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Sunday February 4, 2007
Basics of High Frequency Oscillatory Ventilation (HFOV)


Editors' note: Setup and management of HFOV should be done under experienced physician. Following are just basics to understand the dials and simple maneuvers for the beginers. Other details can be found in literature somewhere else.

HFOV is not an initial form of mechanical ventilation. It always get transit from conventional mechanical ventilation.

Indications

Patients with severe ARDS having Pplat more than 30 - 35 cm H2O along with high FiO2 and worsening acidosis and all conventional mode of ventilation is exhausted.

Check for things before initiating HFOV

1. Need of suction. Bronchoscope should be done in conventional mode before switching to HFOV.
2. Good sedation, analgesia, and possible need of neuromuscular blockade.
3. Ensure euvolemia. Intial phase of HFOV may drop BP in hypovolemia.

Initial Settings (RTs usually do the rest but you have to provide MAP, frequency and amplitude)

A) TO IMPROVE OXYGENATION

MAP (Mean Airway Pressure): Initiated at 5 cm H20 higher than the MAP during conventional ventilation. The MAP should be raised only in 2-3 cm H2O increments at 30 – 60 minute intervals to improve oxygenation. Ideal is not to exceed beyond 35 cm H2O.

FiO2: Start at 1.00 and wean slowly towards .4

IT or I:E ratio: Set IT to 33% (can go up to max of 50% if difficulty with oxygenation)

Flow: Start at 15 LPM but increase if oxygenation remains issue.

B) TO CORRECT HYPERCAPNIA

Amplitude of oscillation (P): The amplitude is initiated at either a value where vibration can be seen down to mid-thigh. Amplitude is increased by 10 to 20 cm H2O (value runs from 0-100 cm H2O). You may start arbitrarily somewhere between 70-90 cm H20.

Frequency: It can be set arbitrarily at 5 Hz. Some experts prefer initial setting at 7 and target to go up as oxygenation gets better. (Value runs from 3-15 Hz)

Most important trick to remember

Increasing the amplitude and decreasing the frequency (Hz) will lower the PaCO2. Conversly, decreasing amplitude and increasing frequency (Hz) will increase PaCO2. If PaCO2 worsens, increase amplitude in 10 cmH2O increments every 30 minutes. After maximum amplitude is achieved, and oxygenation remains poor, decrease Hz to the minimum setting of 3 Hz.

Weaning

Wean FiO2 first.
Second wean MAP slowly 2-3 cm H2O at a time till reach at least 24 cm H2O.
Switch to convetional ventilator once patient is stable.

Complications

ETT obstruction - suction
HYPOTENSION - IVF boluses or pressor as clinically indicated
PNEUMOTHORAX - Watch for chest vibration (stethoscope does not work due to noise) or quickly rising PCO2 or desaturation. Confirm PTX with CXR.