Inspired gas humidity during mechanical ventilation: effects of humidification chamber, airway temperature probe position and environmental conditions.

OBJECTIVES

To determine the inspired gas humidity during mechanical ventilation with: (i) four different humidification chambers; (ii) two airway temperature probe (ATP) positions; (iii) five different humidicrib temperatures; and (iv) insulating the inspiratory limb with bubble wrap.

METHODOLOGY

An observational study in the Neonatal Laboratory and Neonatal Intensive Care Unit, Westmead Hospital. The humidity of the inspired gas was measured at the proximal end of the endotracheal tube (ETT) during mechanical ventilation. Inspired humidity measurements were made with four different humidification chambers (Fisher & Paykel (F&P Healthcare Pty Ltd, Auckland, New Zealand) auto refill MR290. F&P manual refill MR310, Suruga (Suruga Inc. Humidifiers, Vincent Medical, Dongguan, China) manual refill MI-20 and MI-10F) with the humidity control (relative humidity setting) set at - 2. Measurements were made with the ATP positioned either; (A) at the distal end of the inspiratory tube inside the humidicrib or (B) outside the humidicrib 50 cm proximal to the ETT. The inspired gas temperatures were set at 36.5 degrees C and at 39.0 degrees C, respectively. For each of the different humidification chambers and ATP positions, inspired humidity measurements were made with the humidicrib temperature set at 30.8, 32.9, 35.2, 36.2, or 37.2 degrees C. Two further sets of measurements were made, one with the inspiratory limb insulated with bubble wrap and the second set without bubble wrap.

RESULTS

There were significant differences in inspired humidity with the four humidification chambers at both ATP positions at all humidicrib temperatures. Both Suruga humidification chambers produced significantly higher inspired gas humidities under most conditions. Positioning the ATP outside the humidicrib produced significantly higher inspired gas humidities than with the ATP inside the humidicrib. Insulating the inspiratory tubing with bubble wrap also significantly improved the inspired gas humidity.

CONCLUSIONS

Significant differences in inspired gas humidity were found with the humidification chambers tested. The position of the ATP and the set temperature had a significant impact on the absolute humidity of the inspired gas. In general, higher inspired gas humidities were obtained with the ATP outside the humidicrib. However, condensation of water close to the ETT appeared at low humidicrib temperatures (< 36.2 degrees C) with the ATP outside the humidicrib and extreme care should be taken that particulate water does not enter the lungs under these conditions.