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
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%.
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
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.