PC3066: Dew Point Considerations, Condensation, and Rainout with High Flow Nasal Cannula

Background: We speculated that using a heated, humidified nasal cannula may produce inadvertent delivery of free water above intended maintenance fluid volumes. If the entrained temperature matches that of the delivery environment, the humidification present will not dissipate through condensation. However, if there is even a slight decrease in the patient or environmental temperature, a significant rainout may occur.
Is the rainout associated with a temperature drop from nasal cannula entrainment to the nares or environment significant to warrant concern?

Methods: A model of humidification of entrained 21% oxygen gas was created. A series of samples were simulated at 35 and 40 oC with the assumption of 100% humidification in the circuit. Relative humidity (RH) is the ratio of the actual amount of water vapor in the air to the amount it could hold when saturated is expressed as a percentage. RH = 100 x (e/ew). Absolute humidity (AH) is the mass of water vapor in a unit volume of air and measures the air's water vapor content. AH = mv/V. The actual vapor pressure from the relative humidity and saturation vapor pressure is e=ew x RH/100. PV = m/M x RT uses the general law of perfect gases. This is eV =mRvT, where R is the universal gas constant defined as 8313.6, and Rv is the specific gas constant for water vapor defined as 461.5. Then, the mass volume ratio is m/V = e/RvT which is absolute humidity. Flows of 1, 5, 10, 20, 30, and 40 LPM were assessed with a presumed temperature delta of 5 oC. Water condensation (rainout) per minute, hour, and day was calculated based on a drop of 5 oC from 35 and 40 oC at the associated flow rates. A closed environment such as an isolette was presumed.

Results: The results are as indicated graphically. Higher temperature gradients and high flow were associated with an increased potential percentage of condensation. Inferentially, for each increase in liter flow, and drop in environmental temperature, a predictable amount of H2O/day was lost to changing dewpoint.

Conclusion: The administration of a high flow nasal cannula into a heated and humidified isolette should not be associated with condensation as long as the dew point is not exceeded and there is sufficient leak to allow the egress of the high flow in excess of normal minute ventilation. However, temperature maintenance within the isolette is not guaranteed, convective cooling may occur, and potential cold spots or the plastic interface to the room can result in “rain out.” Minimization of temperature gradient without the thermoneutral environment is critical to prevent rainout