High Frequency Oscillatory ventilation (HFOV)

general concept

  • The theory behind HFOV includes the following aspects:

    • Use of supra-physiologic ventilation frequencies and low tidal volumes (less than dead space)

    • Instead of bulk flow (as in conventional mechanical ventilation), gas flow ad therefore ventilation occurs due to

      • Axial dispersion

      • Collateral flow through pores of Kohn

      • Pendelluft phenomenon

      • Taylor dispersion

      • Asymmetric gas profiles,

      • Gas mixing due to  pressure-diameter relationship of the bronchi

    • Delivery of a constant mean airway pressure (MAP) without the high peak pressures of conventional mechanical ventilation that is directly related to oxygenation 

    • Uncoupling of oxygenation and ventilation allowing separate adjustment of either variable 

  • As pressure increases, lung volume increases depending on the tissues' compliance

  • Low pressure go along with atelectasis / collapse, while high pressure cause over distension / volutrauma

  • In order to minimise ventilator-induced lung injury (VILI), HFOV operates in the "safe zone"

  • Note the hysteresis effect between in- and expiration

  • Given the very small tidal volumes during HFOV this mode undulates around a small "safe" window on the expiratory limb of the pressure-volume curve

  • APRV is similar in this as it used high MAPs and small tidal volumes on the inspiratory limb of the pressure-volume curve

Physics involved in gas-exchangE during HFOV

  • As opposed to conventional mechanical ventilation (CMV) which uses bulk flow during in and expiration for gas exchange - HFOV works as a result of a set of physical phenoma

  • The circuit features gas inflow as well as outflow. The mean airway pressure is generated through gradual changes in obstruction to gas outflow via a diaphragm. 

  • The membrane generates gas oscillations inside the circuit. The initial amplitude is dampened as it progresses from the membrane down to the alveoli. 

HFOV Principle and Ventilator Set-Up

General HFOV Ventilator Set-Up

  • The operator sets

    • mPaw (Mean Airway Pressure)​

    • Frequency (ƒ)

    • Amplitude (∆P)

    • Inspiratory time (Ti) in % of respiratory cycle

  • This results in a waveform that undulates around a mean airway pressure.​

  • Half of the amplitude generates positive pressure (inspiration) while the other half generates negative pressure (expiration)

Settings / Variables

  • Increase mPaw

  • Increase FiO2

  • Increase inspiratory time (Ti)

To improve Oxygenation

To improve Ventilation

  • Increase amplitude (∆P)

  • Decrease frequency (ƒ)

  • Decrease inspiratory time (Ti)

  • Deflate ETT cuff

vyaire(TM) 3100A/B

3100A High-frequency-oscillatory-ventilator
  • This is the most commonly used HFOV ventilator

  • The manufacturing companies have changed over time but the model remains the same

  • There are two models:

    • 3100A​

      • For children and adults (initially aimed at patients < 35kg)

      • Currently manufactured

      • Consumables available

    • 3100B​

      • For children / Adults > 35kg

      • Currently no longer manufactured

      • Consumables available 

Contact/Suggestions

Juerg Burren, MBBS MD FMH (CH) EPIC FCICM

Paediatric Intensivist and Ventilation Enthusiast

​​

Tel: +61 (0) 481 751 073

info@pivares.com

© 2019-2020 by Juerg Burren for PIVARES

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