With regards to High Frequency Oscillatory Ventilation (HFOV),
a. What are the indications for HFOV in the ICU?
b. What ventilation principles should be considered when using a high frequency oscillator?
c. When using a high frequency oscillator, what parameters determine thePaO2?
d. When using the high frequency oscillator, what parameters determine thePaCO2?
e. Briefly outline the mechanisms of gas transport during HFOV.
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.
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:
b. What ventilation principles should be considered when using a high frequency oscillator?
c. When using a high frequency oscillator, what parameters determine the PaO2?
d. When using the high frequency oscillator, what parameters determine the PaCO2?
e. Briefly outline the mechanisms of gas transport during HFOV.
The college lists a series of eponymous mechanisms, which are incomprehensible to the savage.
This topic is explored elsewhere:
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.