CHAPTER 13. DRUGS AFFECTING THE RESPIRATORY SYSTEM

According to the direction of their action, respiratory stimulants are subdivided into the following groups (Fig. 13.1):

• Drugs affecting the respiratory center directly

- Bemegride

- Caffeine

- Aethimizolum

• Reflex respiratory stimulants

- Cytiton

- Lobeline

• Drugs of the mixed type of action

- Nikethamide (cordiaminum)

- Carbon dioxide

Drugs that stimulate the respiratory center directly are caffeine (that is known to be a psychostimulating agent and an analeptic [See Chapter 11.6]), bemegride (analeptic) and aethimizolum¹. The last agent has a special place among centrally acting respiratory stimulants. It activates both the subcortical structures of the brain and medulla oblongata. Aethimizolum differs from typical analeptics because it suppresses the cerebral cortex, which is why it is used not only as a respiratory stimulant, but also as a sedative to treat anxiety. This drug can be taken orally or parenterally.

Reflex respiratory stimulants include N-cholinomimetics such as cytiton and lobeline (see Chapter 3.4). The mechanism

¹ It is a derivative of imidazole dicarboxylic acid (see structure).

Fig. 13.1. The main directions of action of the drugs affecting the functions of the respiratory organs.

of their action is the activation of N-cholinoceptors of the carotid body from where afferent impulses run to the medulla oblongata and increase the activity of the respiration center. The above mentioned cholinomimetics have a short-term effect (minutes). They are used for intravenous injections only.

The central effect of the drugs of the mixed type of action is supplemented by the stimulating influence on carotid chemoreceptors. Nikethamide (see Chapter 12) and carbon dioxide belong to such agents. The last one is known to be a physiological respiration stimulant.

In medical practice the combination of 5-7% CO₂ and 93-95% O₂ is usually used. This mixture is called carbogen. When a healthy person inhales carbogen, his respiratory volume increases 5-7 times. Stimulation of respiration occurs mainly due to a decrease in pH and an accumulation of H⁺ in the respiratory center. Hydrogen ions are believed

to stimulate the cells of the respiratory center indirectly, through some specific chemoreceptors which are situated in the medulla oblongata near the respiratory center. The reflexes of the carotid sinus underlie respiratory response to CO₂. Stimulation of respiration caused by CO₂ develops within the first 5-6 minutes. It also improves the general circulation. There is evidence that CO₂ also increases the cortical blood flow.

Respiratory stimulants are used to treat mild intoxication with the opioid analgesics and carbon oxide as well as asphyxia of newborns. They are also used to improve the essential levels of lung ventilation in the postanesthetic period. In general, respiratory stimulants are used very rarely. Hypoxia is usually treated with assisted or artificial respiration.