Ventilation and End‐tidal Carbon Dioxide Tension

It is well known that considerable deviations from the normal minute volume may occur during anasthesia. During spontaneous respiration there have been reports of hypo-ventilation and occasionally slight hyperventilation 1 2 3 4 5 6 . Fewer measurements have been made during artificial ventilation but it is apparent that, as the hand of the anasthetist is the controlling factor, wide variations may be expected6. When there is only negligible carbon dioxide in the inspired gas, the alveolar carbon dioxide concentration is dependent upon the alveolar ventilation to which it bears an inverse relation. The carbon dioxide tension of the arterial blood is generally close to the value for the alveolar pas and in recent years very high tensions have been reported during anasthesia’ 8 9. However the relationship between ventilation and carbon dioxide tension during ansesthesia has seldom been investigated under defined and reproduceable conditions. In particular, re-inhaled carbon dioxide and apparatus dead space have a considerable influence upon the carbon dioxide tensions produced by a particular minute volume. Not only is the relationship between ventilation and carbon dioxide tension of interest from the standpoint of pulmonary function during ansesthesia, but it is also of practical importance for predicting the carbon dioxide tension from the ventilation. The measurement of ventilation is considerably easier than the determination of the carbon dioxide tension in either arterial blood or alveolar gas and it would therefore be valuable if the minute volume could be used as an approximate indication of the respiratory acidbase balance. The present study reports the simultaneous measurement of ventilation and end-tidal carbon dioxide tension during anasthesia. Normal methods of anzesthesia were employed by anzsthetists who were not directly concerned with the investigation. Thus data were obtained on