On the other hand, a slight decrease in ventilation, e.g., during permissive hypercapnia that is also recommended during HFOV, was associated with deterioration in oxygenation compared to the previous normocapnic period. The authors also noticed that in contrast to CV, any degree of hyperventilation led to an appreciable increase in oxygenation. It was noticed that V T during HFOV was not only important for the control of CO 2 elimination but that V T affected oxygenation as well, even if mean airway pressure was kept constant. When the authors started to use HFOV in adult patients with acute respiratory distress syndrome (ARDS) more than 15 years ago, they used a custom-made respiratory monitor for the measurement of V T delivered during HFOV. Ventilation and thus CO 2 elimination during HFOV are complex and determined by changes in power, oscillatory frequency, and by washout of CO 2 around the endotracheal tube cuff. In addition, the combination of a high Δ P and a low mean airway pressure (e.g., during weaning) may result in the entrainment of CO 2 in the inspiratory limb of the HFOV ventilator circuit. The portion of tidal volume delivered directly to the alveolar space may be further affected by the presence or absence of an ETT cuff leak. At any given power and frequency combination, V T delivery into the lungs is also influenced by relative inspiratory time (e.g., ‘% inspiratory time’, in fact inspiratory-to-expiratory ratio, I:E), and the diameter of the endotracheal tube (ETT). V T is increased by increasing Δ P (i.e., increasing ‘power’) or decreasing the frequency of oscillations. Since tidal volume cannot be set directly on an adult HFOV ventilator, delivered tidal volume is set indirectly using pressure amplitude Δ P (controlled by the parameter called ‘power’) and the frequency of oscillations (Hz). As with conventional mechanical ventilation, oxygenation during HFOV is primarily determined by mean airway pressure and by FiO 2. One of the theoretical advantages of high-frequency oscillatory ventilation (HFOV) over other high-frequency modes is the relative decoupling of oxygenation and CO 2 elimination. The trial conducted by the ARDS Network showed better oxygenation (but worse mortality) at V T = 12 mL/kg versus V T = 6 mL/kg. Tidal volume, by opening lung alveolar units, also affects oxygenation, although V T is not typically increased for this purpose because of concerns regarding ventilator-associated lung injury (e.g., volutrauma). Oxygenation, on the other hand, is primarily controlled by the fraction of inspired oxygen (FiO 2), mean airway pressure and positive end-expiratory pressure (PEEP). The effective removal of CO 2 is therefore proportional to respiratory rate and tidal volume ( V T). In clinical practice, the elimination of carbon dioxide (CO 2) during conventional mechanical ventilation (CV) depends on alveolar ventilation, defined as minute ventilation reduced by dead space ventilation. ConclusionĪ change in P aCO 2 induced by the manipulation of tidal volume inevitably brings with it a change in oxygenation, while this effect on oxygenation is significantly greater in HFOV compared to CV. Any decrease in V T during HFOV caused a rapid worsening of oxygenation compared to CV. Increasing V T above its normocapnic value during HFOV caused a significant improvement in oxygenation, whereas improvement in oxygenation during CV hyperventilation was limited. ResultsĬhanges in P aCO 2 intentionally induced by adjustment of V T affected oxygenation more significantly during HFOV than during CV. The same procedure was repeated for CV and HFOV in random order. Arterial partial pressures of oxygen (P aO 2) and carbon dioxide (P aCO 2) were recorded. Then, V T was repeatedly changed over a wide range while keeping constant the levels of PEEP during CV and mean airway pressure during HFOV. In each animal, we found a normocapnic tidal volume ( V T) after the lung recruitment maneuver. To investigate a different level of coupling or decoupling between oxygenation and carbon dioxide elimination during CV and HFOV, we conducted a prospective crossover animal study in 11 healthy pigs. One of the accepted explanations is that CV and HFOV act differently, including gas exchange. Numerous studies documented its benefits, whereas several more recent studies did not prove superiority of HFOV over protective conventional mechanical ventilation (CV). The role of high-frequency oscillatory ventilation (HFOV) has long been debated.
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