Influence of anaesthesia and analgesia on the control of breathing.

Most agents used to induce and maintain general anaesthesia or to relieve pain or suppress the responses to pain change the control of breathing drastically. They affect the chemical control of breathing, behavioural control or, most often, both. Chemical or metabolic control of breathing is coupled to metabolism and depends on the chemical composition of arterial blood (pH, PCO2, PO2) and brainstem interstitial fluid (pH, brain tissue PCO2). Chemical control occurs during non-rapid-eye-movement sleep and anaesthesia. Behavioural control adjusts breathing in specific situations such as speech, exercise, pain, arousal and stress. An example of agents that affect both control systems are the volatile halogenated anaesthetics. For example, halothane depresses ventilation by abolishing peripheral drive from the chemoreceptors at the carotid bodies, by general depression of respiratory centres in the central nervous system (CNS), and by the suppression of the function of motor neurones, intercostal muscles and the diaphragm (all involved in the chemical control of breathing) and also by the loss of wakefulness drive (behavioural control). 13‐15 30 31