Control of carbon dioxide in modified Mapleson A and D (Hafnia) anaesthetic systems. An experimental model.

The effects of varying ventilations (VE) and fresh gas flows (FGF) on end-expiratory CO2 (FECO2) levels were investigated in an experimental model lung, employing the Hafnia modification of the Mapleson A and D anaesthetic systems during CO2-absorption and CO2-wash-out (rebreathing). Identical results were found in both systems: FECO2 was constant and independent of FGF with CO2-absorption and constant VE, whereas rebreathing resulted in increasing FECO2 levels as FGF was decreased. As control of FECO2 in the rebreathing systems by regulating FGF could only take place within FECO2 levels higher than that determined by VE at complete CO2-absorption, e.g. for the Hafnia A and D rebreathing systems, control of FGF necessitates relative hyperventilation. FECO2 with constant FGF decreased with increasing VE during CO2-absorption, as well as during rebreathing, although this decrease was less in the rebreathing systems. Thus a decrease in FECO2 with rising VE can be avoided and hypocapnia prevented. The results agree with those obtained in clinical studies.