TITLE: THE EFFECT OF HEATING WIRE ON RELATIVE HUMIDITY OF INSPIRED GAS

AUTHORS: Yoshiro Kobayashi, MD, Kimiaki Ai, MD, Yasuhiro Innami, MD, Norihito Nakamura, MD, Junichi Noguchi, MD
AFFILIATION: Kawasaki Municipal Hospital, Kawasaki, Japan.

Recent studies demonstrated that relative humidity (RH), rather than absolute humidity (AH), was a dominant factor in the case of drying secretions in the upper airway when using a humidifier with heated wire circuits. The Hudson RCI Concha-Therm IV (Hudson Respiratory Care, Inc., Temecula, CA) has the control function to set the difference (deltaT) between the gas temperature delivered to the patient and the temperature at the humidification chamber outlet by a heating wire. We performed this study to measure the effect of heating wire on the humidity of delivered gas in laboratory simulations of clinical circumstances.
METHODS: The hygrometric performance of Concha-Therm IV was tested under intermittent flow condition provided by a Servo 300 ventilator (Siemens Elema AB, Sweden) with the following settings: inspiratory flows (IF) of 30L/min and 45L/ min; inspiratory time of 2s; respiratory rate of 12 breaths/min. Servo control was adjusted to maintain a distal circuit temp- erature (DCT) of both 34_C and 37_C. Measurements were made at each DCT with the following deltaT settings: -3, 0, and +3. Delivered humidity and circuit temperature were measured with electric hygrometer, MOISCOPE (S.K.I.Net, Japan) at the distal end of the circuit.
RESULTS: Higher RH was observed at 37°C than at 34°C in each (DCT and IF) setting. RH decreased with increasing deltaT. Humidifier performance was satisfactory (near 100%RH), when DCT was maintained at 3 degrees lower than the temperature of the humidification chamber outlet (i.e. deltaT: -3). In contrast, the results of RH (deltaT: +3) were mean51.4+/-SD3.9 (DCT: 37°C; IF: 45L/min), 71.6+/-1.5 (37°C; 30L/min), 37.3+/-3.0 (34°C; 45L/min), and 58.6+/-1.8 (34°C; 30L/min). Humidifier perform- ance was unsatisfactory (RH<75%, AH<33mg/L) when deltaT was set to +3.
CONCLUSION: The optimum level of humidity necessary to prevent drying of secretions and deleterious effects on ciliary function is still a matter of debate. However, the elimination of unwanted condensation by heated wire circuits can be associated with a marked decrease in relative humidity of inspired gas. We conclude that, if information about inhaled water content is unavailable, the gas temperature delivered to the patient must not be set above the temperature of the humidification chamber outlet in using a humidifier with a heating wire.