Humidity in anaesthesiology. II. Evolution of heat and moisture in the large carbon dioxide absorbers.

Abstract

The evolution of heat and moisture in the large modern absorbers have been investigated both in the laboratory and in the operating room, using the semi-closed circle. Our results suggest the following conclusions: 1. Little heat is evolved at the inflow site of the canister during clinical anaesthesia, with a 4 L./min. fresh gas inflow from the debimeters. In our experiments with a standard ventilation of 10 L./min., carbon dioxide averaged 2 per cent at the gas inlet to the canister. Heat produced by the exothermic reaction in this area is in part cancelled by the latent heat of vaporization. 2. Accordingly, the temperature at the lower end of the canister is little influenced by CO2 absorption and remains near room temperature. 3. Within certain limits, perigranular PH2o is unaffected by flow. However, it is highly dependent upon temperature changes. At a room temperature of 24° C, perigranular water vapour tension is constant at 17.8 mm. Hg. This corresponds to a relative humidity of 80 per cent in the perigranular space. 4. Gas mixtures reach the outflow tract of the absorbent in a hygroscopic equilibrium with the perigranular PH2o found in the lower chamber of the soda-lime. 5. The humidity content of the inhaled gas mixture is supplied by two main sources: the moisture content of the granules and the water vapour from exhaled air. The contribution of each source is determined by the volume of the dry diluent gases added to expired gases at the top of the absorber. 6. Carbon dioxide absorption probably does not directly contribute water to humidify the inspired gas. However, it may stimulate water evaporation from the granules warmed by this exothermic reaction. 7. The relative humidity of inspired air may be predicted by monitoring the temperature in the lower part of the absorbent.