High-frequency oscillatory ventilation

A patient with severe acute respiratory distress syndrome (ARDS) develops a pneumothorax requiring insertion of a chest drain. You decide to institute high-frequency oscillatory  ventilation (HFOV). Regarding this case which of the following options is CORRECT?

a) Positive end expiratory pressure (PEEP) levels during HFOV would be similar to those in an optimal conventional ventilator strategy

b) Tidal volumes employed in HFOV are generally only 1 to 2 mL/kg more than the physiological dead-space volume

c) Maximum ventilation frequency may be up to 300 per minute

d) On commencement of HFOV a drop in cardiac output and central venous pressure and a rise in pulmonary artery pressure would be expected

e) The tidal volume generated during HFOV is directly related to both the driving pressure and ventilator frequency, both of which are controlled by the operator

Answer: a

Explanation

High-frequency oscillatory ventilation (HFOV) delivers small tidal volumes at extremely high frequencies (anywhere from 3 to 15 Hz). Tidal volumes are usually 1 to 3 mL/kg less than the physiological dead space but gas exchange still occurs via a number of mechanisms including direct bulk flow, molecular diffusion, cardiogenic mixing and pendelluft, the latter due to regional differences in lung compliance and airway resistance. When initiating HFOV the frequency, I:E ratio, driving pressure and mean airway pressure are all set by the operator with the tidal volumes generated being directly related to the driving pressure and inversely related to the frequency. The potential advantages of HFOV over conventional ventilation include the delivery of smaller tidal volumes thus limiting alveolar overdistension, the application of a higher mean airway pressure (mPaw) than in conventional ventilation so promoting more alveolar recruitment and the maintenance of a constant mPaw during inspiration and expiration, thus preventing end-expiratory alveolar collapse. Patients treated with HFOV generally have an early and non-persistent increase in pulmonary artery occlusion pressure, a small persistent increase in central venous pressure and a small decrease in cardiac output compared with baseline. A small number of studies show that the use of HFOV in adult patients with ARDS is associated with improvements in oxygenation, without a significant reduction in mortality. Application of HFOV early in the course of ARDS may be associated with improved outcome but more trials are needed.

Reference

Krishnan JA, Brower RG. High-frequency ventilation for acute lung injury and ARDS.

Chest 2000; 118(3): 795–807.