Gas exchange in highfrequency ventilation

How HFV achieves effective gas exchange despite generating tidal breaths less than the anatomical dead-space remains a mystery. Many theories exist and involve extremely complex physiology and fluid mechanics. Involved processes include augmented gas diffusion in the distal airways, Taylor (lateral) dispersion of gas molecules in more proximal small airways, and high-flow CO2 washout of central compartments. In addition, Pendelluft (cross-ventilation between lung units of different time constants) undoubtedly contributes to effective gas exchange.

The distribution of inspired gas in HFV varies significantly from conventional ventilation. In contrast with low-frequency techniques, where the lower lobes may be preferentially ventilated due to regional lung capacitance, HFV results in more uniform distribution of gas throughout all lobes. Gas distribution in HFV is influenced more by airway resistance and results in more homogeneous gas distribution in both healthy and diseased lungs.

Principles of oxygenation with HFV do not vary significantly from conventional techniques. Effective oxygenation in normal lungs is easily achieved with HFV at low airway pressures but is unpredictable in the presence of pneumonic consolidation or chronic obstructive airways disease. In contrast, in pulmonary edema from any cause, intrapulmonary shunt can be manipulated, as in conventional ventilation, by titration of mean airway pressure, the clinical measure which most usefully reflects lung volume.

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