The Science Journal of the American Association for Respiratory Care

2008 OPEN FORUM Abstracts


Daniel P. Dwyer1

Background: Heated passover style humidification is used to add heat and moisture to inspired gases. This study evaluates the humidity levels at the end of the patient circuit during continuous flow applications. Humidification systems analyzed were the Hudson RCI Neptune heater with 2410 Comfort Flo Circuit, Fisher & Paykel MR850 heater with RT329 Continuous Flow Circuit and Vapotherm 2000i with DTPV9007 Heated Delivery Tube.

Methods: Real time humidity levels exiting the circuits were measured utilizing a calibrated Vaisala HMT337 Humidity and Temperature Monitoring Sensor, which has an accuracy of ± 1.0% RH at 0-90% RH and ± 1.7% RH at 90-100%RH. The sensor utilizes a warmed probe to prevent condensation in high humidity applications. The air exiting the circuit was directed into an air chamber placed in a water bath that was adjusted to the heater's indicated patient temperature. In addition, a patient interface, such as a nasal cannula, was not used, in order to negate the affect of its cooling and the subsequent rise in relative humidity. The Hudson RCI Neptune heater includes gradient control which allows for different temperature settings at the proximal and distal ends of the circuit by adjusting the amount of heat supplied to the circuit wires. The Fisher &smp; Paykel heater is preset to 40C at the patient probe and 37C at the heater probe. The Vapotherm heater does not have gradient control.

Conclusion: Using these 3 systems to warm and moisten inspired air results in high humidity levels (79-97%RH). Adjusting the temperature gradient will change the humidity levels and therefore allow control of humidity and condensation in the circuit and patient interface. Humidifiers that have this gradient feature allow various humidification levels for different patient needs and environmental conditions. The humidity levels presented here are measured at the end of the heated circuit and do not take into account the reduction in heat across the unheated patient interface (cannula, tent, etc.) and the subsequent increase in humidity levels as a result of this cooling. Once the humidified air leaves the circuit, there are numerous variables that could change the temperature and therefore change humidity levels leaving the patient interface. Further studies could determine the impact of the patient interface, higher flowrates and environment on humidity levels.