetic agents

Table 8.5.Factors altering MAC.

Factors increasing MAC Factors decreasing MAC

Infancy During the neonatal period

Increasing age

Pregnancy

Hypotension

Hyperthermia Hypothermia

Hyperthyroidism Hypothyroidism

Catecholamines and sympathomimetics α 2 -agonists

Sedatives

Chronic opioid use Acute opioid use

Chronic alcohol intake Acute alcohol intake

Acute amphetamine intake Chronic amphetamine intake

Hypernatraemia Lithium

Physical


Stable to light and heat


Inert when in contact with metal, rubber and soda lime


Preservative free


Not flammable or explosive


Pleasant odour


Atmospherically friendly


Cheap

Biochemical


High oil:gas partition coefficient; low MAC


Low blood:gas partition coefficient


Not metabolized


Non-toxic


Only affects the CNS


Not epileptogenic


Some analgesic properties

Kinetics of inhaled anaesthetic agents

Atsteady-state, the partial pressure of inhaled anaesthetic within the alveoli (PA)is

in equilibrium with that in the arterial blood (Pa) and subsequently the brain (PB).

Therefore, PAgives an indirect measure of PB. However, for most inhaled anaesthetics

steady-state is rarely achieved in the clinical setting as the process may take many

hours (Figure8.7).

Physiological and agent-specific factors influence the speed at which inhaled

anaesthetics approach equilibrium.