Performance Characteristics of Low-Flow Humidifiers

Abstract

Performance of 18 models of low-flow humidifiers was studied during 8 h operation at flowrates from 2-10 l/min. Amounts of water vaporized varied from 18.1 g at 2 1/min to 62.2 g at 10 l/min. Output range was smaller at low flowrates. Increasing flow from 2 to 4 l/min increased vaporization 69 per cent, but further flow increases produced smaller percentages of increase. Reservoir temperature dropped after 8 h by amounts from 4.4° C at 2 1/min to 10.2° C at 10 l/min, but temperature decrease was not proportional to vaporization decrease, indicating other factors affect vaporization. At flowrates > 2 1/min, 93 to 98 per cent of total temperature drop was not reached until 5th h. Major cause of temperature drop is evaporation heat loss. Effluent gas temperature was not < than incoming gas temperature at 2 1/min but was < at higher flows, up to 2.4° C less at 10 l/min. Reservoir pressure at 2 1/min was 0.16 cm H₂O average for all devices but ranged from 0.5-1.5 cm H₂O in 3 units. At 10 l/min, mean pressure in 13 devices was 10.7 cm H₂O (range 2-32). Initial RH was highest at high flows and lowest at low flows, ranging from 90.6 per cent at 10 l/min to 86.5 per cent at 2 1/min. At 8 h, RH was 68.7 per cent at 10 l/min and a higher 78.6 per cent at 2 1/min. RH at body temperature initially ranged from 46.6 per cent at 10 l/min to 44.7 per cent at 2 1/min, declining at 8 h to 30.2 per cent at 10 l/min and 40.1 per cent at 2 1/min. The most rapid decreases in volume and saturation output occurred in 1st 3 h, indicating that data collected during briefer periods might not reflect longer-range, clinically significant performance. Wide variations exist in performance of various models. Maximal performance occurs at flows of 2 1/min and deteriorates rapidly at higher flows. Reservoir temperature appears to be most important variable. Product literature should specify RH at body temperature after 4 h operation to permit judgment of device's clinical value.