Abstract
Soda lime-based CO2 absorbents are safe, but not ideal for reasons of ecology, economy, and dust formation. The Memsorb (TM) 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 (TM) used with a Zeus IE (R) anesthesia machine when administering sevoflurane and desflurane in O-2/air mixtures. The Zeus IE (R) equipped with Memsorb (TM) 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 (TM) sweep gas flow, and Zeus IE (R) fresh gas flow (FGF) and ventilator settings. More specifically, it was determined under what circumstances the inspired CO2 concentration (FICO2) could be kept < 0.5%. O-2 kinetics were studied by measuring the inspired O-2 concentration (FIO2) resulting from different combinations of Memsorb (TM) sweep gas flow and O-2 concentrations, and Zeus IE (R) FGFs and O-2 concentrations. Memsorb (TM)'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 (R) FGF increased FICO2 in a non-linear manner. Sweep gas flow adjustments kept FICO2 < 0.5% over the entire Zeus IE (R) 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 FICO2 <= 0.5%. When the O-2 concentration in the Zeus IE (R) FGF and the Memsorb (TM) sweep gas flow differed, FIO2 drifted towards the sweep gas O-2 concentration, and more so as FGF was lowered; this effect was absent once FGF > minute ventilation. During sevoflurane and desflurane TCCCA, the Zeus IE (R) FGF remained zero while agent usage per % end-expired agent increased with increasing end-expired target agent concentrations and with a higher target FIO2. Agent waste during target-controlled delivery was higher with Memsorb (TM) than with the soda lime product, with the difference remaining almost constant over the FGF range studied. With a 5 L/min MV, Memsorb (TM) successfully removes CO2 with inflow rates up to 240 mL/min if an FICO2 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 FICO2 <= 0.5%. To avoid FIO2 deviating substantially from the O-2 concentration in the fresh gas, the O-2 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.