Pharmacology for Anaesthesia and Intensive Care

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Section IICoredrugs in anaesthetic practice

10

FA/FI 05

03060

N 2 O

Desflurane

Sevoflurane

Isoflurane

Halothane

360

Time(minutes)

Figure 8.7.Different agents approach a FA/FIratio of 1 at different rates. Agents with a low

blood:gas partition coefficient reach equilibrium more rapidly. (FA/FIrepresents the ratio of

alveolar concentration to inspired concentration.)

Alveolar ventilation

Increased alveolar ventilation results in a faster rise in PA.Consequently, PBincreases

more rapidly and so the onset of anaesthesia is faster. A large functional residual

capacity (FRC) will effectively dilute the inspired concentration and so the onset of

anaesthesia will be slow. Conversely, those patients with a small FRC have only a

small volume with which to dilute the inspired gas and so PArises rapidly resulting

in a fast onset of anaesthesia.

Inspired concentration

Ahigh inspired concentration leads to a rapid rise in PAand so onset of anaesthesia

is also rapid.

Cardiac output

Ahigh cardiac output will tend to maintain a concentration gradient between the

alveolus and the pulmonary blood so that PArises slowly. Conversely, a low cardiac

output favours a more rapid equilibration and so onset of anaesthesia will also be

more rapid. However, modern anaesthetic agents, which are relatively insoluble in

blood, are affected to a much lesser extent by cardiac output when compared with

agents of greater blood solubility.

Blood:gas partition coefficient

The blood:gas partition coefficient is defined as 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◦C.