Carbon dioxide rebreathing with the anaesthetic conserving device, AnaConDa

The anaesthetic conserving device (ACD) AnaConDa was developed to allow the reduced use of inhaled agents by conserving exhaled agent and allowing rebreathing. Elevated has been observed in patients when using this ACD, despite tidal volume compensation for the larger apparatus dead space. The aim of the present study was to determine whether CO2, like inhaled anaesthetics, adsorbs to the ACD during expiration and returns to a test lung during the following inspiration. The ACD was attached to an experimental test lung. Apparent dead space by the single-breath test for CO2 and the amount of CO2 adsorbed to the carbon filter of the ACD was measured with infrared spectrometry. Apparent dead space was 230 ml larger using the ACD compared with a conventional heat and moisture exchanger (internal volumes 100 and 50 ml, respectively). Varying CO2 flux to the test lung (85375 ml min(1)) did not change the measured dead space nor did varying respiratory rate (1224 bpm). The ACD contained 3.3 times more CO2 than the predicted amount present in its internal volume of 100 ml. Our measurements show a CO2 reservoir effect of 180 ml in excess of the ACD internal volume. This is due to adsorption of CO2 in the ACD during expiration and return of CO2 during the following inspiration.