To the Editor:—

Amsorb (Armstrong Medical Ltd., Coleraine, United Kingdom) is a new carbon dioxide (CO2) absorbent that does not degrade the inhalation anesthetics into compound A and carbon monoxide. According to the report by Murray et al. , the CO2absorptive capacity of Amsorb was retained at 85–90% of that of currently available CO2absorbents. 1However, we had unexpected clinical occurrences of CO2rebreathing caused by rapid exhaustion of Amsorb during low-flow anesthesia.

We studied the CO2absorption capacity of Amsorb and of two currently available brands of soda lime: Medisorb (Datex Ohmeda, Bromma, Sweden) and Dragersorb800plus (Drager, Lübeck, Germany) in a model semiclosed breathing system at low flow rates of fresh gas. This study has been performed in two anesthetic machines, Excel (Ohmeda, Madison, WI) and Cato (Drager). Before each trial, two Excel canisters and one Cato canister were filled with 2.4 kg and 1.2 kg of each absorbent, respectively. Oxygen was used as fresh gas at flow rates of 0.5, 1.0, and 2.0 l/min. The anesthetic ventilator was set at an inspiratory:expiratory ratio of 1:2, a respiratory rate of 12 breaths/min, and a tidal volume of 500 ml. A 3-l reservoir bag with a CO2inflow of 0.2 l/min was ventilated mechanically with an anesthetic ventilator. Gas was sampled from the Y-piece at a speed of 200 ml/min and analyzed with use of a capnograph (BP-508; Nihon Colin, Komaki, Japan). The data were recorded at 5-min intervals. The sampling gas was sent to the inspiratory limb of the circuit, and CO2absorptive capacity was determined as the time taken for the inspired CO2tension (Pico2) to increase from 0 to 5 mmHg. The time interval of CO2absorption in Amsorb was approximately 50% less than the conventional absorbents with the previously mentioned fresh gas flow rates and anesthetic machines (table 1).

Table 1. CO2Absorption Capacity in Three Absorbents

Data are given as mean ± SD.

CO2= carbon dioxide; time = time for inspired CO2tension to increase from 0 to 5 mmHg.

Table 1. CO2Absorption Capacity in Three Absorbents
Table 1. CO2Absorption Capacity in Three Absorbents

In the report of Murray et al. , 1the CO2absorptive capacity was measured simply by continuously administrating CO2containing fresh gas through the canister, and CO2tension was measured at the exit port of the canister. We believe that our model, the mechanically ventilated semiclosed breathing system, is more reasonable for the estimation of CO2absorptive capacity because it is based on clinical practice. Although Amsorb is chemically unreactive with inhalational anesthetics, the CO2absorptive capacity is far less from conventional absorbents in low-flow anesthesia.

It is well-known that change of color in soda lime is not a good indicator when it is exhausted in low-flow anesthesia. 2Therefore, the use of Amsorb in cases in which high expiratory CO2is expected, such as in laparoscopic surgeries, must be performed with caution.

Murray JM, Renfrew CW, Bedi A, McCrystal CB, Jones DS, Fee JP: Amsorb: A new carbon dioxide absorbent for use in anesthetic breathing systems. A nesthesiology 1999; 91: 1342–8
Ohrn M, Gravenstein N, Good ML: Duration of carbon dioxide absorption by soda lime at low rates of fresh gas flow. J Clin Anesth 1991; 3: 104–7