• High
frequency jet ventilation (HFJV) involves a high pressure jet of gas entraining
further fresh gas which is directed by the jet towards the lungs.
• Respiratory
rates of 100–300/min ensure minute ventilation of about 20L/min although tidal
volume may be lower than dead space. CO2
elimination is usually more efficient than conventional IPPV.
• The
method of gas exchange is not fully elucidated, but includes turbulent gas
mixing and convection.
• Oxygenation
is dependent on mean airway pressure. Peak airway pressures are lower than with
conventional mechanical ventilation, but auto-PEEP and mean airway pressures
are maintained.
• SaO2
often falls when starting HFJV, but usually improves with
time.
• The
high gas flow rates employed require additional humidification (30–100mL/h);
this is usually nebulised with the jet.
Indications
Bronchopleural fistula is the only
proven ICU indication for HFJV. It may assist weaning from mechanical
ventilation as the open circuit allows pontaneous breaths without the
drawbacks of demand valves. HFJV also ensures adequate ventilation if the
patient fails to breathe adequately. Reducing the driving pressure and
increasing the respiratory rate may facilitate weaning further. In ARDS,
conventional ventilation can lead to ventilator trauma if a high VT is used. HFJV avoids problems associated with high VT, but is often unable to provide adequate ventilation in
isolation for patients with severe ARDS.
Setting
up HFJV
• A jet must be provided via a modified
endotracheal tube or catheter mount. Entrainment gas is provided via a
T-piece.
• VT cannot
be set directly, but is altered by adjusting jet size, I:E ratio, driving pressure, and respiratory rate
from inbuilt algorithms.
• Respiratory rate is usually set between
100–200/min. As the rate increases at a constant driving
pressure, PaCO2
may increase (increasing PEEPi increases effective
physiological dead space).
• The I:E ratio is usually set between 1:3
and 1:2. VT
is determined by airway pressure and I:E ratio.
• Driving pressure is usually set between
1–2bar. These pressures are much higher than the 60–100cmH2O used in conventional ventilation.
PEEPi is related to the driving
pressure, I:E ratio, and respiratory rate.
• External PEEP may be added to increase
mean airway pressure should this be necessary to improve
oxygenation.
Combined
HFJV and conventional CMV
May be useful in conditions of poor lung
compliance where HFJV alone cannot provide adequate gas exchange. Low
frequency pressure limited ventilation with PEEP provides an
adequate mean airway pressure to ensure oxygenation while CO2 clearance is effected by HFJV. Care must be taken to avoid excessive peak airway
pressure when HFJV and CMV breaths stack.
Adjusting
HFJV according to blood gases
• Increasing PaO2
• Increase FIO2.
• Increase I:E ratio.
• Increase driving pressure.
• Add external PEEP.
• Consider reducing respiratory rate.
Decreasing PaCO2
• Increase driving pressure.
• Decrease respiratory rate.
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