1. High Frequency Oscillatory
Ventilation
GAS EXCHANGE
Department of Respiratory Care
John Priest RRT-NPS
JP 01/2012

2. High Frequency Oscillatory
Ventilation
• Defined by FDA as a ventilator that delivers more
than 150 breaths/min.
• Delivers a small tidal volume, usually less than or
equal to anatomical dead space volume.
• Sinusoidal waveform

3. Theoretical Advantages
• Smaller Tidal Volume
• limits alveolar overdistention
• Higher mean airway pressure
• alveolar recruitment
• Constant mPaw during inspiration and expiration
• Preventing end-expiratory alveolar collapse

4. Mechanisms of Gas Exchange
Convective Ventilation (Bulk Flow)
Asymmetrical Velocity Profiles
Taylor Dispersion
Pendeluft
Molecular Diffusion
Cardiogenic Mixing

5. Convective Ventilation
(Bulk Flow)
• Even with small tidal volumes high frequency
oscilatory ventilation can provide direct alveolar
ventilation to short path length units that branch off
of the primary airways

6. Asymmetrical Velocity Profiles
Inspiration
The high frequency bulk flow creates a “bullet” shaped flow
profile, with the central molecules moving further down the
airway than those molecules found on the periphery of the
airway.
Expiration
The velocity profile is blunted so that at the completion of
each return, the central molecules remain further down the
airway and the peripheral molecules move towards the
mouth of the airway.

7. Taylor Dispersion
• Augmented diffusion that occurs because of
turbulent flow between the axial and radial gas
concentrations in the airways.

8. Pendeluft
Transient movement of gas out of some alveoli and
into others when flow has just stopped at the end of
inspiration, or such movement in the opposite
direction just at the end of expiration; occurs when
regions of the lung differ in compliance, airway
resistance, or inertance so that the time constants of
their filling (or emptying) in response to a change of
transpulmonary pressure are not the same.

9. Molecular Diffusion
Responsible for the gas exchange across the alveolar-capillary
membrane and also contributes to the transport of O2 and CO2 in
the gas phase near the membrane.
Due to the increased turbulence of molecules.
One of the major mechanisms for alveolar ventilation.

10. Cardiogenic Mixing
The heart beat adds to the peripheral gas
mixing.

11. Oxygenation and Ventilation
• Oxygenation and CO2 elimination have been
demonstrated to be decoupled with HFOV.
Primary Controls for Oxygenation
Mean Airway Pressure
FiO2
Secondary Control for Oxygenation
Recruitment Maneuver
Primary Controls for Ventilation
Amplitude
Hertz
Secondary Controls for Ventilation
Inspiratory Time %
Cuff Deflation
Permissive Hypercapnia

12. Oxygenation
Mean Airway Pressure
• Used to optimize lung volume which increases
alveolar surface area for gas exchange.
• Manipulation of Mean Airway Pressure results in:
– Recruitment of atelectatic alveoli
– prevents alveoli from collapsing (derecruitment)
• Possible complications from Mean Airway Pressure
– Overdistension.
– Alveolar atelectasis or overdistension can result in
pulmonary vascular resistance (PVR).

13. Oxygenation
FiO2
• The goal FiO2 for the ARDS patient
population is .5
• If the goal FiO2 is not able to be met then
the lung has not been adequately recruited

14. Oxygenation
Recruitment Maneuver
• When initiating HFOV to recruit lung
• After a disconnect or loss of FRC/Paw
• After suctioning (even with a closed suction system)
• Inability to wean FiO2
• When considering increasing Paw
– A recruitment maneuver may recruit lung
allowing you to maintain the baseline Paw and,
thus, not increase support.

15. Ventilation
• Alveolar Ventilation during CMV is defined as:
f x Vt
• Alveolar Ventilation during HFV is defined as:
f x Vt 2
• Changes in volume delivery (as a function of Delta-
P, Frequency, or % Insp. Time) has the most
significant affect on CO2 elimination.

16. Ventilation
Amplitude
• The Amplitude is controlled by the force with which
the oscillatory piston moves. (represented as stroke
volume or P)
• Described as the peak-to-trough swing across the
mean airway pressure.
• Dependent on the Power setting
• Amplitude numbers are arbitrary
• The degree of deflection of the piston (amplitude)
determines the tidal volume.

17. Ventilation
Hertz
• Controls the frequency and distance the
piston moves. The lower the frequency, the
greater the volume displaced, and the higher
the frequency, the smaller the volume
displaced.
• 1 Hertz = 60 breaths per minute

18. Ventilation
% Inspiratory Time
The % Inspiratory Time also controls the time for
movement of the piston, and therefore assists with
CO2 elimination.
Increasing % I-Time is used as the third maneuver
to control CO2 elimination.
Increases in Inspiratory Time increases delivered
mPAW

19. Bias Flow
Bias Flow produces the mPaw of the system,
and also helps to flush the CO2 that is
actively pulled back into the circuit during the
expiratory phase.
The bias flow may also maintain the mean
airway pressure in circumstances that a cuff
leak is used