EFFECTS OF HALOTHANE, ETHER AND CYCLOPROPANE ON RESPIRATION BY

Dogs have been anaesthetized with halothane, ether and cyclopropane, and the depth was kept constant at any desired level by maintaining an unvarying gas concentration in alveolar air. Under these steady-state conditions carbon dioxide response curves have been elicited at different depths of anaesthesia. All three anaesthetic drugs caused a progressive reduction of ventilation volumes as the concentration was increased. In one subject there was an augmentation of ventilation at moderate levels of ether anaesthesia. Halothane, ether and cyclopropane all produced apnoea at an alveolar concentration approximately 2.2 times the minimum concentration required to main-tain surgical anaesthesia. Anaesthetic drugs are generally recognized as respiratory depressants. Though this effect is pre-sumed to have a dose-response relationship, neither dose nor response has lent itself to easy definition. Most early attempts to define respira-tory responses consisted of a simple descriptive account of changes in rate and depth of respira-tion under general anaesthesia. Such observations later gave place to the measurement of tidal volume and minute ventilation, the latter being a useful measure of the total output of the res-piratory control centres. The calculation of alveolar ventilation offered a refinement which is more closely related to carbon dioxide excretion rate (Tenney, 1956; Bellville et al., 1959). When minute ventilation (VE) is plotted against arterial carbon dioxide tension (Paoos), the result is the familiar carbon dioxide response curve. This curve permits a distinction to be made between changes in sensitivity of the respiratory control centres, evidenced by changes in slope of the curve, and changes in respiratory threshold to carbon dioxide, which result in parallel displace-ment