Memsorb™, a novel CO₂ removal device part I: in vitro performance with the Zeus IE®

Soda lime-based CO₂ absorbents are safe, but not ideal for reasons of ecology, economy, and dust formation. The Memsorb™ is a novel CO₂ removal device that uses cardiopulmonary bypass oxygenator technology instead: a sweep gas passes through semipermeable hollow fibers, adding or removing gas from the circle breathing system. We studied the in vitro performance of a prototype Memsorb™ used with a Zeus IE® anesthesia machine when administering sevoflurane and desflurane in O₂/air mixtures. The Zeus IE® equipped with Memsorb™ ventilated a 2L breathing bag with a CO₂ inflow port in its tip. CO₂ kinetics were studied by using different combinations of CO₂ inflow (VCO₂), Memsorb™ sweep gas flow, and Zeus IE® fresh gas flow (FGF) and ventilator settings. More specifically, it was determined under what circumstances the inspired CO₂ concentration (F_ICO₂) could be kept < 0.5%. O₂ kinetics were studied by measuring the inspired O₂ concentration (F_IO₂) resulting from different combinations of Memsorb™ sweep gas flow and O₂ concentrations, and Zeus IE® FGFs and O₂ concentrations. Memsorb™'s sevoflurane and desflurane waste was determined by measuring their injection rates during target-controlled closed-circuit anesthesia (TCCCA), and were compared to historical controls when using a soda lime absorbent (Draegersorb 800+) under identical conditions. With 160 mL/min VCO₂ and 5 L/min minute ventilation (MV), lowering the sweep gas flow at any fixed Zeus IE® FGF increased F_ICO₂ in a non-linear manner. Sweep gas flow adjustments kept F_ICO₂ < 0.5% over the entire Zeus IE® FGF range tested with VCO₂ up to 280 mL/min; tidal volume and respiratory rate affected the required sweep gas flow. At 10 L/min MV and low FGF (< 1.5 L/min), even a maximum sweep flow of 43 L/min was unable to keep F_ICO₂ ≤ 0.5%. When the O₂ concentration in the Zeus IE® FGF and the Memsorb™ sweep gas flow differed, F_IO₂ drifted towards the sweep gas O₂ concentration, and more so as FGF was lowered; this effect was absent once FGF > minute ventilation. During sevoflurane and desflurane TCCCA, the Zeus IE® FGF remained zero while agent usage per % end-expired agent increased with increasing end-expired target agent concentrations and with a higher target F_IO₂. Agent waste during target-controlled delivery was higher with Memsorb™ than with the soda lime product, with the difference remaining almost constant over the FGF range studied. With a 5 L/min MV, Memsorb™ successfully removes CO₂ with inflow rates up to 240 mL/min if an F_ICO₂ of 0.5% is accepted, but at 10 L/min MV and low FGF (< 1.5 L/min), even a maximum sweep flow of 43 L/min was unable to keep F_ICO₂ ≤ 0.5%. To avoid F_IO₂ deviating substantially from the O₂ concentration in the fresh gas, the O₂ concentration in the fresh gas and sweep gas should match. Compared to the use of Ca(OH)₂ based CO₂ absorbent, inhaled agent waste is increased. The device is most likely to find its use integrated in closed loop systems.