The erect "active" position is man's normal or "physiological" state. Gravitational stresses, acting directly on the arterial and venous circulations, and indirectly through diaphragmatic mechanics and intra-abdominal transmitted pressures on the lung, are important in determining the net cardiorespiratory response to change in body position. In normal active man, homeostatic compensatory responses maintain physiological normal ranges. In disease states, after immobility or prolonged bed rest, and in the presence of pharmacological agents, these compensatory responses may be attenuated. Under anaesthesia, the degree of compromise which will occur will be determined in part by the baseline against which the anaesthesia is induced. Consideration of all of these factors is thus important in anticipating or predicting the effects of changes in body position during anaesthesia,

Departments of Anaesthesia and Radiology, University of Toronto and Toronto General Hospital, Toronto, Canada

Althesin in dose ranges of 40, 60 and 80 mul/Kg was administered to six healthy unpremedicated volunteers according to a balanced incomplete block design. Both subjective and objective changes resulting from the administration of the agent were studied and analyzed. In the dose ranges used, Althesin was found to have no deleterious effects on heart rate, respiratory rate or blood pressure and apnoea only occurred in one patient who had an end-expiratory CO2 level lower than normal, due to preanaesthetic hyperventilation. The onset of sleep is prolonged with Althesin at the injection rate of 1 ml/10 secs. The onset of anaesthesia is not influenced by the total dose, but the sleep time is directly proportional to the dose injected. Auditory evoked responses tended to lie on a curve which might indicate that with increasing dose the time to reappearance of these responses is not correspondingly prolonged. However, this last observation must be confirmed by studies on a larger scale.