High frequency oscillatory ventilation

High frequency oscillation (HFO) is an extreme form of standard ventilation with high rates, sub-deadspace tidal volumes (1–3mL/kg), and significantly higher levels of PEEP (equal to the continuous distending pressure or mean airway pressure during HFO). It may thus be viewed as a CPAP device that allows generation of pressure oscillations around a continuous distending pressure eliminating CO2 by accelerating molecular diffusion processes. Precise mechanisms of action on gas exchange are uncertain. Whereas bulk convection and diffusion predominate, during standard ventilation, HFO may provide:

• Inter-regional gas mixing between respiratory units with different time constants (Pendelluft ventilation).

• Convective transport from asymmetric inspiratory and expiratory velocity profiles.

• Longitudinal dispersion via interaction between the axial velocity profile and the radial concentration gradient.

During HFO, high frequencies (180–360bpm = 3–6Hz) generally maintain normal PaCO2 levels. At lower frequencies (3Hz), CO2 clearance usually improves because of the larger VT generated. CO2 clearance can also be enhanced by higher proximal driving pressures (range 60–90cmH2O) and longer inspiratory times (range 30–50%)—both have a similar effect on VT.

Indications

This technique is used as a rescue therapy for refractory hypoxaemia or ventilatory failure in ARDS. It is now being considered as an early strategy in patients with milder forms of acute lung injury to prevent further deterioration. However, there are no controlled data showing superiority over conventional techniques. Theoretically, low VT and high PEEP in HFO reduce the risk of cyclical alveolar collapse and over-distension, both important factors in ventilator-induced lung injury (VILI). The higher mean airway pressure, but lower cycling and plateau pressures, will also improve oxygenation and allow reduction in the FIO2.

Lung recruitment can often be achieved by temporarily increasing lung volumes by a stepwise increase in continuous distending pressure to an oxygenation or chest X-ray target.

The incidence of pneumothorax is thought to be similar to conventional ventilation in adults. HFO may reduce the size of an air leak and promote healing by reducing high peak airway pressures and the alveolar–pleural pressure gradient. Reducing the diameter of the leak increases resistance to gas flow and this facilitates lung healing. Changes in mean airway pressure will result in the greatest percentage change in the size of the air leak.

Potential problems with HFO

• Inability to maintain spontaneous breathing such that the patient often requires heavy sedation 9 paralysis. Novel technical developments incorporating a flow-demand system to enable flow compensation may reduce the imposed work of breathing, increase patient comfort, and allow continued spontaneous breathing.

• Haemodynamic compromise that usually responds to volume loading. This is more common during recruitment manoeuvres.