High frequency ventilation

Currently HFOV is used as salvage therapy for patients failing conventional mechanical ventilation. It should restricted to centres with training and experience in this mode of ventilation. HFOV provides tidal volume below the anatomic dead space at frequencies greater than 60 breaths per second.

Benefits include reduce barotrauma, improve V/Q matching and less respiratory compromise. Complications include dessication and inspissation of mucus, airway damage due to high gas velocities, air trapping and high shear forces at interfaces between areas of the lung at different impedences.

Gas exxchange during HFOV:

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  direct bulk flow


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  longitudinal (taylor) dispersion


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  pendeluft


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  asymmetric velocity profile


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  cardiogenic mixing


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  molecular diffusion

1. Two observational studies, adults who failed to respond to conventional ventilation were managed with HFOV at 5 Hz. Improvement in oxygenation were noted within 8 hours in both studies. (Fort CCM 1997, Mehta CCM 2001).

2. HFOV for ARDS in adults: a RCT. Derdak AJRCCM 2002.

Multicentre Oscillatory Ventilation for ARDS Trial (MOAT)

148 adults were randomised to HFOV or conventional ventilation. The HFOV group had significant improvement in oxygenation within 16 hours,compared to conventional ventilation. However the improvement did not persist and oxygenation was the same in both groups by 24 hours. The survival trend favour HFOV over CV but is underpowered (would need n=199 to evaluate mortality)

The difference in survival rate did not reach significance at 30 or 90 days.

3. High frequency oscillatory ventilation in adults: the Toronto experience. Mehta Chest 2004 Canadian experience, Retrospective chart review of 156 patients treated with HFOV. The authors suggested that HFOV might be an effective rescue therapy for patients with severe oxygenation failure. Because mortality was associated with a greater number of days receiving conventional ventilation prior to HFOV, the authors also suggested that earlier institution of HFOV could be beneficial.

4. High Frequency Oscillatory Ventilation Compared to Conventional Ventilation in ARDS: a RCT. Bollen et al. Crit Care 2005

ICU in London, Cardiff, Paris, Mainz. n = 61

Study stopped prematurely because of la ow inclusion rate and the completion of similar MOAT trial.

No difference in 30 day mortality. Post hoc analysis- better treatment effect of HFOV in patients with higer baseline oxygen index (OI).

Critics: Small number of patients, lack of explicit ventilation protocol and underpowered to show differences in efficacy or safety.

Conclusions:

1. HFOV as safe and efficacious as lung protective controlled ventilation in RCTs.

2. There is a trend of improved mortality with HFOV but this needs to be repeated in a fully powered, properly controlled RCT.

3. HVOV may be more effective in patients with high baseline OI, but this should be studied directly.