An Evaluation of Water-Vapor Output from Four Brands of Unheated,Prefilled Bubble Humidifiers

We evaluated the water-vapor output from four brands of unheated, prefilled bubble humidifiers—the Aerwey 300, the Aquapak 301, the McGaw 250, and the Travenol 500—at oxygen flowrates of 2, 4, 6, and 8 l/min. We recorded relative humidity (RH) and temperature in a test chamber at intervals over a minimum period of 24 h, and we calculated water-vapor content (WVC) and corresponding RH at 37°C. Ambient RH was 53.4% ± 5.25% (mean ± 1 SD) at a test-chamber temperature of 26.2°C ± 0.79°C. Mean 24-h WVC at 2 l/min (n = 20) ranged from 20.4 ± 0.63 mg/l (McGaw) to 17.2 ± 0.76 mg/l (Aerwey). This corresponded to a range of RH at 37°C of 46.6% ± 1.43% (McGaw) to 39.1% ± 1.72% (Aerwey). A statistically significant difference in mean WVC existed between brands of humidifiers (P < 0.001). The McGaw and Travenol humidifiers consistently delivered hydrated gas with a greater WVC than did either the Aerwey or Aquapak humidifiers (P < 0.001). A statistically significant decrease in mean WVC and RH at 37°C occurred as the oxygen flowrate increased (P < 0.001). None of the humidifiers was able to hydrate the source gas to a mean equivalent of 50% RH at 37°C, at any flowrate. The design of the humidifier bubble diffuser and the source-gas flowrate appear to be the prime determinants of the humidification efficiency of unheated, prefilled bubble humidifiers.