Collateral Ventilation

Collateral ventilation occurring through nonairway connections between neighboring alveoli has also been proposed as an additional mechanism of gas transport during both CV and HFOV. It is thought that this collateral ventilation occurs via the pores of Kohn (discrete holes in walls of adjacent alveoli) and canals of Lambert (accessory connections in the lungs between some bronchioles and their adjacent alveoli). The relatively high resistance of the collateral channels to gas flow is likely to limit the extent to which this mechanism contributes to gas mixing during HFOV [17].

Whereas gas transport is driven by convective fluid motion, the mechanics of the respiratory system are also an important consideration as convective fluid motion is driven by the pressure differences imposed by the chest wall or the ventilator. In this respect, the impedance of the combined ventilator, circuit, tracheal tube, and respiratory system is an important determinant of the efficiency of ventilation during HFOV. Impedance is a global term that encompasses the mechanical properties of elastance (i.e., the reciprocal of compliance), resistance, and the inertance. Although inertance is essentially negligible at conventional breathing frequencies, it assumes a much greater role at higher frequencies and cannot be ignored during HFOV [18].

Studies in both theoretical models and in healthy animals and humans have demonstrated that V_t has a greater effect on gas exchange than frequency during HFOV. As such, ventilation efficiency (Q) may be expressed as:

The values for a and b in this equation approximate 1 and 2, respectively, although the absolute values may be influenced by other factors such as the shape and complexity of the oscillatory pressure waveform [19, 20].