THE VARIOUS LEVELS of anaesthesia to which we are wont to refer as stages and planes are defined by means of certain symptoms and physical signs. Unfortunately these vary widely in different individuals. They are also subject to modification by the preliminary medication which may have preceded anaesthesia, by the anaesthetic agent in use, and by the technique by which the agent is administered. Almost all the drugs used as anaesthetic agents bring about changes in the function of respiration. Most of them cause a decrease of respiratory exchange in the deeper strata of anaesthesia, and there naturally result an increase in the carbon dioxide and a diminution of the oxygen contained in the alveolar air. Too often in the past has the drug itself been regarded as the cause of these physiological changes. With every passing day, however, anaesthetists become increasingly conscious of the fact that it is these physiological changes which are directly responsible for the variations in the objective signs of anaesthesia, whereas the agent is only their primary or indirect cause.
*From the Department of Anaesthesia, State of Wisconsin General Hospital.
© 1943 International Anesthesia Research Society