Humidification of rapidly flowing gas

Modern ventilatory systems that provide intermittent mandatory ventilation (IMV) and continuous positive airway pressure (CPAP) require high rates of flow of dry gases. Because inadequately humidified gas may contribute to respiratory morbidity, the authors conducted a laboratory study of the performance characteristics of four commercially available humidifiers during simulated clinical conditions. Performance was assessed with gases flowing continuously during simulated IMV and during controlled mechanical ventilation (CMV). A separate investigation assessed the thermal stability of and the resistance to flow through each device. It was found that the performances of the devices tested varied widely. With CMV, three of four humidifiers delivered humidified gas at 37 degrees C; however, with the greater total rate of flow of gas during simulated IMV, only one device provided humidified gas at 37 degrees C. Similarly, the thermal stability of the devices varied widely: two humidifiers' temperatures varied little during the study period; the temperatures on the other two devices varied more than 10 degrees C. The effectiveness of humidification and warming varied inversely with the rate of flow. For all devices, resistance to flow of gas was less than than 5 cm H2O/L . sec at rates of flow less than or equal to 60 L/min. The authors conclude that the performance characteristics of commercially available humidifiers vary widely.