Pharmacology for Anaesthesia and Intensive Care

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Section IICoredrugs in anaesthetic practice

Table 8.4.Pharmacological properties of some intravenous anaesthetics.

Thiopental Methohexitone Propofol Ketamine Etomidate

BP ↓↓ ↓↓↑→

CO ↓↓ ↓↓↑→

HR ↑↑ ↓→↑→

SVR ↑↓ ↑↓ ↓↓ → →

RR ↓↓ ↓↑↓

ICP ↓↓ ↓↑→

IOP ↓↓ ↓↑→

Pain on injection no yes yes no yes

Nausea and vomiting no no?

reduced

yes yes

renal excretion. Non-specific hepatic esterases and possibly plasma cholinesterase,

hydrolyse etomidate to ethyl alcohol and its carboxylic acid metabolite. It may also

inhibit plasma cholinesterase.

Inhaled anaesthetic agents

Inhaled anaesthetic agents in current use include nitrous oxide (N 2 O) and the volatile

liquids isoflurane, halothane, sevoflurane, desflurane and enflurane. Xenon has use-

ful properties but is expensive to extract from the atmosphere, which currently limits

its clinical use.

Minimum alveolar concentration

Minimum alveolar concentration (MAC) is a measure of potency and is defined as

the minimum alveolar concentration at steady-state that prevents reaction to a stan-

dard surgical stimulus (skin incision) in 50% of subjects at one atmosphere. Because

the majority of anaesthetics involving inhaled agents are given at approximately one

atmosphere the indexing of MAC to atmosphereic pressure may be forgotten and lead

the unwary to conclude that concentration is the key measure. However the key mea-

sure is thepartial pressureof the agent. When measured using kPa the concentration

and partial pressure are virtually the same as atmsopheric pressure approximates to

100 kPa.

MAC is altered by many physiological and pharmacological factors (Table8.5) and

is additive when agents are administered simultaneously.

The ideal inhaled anaesthetic agent

While the agents in use today demonstrate many favourable characteristics, no single

agent has all the desirable properties listed below. ‘Negative’ characteristics (e.g. not

epileptogenic) are simply a reflection of a currently used agent’s side effect.