We performed this study to determine the effect of heated wire circuit
s on humidity delivered by several humidifiers commonly used during an
esthesia. Humidifier bases from three manufacturers with several humid
ifying chambers for each base were tested using both heated and unheat
ed circuits. Delivered humidity and circuit temperature were measured
at the distal end of the circuit. Each chamber was tested under both c
ontinuous flow (CF) and intermittent flow (IF) conditions. IF was prov
ided by a Servo 900 B ventilator. Adjustments of flow and volume were
made for pediatric chambers. At 90 L/m CF, humidifier performance was
unsatisfactory with both heated and unheated circuits. At 15 and 45 L/
m CE both distal circuit temperature (DCT) and humidity were within ac
ceptable limits; unheated circuits delivered more humidity than heated
circuits (P < 0.05). Relative humidity was lower at 45 L/m CF than at
15 L/m (P < 0.05). During IF while all chambers provided satisfactory
humidity, humidity was greater when unheated circuits were used. We c
onclude that when heated circuits are used, humidifying systems may fa
il to deliver 100% humidity to patients.