Question 23

College Answer

a. What are the indications for HFOV in the ICU? 
•    Oxygenation failure:        Unable to maintain FiO2 < 0.6
•     Ventilation failure:           pH < 7.25 with Vt  > 6 mls/kg and Plateau pressure > 30 cm H2O

b.    What  ventilation   principles   should  be  considered   when  using  a  high frequency oscillator?

Target pH > 7.25 -7.35
Utilise the highest possible frequency to minimise the tidal volume and only decrease for CO2 control if delta P maximal.
Aim for saturations > 88% or PaO2  > 55 mm Hg to minimise the risk of oxygen
toxicity.

c.    When  using  a high  frequency  oscillator,  what  parameters  determine  the 
PaO2?

•    Mean airway pressure.
•    FiO2.

d.    When using the high frequency oscillator, what parameters  determine the 
PaCO2?

•    Amplitude of oscillations (∆ P).
•    Frequency of oscillations.
•    Inspiratory time
•    Cuff leak

e.     Briefly outline the mechanisms of gas transport during HFOV.

Gas transport during HFOV is thought to occur via

•    Bulk flow of gas in alveolar units close to proximal airways
•    Asymmetrical velocity profiles and Taylor dispersion.
•    In addition, asymmetrical filling of adjacent alveoli (termed pendelluft) due to differing emptying times, collateral ventilation through non-airway connections and cardiogenic mixing are other postulated mechanisms.

Discussion

This is a delicate question, as it is likely that it would not be asked in the post-OSCILLATE era.

Rather, one might expect something like "critically evaluate the use of HFOV in the management of respiratory failure".

But, anyway, let us proceed.

a. What are the indications for HFOV in the ICU?

This is an area now open for debate, and the correct answer may be "none".

However, I draw upon Google searches to suggest the following indications:

• High FiO2 requirements (arbitrarily, 50% or 70% FiO2)
• High PEEP requirements (arbitrarily, 14mmHg or 16mmHg- levels at which alveolar distension is maximal)
• Poor lung compliance (plateau pressures over 30mHg)
• Bronchopleural fistula

b.    What  ventilation   principles   should  be  considered   when  using  a  high frequency oscillator?

• Minimise FiO2, aiming for sats of 88%
• Tolerate high CO2 to minimise leak
• Maximise frequency and minimise tidal volume
• Tolerate a respiratory acidosis with a pH of 7.25-7.35

c.    When  using  a high  frequency  oscillator,  what  parameters  determine  the PaO2?

• Mean airway pressure is the driving pressure which maintains open alveoli, and is the governing principle of oxygenation in HFOV.
  • P_aw largely determines lung volume, which is why it is so important (lung volume being representative of the gas exchange surface)
• FiO2 manipulates the O2 concentration gradient between the machine and the pulmonary circulation

d.    When using the high frequency oscillator, what parameters  determine the PaCO2?

• Amplitude of oscillation (delta P) determines the nearest thing you have to "tidal volume".
• Frequency of oscillations determines the brevity of the "expiratory" period, and thus the lower the frequency, the better the CO2 clearance
• Cuff leak represents the amount of gas intentionally allows to exchange (one way) with the ouside environment, and thus represents an additional mechanism of CO2 clearance.
• Inspiratory time is the time spent by the piston in forward motion; the lower the inspiratory time, the higher the "expiratory time" and thus the higher the CO2 clearance.

e.     Briefly outline the mechanisms of gas transport during HFOV.

The college lists a series of eponymous mechanisms, which are incomprehensible to the savage.

• Bulk flow of gas in alveolar units close to proximal airways
• Asymmetrical velocity profiles
• Taylor dispersion
• Pendelluft
• Collateral ventilation through non-airway connections and cardiogenic mixing are other postulated mechanisms.

This topic is explored elsewhere:

• Physiology of gas exchange in HFOV
• Principles of high frequency oscillatory ventilation (HFOV)

Additionally, an excellent free article is available. The savvy candidate will recall that a detailed knowledge of these principles is probably not expected, as the oscillators in adult ICUs worldwide are gathering layers of dust.

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.