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 (⁠V˙CO2⁠), fresh gas flow, V˙E⁠, end-tidal carbon dioxide (ETco2) fraction, and the type of workstation used.


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 V˙CO2 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 V˙CO2 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 V˙CO2 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 V˙CO2 and fresh gas flow. These measured FCU values were compared to those predicted by the four models using Varvel’s performance criteria.


With 0.3 l/min fresh gas flow, FCU0.5 was proportional with V˙CO2 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.


The models predict prepacked canister performance in vitro over the range of V˙E⁠, fresh gas flow, and V˙CO2 likely to be encountered in routine clinical practice. In vivo validation is still needed.

Editor’s Perspective
What We Already Know about This Topic
  • Anesthesia workstations are equipped with canisters that contain calcium hydroxide to absorb exhaled carbon dioxide from rebreathed gases to make rebreathing during low-flow anesthesia possible
  • Fractional canister use, the fraction of a canister that is used per hour, has been reported to have a negative linear relationship with fresh gas flow when studied with a single carbon dioxide inflow and minute ventilation setting
What This Article Tells Us That Is New
  • Four theoretical models of prepacked carbon dioxide absorbent canister life were derived from first principles that incorporate the effects of the clinically encountered range of fresh gas flows, minute ventilation, carbon dioxide production, inspired and expired carbon dioxide concentrations, and dead space ventilation
  • The performance of each model for predicting fractional canister usage was tested in vitro using a single anesthesia workstation and absorbent prepack combination
  • The two extended models that also incorporated an empirical workstation specific factor predicted fractional canister use over a 20 to 100% rebreathing range with excellent bias and accuracy