Inspiratory gas temperature in the endotracheal tube when ventilated with a heated humidifier

International Organization of Standardization recommends that inspiratory gas should possess absolute humidity of more than 33 mg/l. In this study we measured the temperature in the endotracheal tube (ETT) with a hypothesis that actual temperature of inspiratory gas is lower than that set by heated humidifier resulting in an inadequate humidification.

Methods

A lung model (TTL, Michigan Instruments) was ventilated using a PB840 ventilator (Puritan-Bennett) via an 8 mm ETT. A heated humidifier (MR290, Fisher & Paykel) was set in the inspiratory limb of a standard ventilator circuit with a target temperature of 37°C. The ventilator settings were: volume control mode, rate 10/min, PEEP 0 cmH₂O with various tidal volumes (V_T). The temperature of inspiratory gas was measured at 22 cm, 27 cm and 32 cm from the distal end of the ETT using a thin wire thermocouple thermometer.

Results

Mean inspiratory temperatures (°C) are shown in Table 1.

Table 1

Conclusion

The actual mean inspiratory gas temperature was lower than the target temperature set by the humidifier, especially with low V_T, which may result in an inadequate humidification.