An investigation into factors influencing the application of high frequency jet ventilation.

The clinical application of high frequency jet ventilation (HFJV) is influenced by respiratory system mechanics, gas entrainment, adjustment of the ventilator controls and the configuration of the jet-airway interface. These factors have been investigated using an animal model, bench models and anaesthetised patients. The problems of airway pressure and gas flow measurement have also been examined. The results show that optimum oxygenation and carbon dioxide elimination occur between 1-4 Hz. Chest wall resonance was obtained at ~ 5 Hz but this was not associated with any improvement in gas exchange. An anti-resonant mode was also noted at lower frequencies (~ 0.8 Hz) which was associated with a deterioration in gas exchange. At higher frequencies (>2 Hz), abdomen and thorax responded as separate compartments. A multi-compartment visco-elastic model is presented for the respiratory system. Entrainment was inversely dependent on jet duty cycle (DC) between 10-60%, and ventilation frequency (between 1 - 10 Hz), but independent of drive pressure (DP). Maximum entrainment was obtained with DC values between 10 - 40% and frequencies between 1 - 6Hz. Airway pressure signals vary in morphology and peak, mean and trough levels according to position in the central airway. Pressure measurements have been standardised to a point 5 cm distal to the jet orifice. The morphology of this pressure signals reflects variations in load compliance, and flow resistance as well as local gas flow conditions. Peak pressures at this point are representative of more distal values. Trough (end expiratory) pressures are greater distally than proximally and are increased by increasing frequency, DC and DP controls. A low dead space, light weight pneumotachograph head has been designed and used to measure of expired and entrained volumes. This is based on a modified Pitot tube (MPT). Tidal volumes were dependent on DC and DP controls but inversely related to frequency. Tidal volumes and entrainment were both affected by the configuration of the jet-endotracheal tube (ETT) interface. The jet should not be situated <5 cm from the distal end of ETT because of greatly reduced tidal and entrained volumes. Distal and proximal location of the jet represent two main variations in configuration. Distal injection was found to be less sensitive to reductions in load compliance, while proximal injection gave higher tidal volumes and entrainment ratios which were attenuated by reductions in load compliance more easily than in distal injection. Recommendations for clinical application of HFJV following from these results have been made and topics for future work are suggested.