Recovery of tracheal epithelium following high frequency ventilation at low inspired humidity.

We used scanning electron microscopy (SEM) and light microscopy (LM) to study the recovery of tracheal epithelium in newborn lambs damaged by high frequency flow interrupted ventilation (HFFIV) at low inspired humidity (30%). Newborn lambs were mechanically ventilated for 6 h, allowed to recover and subsequently killed at 2, 7 or 14 days. The recovery of the trachea above and below the tip of the endotracheal tube (ETT) was studied at these time periods and compared to a control non-intubated group and a group killed immediately after 6 h of ventilation. Above and below the ETT, SEM and LM revealed deciliation to be greatest 2 days after ventilation. The damaged tracheal mucosa had converted to non-ciliated epidermoid squamous metaplastic cells. Recovery was not complete by 14 days, although the squamous cells had already differentiated into goblet and ciliated columnar epithelial cells. No difference was seen in the rate of recovery of the tracheal mucosa above or below the tip of the ETT.