Principles of high frequency oscillatory ventilation (HFOV)
The practice of high frequency oscillation has fallen out of favour in the post-OSCAR and OSCILLATE era, but deserves a mention as an interesting excursion into the extremes of critical care physiology. Additionally, Question 23 from the second paper of 2010 and Question 15 from the first paper of 2002 have asked about HFOV.
Anatomy of the high frequency oscillator
This thing is essentially a highly pressurised CPAP machine.
Basically, an electromagnetic coil-driven piston pushes a flexible diaphragm, which generates the oscillation in pressure. The gas pressure in the circuit is kept constant by a series of pressure regulators and valves. Expiration generally does not occur except by pressure release valve opening and by cuff leak.
Settings on the high frequency oscillator
Bias Flow
This is the control for the flow rate of humidified fresh gas. The maximum is about 40L/min.
Mean Pressure Adjust
This controls the mean airway pressure (mPaw). Because this is a purely pneumatic valve, increasing the bias flow will increase the mPaw.
Power, or "Delta P"
This is basically the diaphragm excursion. The nearest thing you get to a tidal volume. It ranges from 0 to 100%.
% inspiratory time
The diaphragm excursion can be assymetrical. It can spend more time in the inspiratory (pushed-in) phase, or less. This usually ranges from 50% to 30%.
Frequency
This is the setting of the frequency of oscillation, in Hertz (i.e. oscillations per second). It can range from 3 Hz to 15Hz.
Control of oxygenation with HFOV
FiO2
Well, obviously.
mPaw controls oxygenation
This is the main controlling variable of oxygenation. Typically, by setting a high Paw one can achieve maximal lung inflation, and thus maximal gas exchange. A typical mPaw for an adult (to start with) is about 34 cmH2O.
Dont even think about reducing your mPaw targets until the FiO2 is well below 60%.
Control of CO2 removal with HFOV
Increasing the Delta P
Because more gas will be moved by the excursion of the diaphragm, more gas mixing will take place and thus more CO2 will be removed from the alveoli.
Decreasing the frequency
Because CO2 escapes only during the "expiratory" phase, a slower frequency allows for a longer expiratory time, and thus more CO2 removal
Increase the % inspiratory time
The higher the % inspiratory time, the greater the degree of diaphragm excursion for any given delta P. Thus, one can get bigger "tidal volumes" by increasing the %inspiratory time.
Increase the cuff leak
Yes, this is a means of getting gas out of your patient, and thus may be your last-ditch measure for clearing CO2 in a desperately hypercapneic ARDS patient.
References
The Carefusion HFOV 3100A machine operator's manual is available for free online
The OSCAR trial didnt find any mortality benefit
Young, Duncan, et al. "High-frequency oscillation for acute respiratory distress syndrome." New England Journal of Medicine 368.9 (2013): 806-813.
The OSCIALLATE trial found INCREASED mortality:
Ferguson, Niall D., et al. "High-frequency oscillation in early acute respiratory distress syndrome." New England Journal of Medicine 368.9 (2013): 795-805.
HFOV is only mentioned on pages 360 and 1114 of Oh's manual.
LITFL have a lucid summary .
Additionally....
Ha, Duc V., and David Johnson. "High frequency oscillatory ventilation in the management of a high output bronchopleural fistula: a case report." Canadian Journal of Anesthesia 51.1 (2004): 78-83.
Stawicki, S. P., Munish Goyal, and Babak Sarani. "Analytic reviews: high-frequency oscillatory ventilation (HFOV) and airway pressure release ventilation (APRV): a practical guide." Journal of intensive care medicine 24.4 (2009): 215-229.
Ritacca, Frank V., and Thomas E. Stewart. "Clinical review: high-frequency oscillatory ventilation in adults–a review of the literature and practical applications." Critical Care 7.5 (2003): 385.
Pillow, J. Jane. "High-frequency oscillatory ventilation: mechanisms of gas exchange and lung mechanics." Critical care medicine 33.3 (2005): S135-S141.