In Vitro Model of Prepacked Carbon Dioxide Absorber Use: Development and Testing.

Background:Carbon dioxide absorbers allow the use of fresh gas flow below minute ventilation (V-E). Models are developed and tested in vitro to quantify their performance with variable carbon dioxide load (VCO2), fresh gas flow, V-E, end-tidal carbon dioxide (ETCO2) fraction, and the type of workstation used. Methods:First principles are used to derive a linear relationship between fresh gas flow and fractional canister usage or FCU0.5 (the reciprocal of the time for the inspiratory carbon dioxide fraction to reach 0.5%). This forms the basis for two basic models in which V-E was measured by spirometry or calculated. These models were extended by multiplying V-E with an empirical workstation factor. To validate the four models, two hypotheses were tested. To test whether the FCU0.5 intercept varied proportionally with VCO2 and was independent of V-E, FCU was measured for 10 canisters tested with a fixed 0.3 l/min fresh gas flow and a range of VCO2 while V-E was either constant or adjusted to maintain ETCO2 fraction. A t test was used to compare the two groups. To confirm whether a change in VCO2 accompanied by a change in V-E to maintain ETCO2 fraction would shift the linear fresh gas flow-FCU0.5 relationship in a parallel manner, 19 canisters were tested with different combinations of VCO2 and fresh gas flow. These measured FCU values were compared to those predicted by the four models using Varvel's performance criteria. Results:With 0.3 l/min fresh gas flow, FCU0.5 was proportional with VCO2 and independent of whether V-E was adjusted to maintain ETCO2 fraction or not (P = 0.962). The hypothesized parallel shift of the fresh gas flow-FCU0.5 relationship was confirmed. Both extended models are good candidate models. Conclusions:The models predict prepacked canister performance in vitro over the range of V-E, fresh gas flow, and VCO2 likely to be encountered in routine clinical practice. In vivo validation is still needed.