Theory of hfv
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Theory of HFV. 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.

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Theory of HFV

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Theory of HFV


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).


Differences between HFOV and CMV

CMVHFOV

Rates0 - 150180 - 900

Tidal Volume4 - 20 ml/kg0.1 - 5 ml/kg

Alv Press0 - > 50 cmH2O0.1 - 5 cmH2O

End Exp VolLowNormalized


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.”


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.


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.


High Frequency Ventilation

  • Types of HFV’s Approved for use in both Neonates and Pediatrics

    • SensorMedics 3100A HFOV

    • Bird Volumetric DiffusiveHFPPV

  • Types of HFV’s Approved for use in Neonates Only

    • Bunnell Life Pulse HFJV

    • Infrasonics Infant Star (discontinued)HFFI


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


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


Bird Volumetric Diffusive Vent

  • 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


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


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


Volume delivery and MAP


Comparison: Volume delivery


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.


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.


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