Long-term effects of different humidification systems on endotracheal tube patency: evaluation by the acoustic reflection method.

Background: Accumulation of mucous secretions in an endotracheal tube (ETT) increases its resistance, and the amount of deposit may be affected by the quality of humidification and heating of the inspired gas.

Methods: The authors assessed the impact of two humidification systems, a heated humidifier (HH) and a hygroscopic-hydrophobic heat and moisture exchanger (HME), on the ETT patency in patients selected to require mechanical ventilation for more than 48 h. This comparison was performed over two consecutive periods and used the acoustic reflection method, which characterizes the amount and site of ETT obstruction and allows estimating ETT inner volume and resistance. Measurements were performed three times a week over the period of mechanical ventilation. Comparisons were performed at mid duration and at the end of the mechanical ventilation period.

Results: The HH was used in 34 patients, and the HME was used in 26 patients. The two groups had similar severity and duration of mechanical ventilation. At mid duration of mechanical ventilation (5.5 +/- 3.3 vs. 4.8 +/- 3.3 days; P = 0.4), no difference was observed in ETT volume and resistance between the two groups. At the end of the study period (10.5 +/- 5.8 vs. 9.6 +/- 6.3 days of mechanical ventilation; P = 0.4), ETT volume was reduced to a greater extent with HME than with HH (-3.3 +/- 2.9 vs. -5.1 +/- 2.5%; P = 0.008), and ETT resistance increased significantly more with the HME than with the HH (8.4 +/- 12.2 vs. 19.4 +/- 17.7%; P = 0.001).

Conclusion: Prolonged use of humidification systems results in progressive reduction of ETT patency, and to a greater extent with HMEs than with HHs.