High frequency jet ventilation HFJV

A high pressure jet of gas entrains further fresh gas which is directed by the jet towards the lungs. Respiratory rates of 100-300/min ensure minute volumes 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 on HFJV, though usually improves with time. The high gas flow rates employed require additional humidification to be provided (30-100 ml/h); this is usually nebulised with the jet.

Indications

Bronchopleural fistula is the only proven ICU indication for HFJV though it has been used to assist weaning from mechanical ventilation as the open circuit allows spontaneous 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. The tidal volume cannot be set directly. Rather it is set by adjusting jet size, I:E ratio, driving pressure and respiratory rate from an in-built algorithm. The respiratory rate is usually set between 100-200/min. As respiratory rate increases at a constant driving pressure the PaCO2 may increase as increasing PEEPi increases the 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 ARDS 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.

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