Controlled respiration: standardization of ventilation.

THE technique of controlled respiration during anaesthesia was developed primarily to deal with respiratory inadequacy enforced on the patient by an open pneumothorax. This paper is concerned with developing means to adjust the volume of passive ventilation to a physiological level, the basis at present appearing to be entirely empirical. Originally, in order to institute controlled respiration, active respiration was abolished in two ways or by a combination of the two (Nosworthy, 1941). (1) By depressing the sensitivity of the respiratory centre, especially to carbon dioxide, by anaesthetics or other drugs. (2) By reducing the carbon dioxide tension in the arterial blood by hyperventilation. In addition it is possible that artificial inflation abolished the patient's respiratory efforts by affecting the Hering-Breuer mechanism. At a later date curarizing drugs came into use and, by paralysing the respiratory muscles, made it easier for the anaesthetist to take over the patient's external respiration. The widespread use of these drugs now provides the most frequent and essential indication for the use of controlled respiration. 23 The first two conditions suggest that assisted respiration may not have much practical application: during anaesthesia the respiratory centre is almost invariably depressed by anaesthetics or other depressant drugs. Recent observations revealed that during anaesthesia by thiopentone, pethidine, nitrous oxide and oxygen the alveolar carbon dioxide tension tended to stabilize at about 50 mm Hg (equivalent to 7 per cent carbon dioxide). If under these conditions the respiration is assisted in any efficient way the carbon dioxide tension will fall, so that the second mechanism will act and apnoea will result with full controlled respiration a necessity. In favour of assisted respiration it has been suggested that retention of some spontaneous respiratory effort is advantageous to the circulation; certainly the manner of artificial ventilation and the mean pressure in the respiratory tract are important if the circulation is not to be impaired (Cournand et al, 1947; Werko, 1947), but the present communication is not concerned with these considerations.