High Frequency oscillatory

ventilation HFOVHigh frequency oscillatory ventilator delivers

breaths at a very high rate with tidal volumes

usually less than or equal to anatomical dead

space volume at a constant high mean airway

pressure.

Main advantages of HFOV over

conventional ventilation Smaller tidal volumes limits alveolar over-distension preventing

volutrauma

Higher mean airway pressure results in better recruitment of

atelectactic alveoli.

Constant mean airway pressure during inspiration and expiration

prevents alveolar collapse.

Peak airway pressures are reduced minimising potential for

barotrauma.

Better gas exchange as the gas molecules are constantly agitated

inside the airway due to the oscillatory mechanism.

Gas exchange mechanism in

HFOV

Direct Bulk flow (Convective ventilation):- Inspired gas directly reaches alveolar regions more proximal to conducting airways (same like in controlled mandatory ventilation)

Taylor Dispersion: – Interplay between convective forces and molecular diffusion which enhances gas mixing.

Pendelluft: - Asynchronous filling of gas adjacent lung units with different time constants. Gas flow from fast to slow filling units at end inspiration. The reverse occurs at end expiration.

Asymmetric velocity profiles – High frequency bulk flow creates a bullet shaped flow profile where the central molecules move further down the airway than the molecules on the periphery of the airway during inspiration. During expiration this profile is blunted where the central molecules remain further down the airway but the peripheral molecules move towards the entry point.

Cardiogenic mixing: - Cardiac contractions promote peripheral gas mixing up to five fold along concentration gradient

Molecular diffusion: - Due to the increased turbulence of molecules gas exchange across the alveolar- capillary membrane occurs more efficiently

Mean airway pressure Mean airway pressure is the constant pressure maintained in the

airway to keep the atelectatic lung area open.

Mean airway pressure is similar to PEEP in conventional

ventilation.

Red arrow represent the constant pressure maintained inside the

airway

Mean airway pressure is set by adjusting the bias flow knob as it is

flow dependent. Mean airway pressure is set 5cms above the mean

airway pressure set on the conventional ventilator.

Amplitude Amplitude (∆P) is the power with which the piston move backwards

and forwards. Higher power results in the piston to move forwards

and backwards more resulting in higher amplitude for the air

oscillating inside the airway.

Mean airway pressure

High Mean airway pressure is used in

HFOV. It is achieved by adjusting the

bias flow.

Mean airway pressure is increased to

improve oxygenation or PaO2

Frequency

Frequency is the respiratory rate and

is expresssed in Hertz.

1 Hertz = 60 breaths per minute..

Frequency is reduced to decrease

PaCO2 as there is more time for

exhalation.

PaO2 – directly proportional to FiO2

and mean airway pressure.

PaCO2 is controlled using amplitude

and Frequency.

PaCO2 can be decreased by

increasing the amplitude or

decreasing the frequency

Tidal volume and PaCO2.

Estimated alveolar ventilation is the product of the device

frequency and the square of the delivered tidal volume.

VCO2 = Frequency x Tidal volume2

Therefore any manoeuvres that alters tidal volume will alter

CO2 removal.

Thus decreasing amplitude decreases the delivered tidal

volume and thereby reduces CO2 elimination resulting in

increase PaCO2.

Respiratory frequency on

PaCO2. VCO2 = Frequency x Tidal volume2

With increasing rate, the inspiratory time is decreased and the

oscillations of the diaphragm become les efficient resulting in reduced

delivered tidal volume.

Based on the above mentioned formula CO2 clearance depends more

on tidal volume than frequency. Hence in HFOV increasing the

frequency decreases CO2 clearance as less tidal volumes are delivered

and conversely decreasing the frequency results in more efficient

oscillations resulting in larger tidal volumes and improved CO2

clearance.

On a patient intubated with size 7.0 ET tube decreasing the rate from 9 to 6 improves the tidal volume from

approximately 125 to 175.

Cuff leak and PaCO2

Cuff leak should be used as a means of improving CO2 clearance

only when a maximum amplitude and a low rate is not improving

hypercarbia.

Cuff leak creates an alternative path outside the ET tube for CO2

clearance.

Inducing the cuff leak results in a decrease in mean airway pressure.

Hence it is important to adjust the bias flow to maintain the mean

airway pressure for this manoeuvre to be effective.

HFOV – SensorMedics 3100B

Indications for HFOV (as a rescue therapy

where conventional ventilation fails to improve

oxygenation

Severe ARDS where conventional ventilation fails to improve oxygenation

Pulmonary contusion

Broncho-pleural fistulas and massive airleaks

Pulmonary contusion

Bronchial injury

Acute brain injury patients with raised ICP because it avoids large swings in peak inspiratory pressure, increases PaO2 and controls PaCO2

Burns: - facilitates early excision and closure of the burn wounds by reversing hypoxaemia.

Complications and limitations

Pneumothorax

Haemodynamic compromise ◦ High pleural pressures can compromise venous

return

◦ High transpulmonary pressure increases right ventricular after load

Prolonged sedation and neuromuscular blockade

Migration of ETT

Infection control

Aerosol delivery

Transport

Monitoring

Staff training