Arterial carbon dioxide tensions during anaesthesia with manual ventilation. A descriptive study of the effects of various non-polluting circuits.

In 660 supine, intubated and anaesthetized, healthy patients scheduled for various elective surgical procedures, the distribution of arterial carbon dioxide tension (PaCO2) was investigated during manual non-monitored ventilation. The study comprised six equal groups: group 1: ventilation with a circle circuit absorber system; group 2: ventilation with the Hafnia A circuit using a total fresh gas flow (FGF) of 100 ml . kg-1 . min-1; groups 3-6: ventilation with a Hafnia D circuit with fresh gas flows of 100, 80, 70 and 60 ml . kg-1 . min-1, respectively. The mean PaCO2's of the first three groups were situated in the lower range of normocapnia (the observations in the first group having the greatest total range), whereas the rebreathing (Hafnia A and D) circuits resulted in a clustering of observed data. Employing the rebreathing circuits, protection against hypocapnia can be achieved by lowering the fresh gas flow. The most satisfying result was obtained with the Hafnia D circuit with a fresh gas flow of 70 ml . kg-1 . min-1 resulting in normocapnia with a modest and limited spread towards hypo- and hypercapnia. FGF in excess of this level must be considered as wasted. The study indicates that corrections of fresh gas flows for age are superfluous. Use of relaxants and type of surgery had no influence on the observations.