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Assignment of a drug to the sedative-hypnotic class indicates that its major therapeutic use is to cause sedation (with concomitant relief of anxiety) or to encourage sleep. Because there is considerable chemical variation within this group, this drug classification is based on clinical uses rather than on similarities in chemical structure. Anxiety states and sleep disorders are common problems, and sedative-hypnotics are widely prescribed drugs worldwide. Drugs in this class include alcohols, benzodiazepines, barbiturates, carbamates, and several newer hypnotics, including eszopiclone, zaleplon, and zolpidem (Figure 13–1).

Figure 13–1.

Drugs used as sedative-hypnotics. Benzodiazepine and barbituratedrug classes are further subdivided into short-, intermediate-, and long-acting groups depending on their respective half-lives.

An effective sedative (anxiolytic) agent should reduce anxiety and exert a calming effect. The degree of central nervous system (CNS) depression caused by a sedative should be the minimum consistent with therapeutic efficacy. A hypnotic drug should produce drowsiness and encourage the onset and maintenance of a state of sleep. Hypnotic effects involve more pronounced depression of the CNS than sedation, and this is achieved with most drugs in this class simply by increasing the dose. Graded dose-dependent depression of CNS function is a characteristic of sedative-hypnotics. However, individual drugs differ in the relationship between the dose and the degree of CNS depression. Two examples of such dose-response relationships are shown in Figure 13–2. The linear slope for barbiturates means an increase in dose above that needed for hypnosis may lead to a state of general anesthesia. At still higher doses, these types of sedative-hypnotics may depress respiratory and vasomotor centers in the medulla, leading to coma and death. Deviation from a linear dose-response relationship, as shown for benzodiazepine drugs, will require much greater dosage increments in order to achieve CNS depression more profound than hypnosis. This greater margin of safety for benzodiazepines and the newer hypnotics (e.g., zolpidem) is an important reason for their widespread use to treat anxiety states and sleep disorders.

Figure 13–2.

Dose-response curves demonstrating the relationships between dose of benzodiazepines and barbiturates and their CNS effects. Benzodiazepines have a higher therapeutic index than barbiturates.

Lipid solubility plays a major role in determining the rate at which a particular sedative-hypnotic enters the CNS. Most of the sedative-hypnotic drugs are lipid soluble and are absorbed well from the gastrointestinal tract, with good distribution to the brain. This property is responsible for the rapid onset of CNS effects of triazolam, thiopental (Chapter 15), and the newer hypnotics. Oral absorption of triazolam and the newer hypnotics is extremely rapid, and that of diazepam and the active metabolite of clorazepate is more rapid than other commonly used benzodiazepines. Clorazepate is converted to its active form, desmethyldiazepam (nordiazepam), by acid hydrolysis in the stomach. Most of the barbiturates and ...

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