In vitro model of prepacked carbon dioxide absorber use: development and testing

CO₂ absorbers allow the use of fresh gas flows (FGF) below minute ventilation (V̇_E). Models are developed and tested in vitro to quantify their performance with variable CO₂ load (V̇CO₂), FGF, V̇_E, end-tidal CO₂ fraction (F_ETCO₂), and the type of workstation used.

First principles are used to derive a linear relationship between FGF and fractional canister usage or FCU_0.5 (the reciprocal of the time for the inspiratory CO₂ fraction (F_ICO₂) to reach 0.5%). This forms the basis for 2 basic models in which V̇_E was measured either by spirometry (V̇_ESpir) or calculated. These models were extended by multiplying V̇_E with an empirical workstation factor. To validate the 4 models, two hypotheses were tested. To test whether the FCU_0.5 intercept varied proportionally with VCO₂ and was independent of V̇_E, FCU was measured for 10 canisters tested with a fixed 0.3 L/min FGF and a range of VCO₂ while V̇_E was either constant or adjusted to maintain F_ETCO₂. A t-test was used to compare the two groups. To confirm whether a change in V̇CO₂ accompanied by a change in V̇_E to maintain F_ETCO₂ would shift the linear FGF – FCU_0.5 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, FCU_0.5 was proportional with V̇CO₂ and independent of whether V̇_E was adjusted to maintain F_ETCO₂ or not (P = 0.962). The hypothesized parallel shift of the FGF – FCU_0.5 relationship was confirmed. Both extended models are good candidate models.

The models predict prepacked canister performance in vitro over the range of V̇_E, FGF, and V̇CO₂ likely to be encountered in routine clinical practice. In vivo validation is still needed.