Theory of HFV

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High Frequency 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.

• While HFV’s are frequently described by their delivery method, they are usually classified by their exhalation mechanism (active or passive).

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Differences between HFOV and CMV

CMV HFOV

Rates 0 - 150 180 - 900

Tidal Volume 4 - 20 ml/kg 0.1 - 5 ml/kg

Alv Press 0 - > 50 cmH2O 0.1 - 5 cmH2O

End Exp Vol Low Normalized

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HFV Gas Exchange

• Henderson first published his findings in 1915, assessing dead space relationship in ventilation.

• He stated, “there may easily be a gaseous exchange sufficient to support life even when Vt is considerably less than dead space.”

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HFV Gas Exchange

• In the 1970’s, Bunnell and his associates demonstrated in animals that adequate alveolar ventilation could be achieved with a frequency between 5 - 30 Hz and a Vt of 20 - 25% less volume than anatomical dead space.

• Slutsky, et al. theorized that the gas exchange mechanism was caused by the “coupled effects” of convection and molecular diffusion.

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HFV Gas Exchange

• Chang theorized that convective processes were more predominant with an increase in Vt and lower frequencies. A diffusive mechanism may be more predominant where there is a decrease in Vt and higher frequencies are used.

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High Frequency Ventilation

• Types of HFV’s Approved for use in both Neonates and Pediatrics• SensorMedics 3100A HFOV• Bird Volumetric Diffusive HFPPV

• Types of HFV’s Approved for use in Neonates Only• Bunnell Life Pulse HFJV• Infrasonics Infant Star (discontinued) HFFI

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Bunnell Life Pulse Jet

• Delivers a pulse of gas into the ETT via a special adapter and pinch valve mechanism

• Exhalation is Passive• Frequency of 4 - 11 Hz• Peak Airway Pressure

of 8- 50 cmH2O

• Used in tandem with a conventional

ventilator• Mean Airway Pressure

limited to conventional ventilator capabilities

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Infrasonics Infant Star HFFI

• Modification of the conventional Infant Star

• Facilitated/Passive Exhalation

• Pressure waveform manipulated by a series of pneumatic valves

• Frequency of 2 - 22 Hz• Paw cannot be adjusted

directly. Usually adjusted by changing end expiratory pressure on CMV (limited to 24 cmH2O)

• Fixed 18 ms inspiratory time

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• A pneumatic cartridge (Phasitron) interrupts the pressurized gas source

• Passive Exhalation• Frequency of 1.6 -

21.6 Hz• Paw is not directly

adjusted• May deliver HFV on

top of a conventional breath

Bird Volumetric Diffusive Vent

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Sensor Medics 3100A

• Electrically powered, electronically controlled piston-diaphragm oscillator

• Paw of 3 - 45 cmH2O

• Pressure Amplitude from 8 - 110 cmH2O

• Frequency of 3 - 15 Hz• Inspiratory Time 30% -

50%• Flow rates from 0 - 40

LPM

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3100A

• “True” Oscillator• Produces an active exhalation, and does not

depend on passive recoil of the chest for CO2 removal

• Stand Alone Ventilator• Does not require nor deliver a conventional

breath through the system• Does not require a special ET tube

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Volume delivery and MAP

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Comparison: Volume delivery

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3100A Ventilator

•Approved in 1991 for Neonatal Application for the treatment of all forms of respiratory failure.

•Approved in 1995 for Pediatric Application, with no upper “weight limit”. For treating selected patients failing conventional ventilation.

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Remember

• Each device has specific labeling restrictions for its use. Many devices are classified as “RESCUE ONLY” or for treating only a specific pathology.

• No clinical comparisons between devices.• Each has its own indications and risks.• Strategies to treat each disease process is

specific to the device used.