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KEY POINTS
High-frequency oscillatory ventilation (HFOV) is a form of ventilatory support which delivers very small tidal volumes (1-2 ml per kg) at very high rates (3-15 breaths per second).
The major indication for HFOV is for patients with severe acute respiratory distress syndrome (ARDS) whose lungs cannot tolerate high tidal distending pressure.
The main determinant of oxygenation during HFOV is the mPaw which is generally initiated at approximately 5 cm H2O greater than the mPaw noted during conventional ventilation.
Carbon dioxide removal during HFOV is directly propportional to the oscillation amplitude and inversely proportional to the oscillation frequency setting.
Two recent multicenter randomized trials showed no benefit (OSCAR) and even harm (OSCILLATE) with the use of HFOV in adult patients with ARDS.
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High-frequency oscillatory ventilation (HFOV) is a form of ventilatory support that delivers extremely small tidal volumes (VTs) at very high rates and maintains a relatively constant and higher mean airway pressure (mPaw) than mechanical ventilation. These properties make HFOV an ideal mode of ventilation for lung protection, because it can allow clinicians to operate in a “safe” zone of the volume-pressure curve, avoiding zones of overdistension and derecruitment/atelectasis, provided an optimal mPaw is set and very small VTs are delivered.
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HFOV was first described in 1972 and was used to improve oxygenation in neonates with severe respiratory distress syndrome. This was gradually expanded into larger, more mature pediatric patients with severe respiratory failure. In 2001, the Food and Drug Administration approved HFOV devices for use in adult patients with acute respiratory distress syndrome (ARDS) who failed conventional mechanical ventilation (CMV).
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PHYSIOLOGY AND MECHANISMS OF GAS EXCHANGE DURING HFOV
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Conventional ventilation mimics the respiratory cycle with inspiration and expiration via positive pressure inhalation compared with the negative pressure that drives normal respiration. HFOV is based on several features of nonlaminar flow of gas into and out of a circuit. High-pressure gas flows down the center of the airway, displacing lower pressure gas via coaxial flow and bulk flow. Between cycles and in distal airways, gases mix uniformly so that delivered, oxygen-rich gas saturates available alveoli to maximize the chances of diffusion of oxygen into the bloodstream. This mixing is called pendelluft and essentially accounts for dead space ventilation of any kind, but is especially useful in HFOV. “Exhaled” gases move by previously mentioned bulk and coaxial flows toward the negative pressure generated by the high-velocity “breaths” where they ultimately exit the endotracheal tube into the exhalation circuit. HFOV can be considered to have separated oxygenation and ventilation into 2 separate mechanisms.
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Similar to CMV when the respiratory frequency exceeds the time for intrinsic exhalation phase, HFOV delivers breaths that can lead to developing auto–positive end-expiratory pressure (auto-PEEP). This can still occur with very small tidal volumes in HFOV. ...