The role of ventilation frequency in airway reopening.

Inhomogeneously compliant lungs need special treatment during ventilation as they are often affected by respiratory insufficiency which is frequently caused by a regional collapse of the airways. To treat respiratory insufficiency atelectatic areas have to be recruited. Beside conventional mechanical ventilation, high-frequency oscillatory ventilation (HFOV) is an efficient method for airway reopening. Using a transparent in-vitro model of the human lung the influence of varying frequencies on the reopening behavior of atelectatic regions is investigated for volume controlled ventilation. The experiments show that higher ventilation frequencies at constant tidal volume enhance the probability of successful reopening of collapsed lung regions and thus, lead to a more homogeneous distribution of air within the lung. This effect can be attributed (i) to larger flow velocities and thus larger pressure losses in the free pathways as the ventilation frequency increases and (ii) to higher inertia effects. In consequence, the static pressure in the branches above the atelectatic regions increases until it reaches a level at which recruitment is achieved.