Humidification in a Modified Circle System

Abstract

An improved circle system, capable of delivering high inspiratory humidity from the onset of anesthesia, was made by introducing a water vaporizer into the soda lime canister and tested on a model patient. Vaporization was facilitated by the exothermic reaction of neutralization of the lime. Humidity loss due to rain out in the absorber and inspiratory limb was minimized by constructional alterations of the absorber circuit assembly: (1) the conduit pipe which leads the moist absorber gases into the inspiratory limb was moved from its conventional outside position to the warm interior of the lime canister; (2) the moisture-laden fresh gases emerging from the vaporizer were delivered close to the patient by a small bore tube placed concentrically inside the inspiratory limb; and (3) both the fresh and inspired gases were thermally insulated by an outermost, moist, warm coaxial expiratory limb. The humidity output of the circuit was high 1 minute after starting ventilation, averaging 19.6 ± 2, 19.3 ± 2.9, and 19 ± 1.1 mg of H2O/L as carbon dioxide output was increased by 100-ml increments from 100 to 300 ml/min (using fresh gas inflow of 5 L/min and a ventilatory minute volume of 6 L/min). After a 2- to 3-hour period of stabilization, humidity reached 22.8 ± 1.5, 24 ± 0.9, and 28 ± 2.4 mg of H2O/L at the carbon dioxide flows quoted above. Vaporizer patent held by Albert Einstein College of Medicine, Bronx, NY (circuit patent pending).