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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

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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

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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

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Physical phenomena during HFIV

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)

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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

3100A Operator Manual

Check out this HFOV Pocket Guide

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