In vitro efficiency of 16 different Ca(OH)₂ based CO₂ absorbent brands

Data directly comparing CO₂ absorbents tested in identical and clinically relevant conditions are scarce or non-existent. We therefore tested and compared the efficiency of 16 different brands of Ca(OH)₂ based CO₂ absorbents used as loose fill or a cartridge in a refillable canister under identical low flow conditions. CO₂ absorbents efficiency was tested by flowing 160 mL/min CO₂ into the tip of a 2 L balloon that was ventilated with an ADU anesthesia machine (GE, Madison, WI, USA) with a tidal volume of 500 mL and a respiratory rate of 10/min while running an O₂/air FGF of 300 mL/min. After the 1020 mL refillable container was filled with a known volume of CO₂ absorbent (derived from weighing the initial canister content and the product’s density), the time for the inspired CO₂ concentration (F_ICO₂) to rise to 0.5% was measured. This test was repeated 4 times for each product. Because the two SpiraLith Ca^® products (one with and one without indicator) are delivered as a cartridge, they had to be tested using their proprietary canister. The time (min) for F_ICO₂ to reach 0.5% was normalized to 100 mL of product, and defined as the efficiency, which was compared amongst the different brands using ANOVA. Efficiency ranged from 50 to 100 min per 100 mL of product, and increased with increasing NaOH content (a catalyst), the exception being SpiraLith Ca^® cartridge with color indicator (performing as well as the most efficient granular products) and the SpiraLith Ca^® cartridge without color indicator (outperforming all others). Results indicated a spherical or bullet shape is less efficient in absorbing CO₂ than broken fragments or cylinders, which in turn is less efficient than a hemispherical (disc) shape, which is in turn less efficient than a solid cartridge with a molded channel geometry. The efficiency of Ca(OH)₂ based CO₂ absorbent differs up to 100% on a volume basis. Macroscopic arrangement (cylindrical wrap with preformed channels versus granules), chemical composition (NaOH content), and granular shape all affect efficiency per volume of product. The data can be used to compare costs of the different products.