Tracheal damage following conventional and high-frequency ventilation at low and high humidity

To compare the degree of tracheobronchial damage in newborn lambs ventilated for 6 hrs with relative humidities of 30% or 90% and continuous positive airway pressure breathing, conventional mechanical ventilation of 25 and 60 breaths/min, or high frequency flow-interrupted ventilation at 600 breaths/min.

Tracheobronchial damage secondary to mechanical ventilation remains a major iatrogenic lesion of the newborn despite substantial advances in both mechanical design and ventilatory techniques. A histologic scoring system was used to compare the damage noted in the tracheobronchial epithelium of newborn lambs after 6 hrs of conventional mechanical ventilation or high-frequency flow-interrupted ventilation at two relative humidities. Three groups of animals were ventilated for 6 hrs with an FIO2 of 0.21 at 36.0 degrees C and relative humidity of 90%. The first group received continuous positive airway pressure of 4 cm H2O, the second group received slow rate, conventional mechanical ventilation at 25 breaths/min, and the third group received fast rate, conventional mechanical ventilation at 60 breaths/min. Two other groups of animals were ventilated for 6 hrs with an FIO2 of 0.21 at 36.0 degrees C and relative humidity of 30%. The first group was ventilated with high-frequency flow-interrupted ventilation at 600 breaths/min and the second group with slow rate, conventional mechanical ventilation at 25 breaths/min. Two additional groups served as nonintubated controls; one group was killed immediately after sedation and the other group was killed after 6 hrs of sedation.

The damage was mild but significantly different from controls when 90% humidity was used and there was no difference in the histology score between continuous positive airway pressure breathing and conventional mechanical ventilation at 25 or 60 breaths/min. Significant inflammation, erosion, necrosis, and blistering occurred with both conventional mechanical ventilation at 25 breaths/min and high-frequency flow-interrupted ventilation at 600 breaths/min when 30% humidity was used. The damage was only found 5 mm below the tip of the endotracheal tube and not at 3.5 cm beyond the endotrachea tube in the trachea nor in the right main bronchus.

These data indicate that endotracheal intubation and mechanical ventilation, regardless of the method of ventilation, cause damage to the tracheal mucosa, but that poorly humidified inspired gases cause significantly greater damage.