CO2 absorbers allow the use of fresh gas flows (FGF) below minute ventilation (V̇E). Models are developed and tested in vitro to quantify their performance with variable CO2 load (V̇CO2), FGF, V̇E, end-tidal CO2 fraction (FETCO2), and the type of workstation used.
First principles are used to derive a linear relationship between FGF and fractional canister usage or FCU0.5 (the reciprocal of the time for the inspiratory CO2 fraction (FICO2) to reach 0.5%). This forms the basis for 2 basic models in which V̇E was measured either by spirometry (V̇ESpir) or calculated. These models were extended by multiplying V̇E with an empirical workstation factor. To validate the 4 models, two hypotheses were tested. To test whether the FCU0.5 intercept varied proportionally with VCO2 and was independent of V̇E, FCU was measured for 10 canisters tested with a fixed 0.3 L/min FGF and a range of VCO2 while V̇E was either constant or adjusted to maintain FETCO2. A t-test was used to compare the two groups. To confirm whether a change in V̇CO2 accompanied by a change in V̇E to maintain FETCO2 would shift the linear FGF – FCU0.5 relationship in a parallel manner, 19 canisters were tested with different combinations of V̇CO2 and FGF. These measured FCU values were comparted to those predicted by the 4 models using Varvel’s performance criteria.
With 0.3 L/min FGF, FCU0.5 was proportional with V̇CO2 and independent of whether V̇E was adjusted to maintain FETCO2 or not (P = 0.962). The hypothesized parallel shift of the FGF – FCU0.5 relationship was confirmed. Both extended models are good candidate models.
The models predict prepacked canister performance in vitro over the range of V̇E, FGF, and V̇CO2 likely to be encountered in routine clinical practice. In vivo validation is still needed.