Lung mucociliary transport during high-frequency ventilation

The effects of high-frequency, low-tidal-volume ventilation (HFV) and conventional intermittent positive-pressure ventilation (IPPV) on lung mucocillary transport were compared in 5 anesthetized, intubated mongrel dogs using a radioactive label. An aerosol of [^99mTc]sulfur colloid was delivered to the dogs' airways and the subsequent clearance of isotope was followed over 4 h of either HFV or IPPV. After 4 h of IPPV, approximately 8 to 10% of the initial activity had cleared from the lung. By contrast, practically all the isotope remained in the lung after 4 h of HFV. Although the reasons for this were not definitely established in this study, several other observations were made that related to this question. First, large amounts of mucus were visible in the trachea immediately after each of the 4-h HFV studies, whereas this was so in only one of the IPPV studies. Second, in 5 of 6 studies, a bolus of radioactive label placed on the posterior trachea after 4 h of HFV was noticed to disperse and move rapidly toward distal airways under the influence of HFV. Third, despite these findings of disturbed mucociliary transport during HFV, measurements of tracheal velocity, using a radioactive bolus technique during conventional ventilation, were the same after 4 h of HFV as after 4 h of IPPV (18.4 ± 2.6 and 15.0 ± 1.6 mm/min, respectively). We conclude that in the anesthetized dog HFV may alter mucociliary transport. This is unlikely to be the result of major structural damage to the mucosal surface, because tracheal mucous velocity measured after 4 h of HFV was not different from that after IPPV. It may be partly explained, however, by the retrograde flow of mucus that was observed during HFV in 5 of 6 studies.