Many drugs act on the central nervous system (CNS), with specific aims in mind. While certain categories of drugs are considered elsewhere (anaesthetic gases and vapours in Section 3, Chapter 5, hypnotics and intravenous agents in Chapter 6) other drugs acting on the CNS have been grouped here. Anti-emetic agents are considered in detail, with specific pharmacology of individual agents to reflect their direct relevance to the practice of anaesthesia.

Anti-emetic agents

The causes of nausea and vomiting (NV) are legion, as illustrated by Figure CN1, and anti-emetic therapy is most effective when directed at the likely origin.

Postoperative nausea and vomiting (PONV) is a specific entity. Its treatment is more appropriately directed when other risk factors are considered, and these are summarised in Figure CN2.

Two distinct sites in the CNS, the vomiting centre and the chemoreceptor trigger zone, are implicated in the causes of NV. The chemoreceptor trigger zone lies in the area postrema outside the blood–brain barrier and possesses dopaminergic (D2) and serotonergic (5-hydroxytryptamine, 5-HT3) receptors. In contrast, the vomiting centre is a complex entity located in the dorsolateral reticular formation of the brain stem that possesses 5-HT3, D2 and muscarinic (M3) receptors. Histaminic (H1) and neurokinin (NK1) receptors are located in the nucleus of the tractus solitarius, which integrates afferent signals associated with emesis. The interaction of various drugs with these sites is shown in Figure CN3.