Tidal volume and airway pressure on high frequency jet ventilation

The principle of jet injector indicates that large tidal volumes may be delivered on high frequency jet ventilation (HFJV) without increasing airway pressure. Fifteen dogs were ventilated on HFJV in 2 separate experiments. In the first one, tidal volume was maintained constant at 10 ml/kg, while PEEP, respiratory rate, and cannula size were changed in 16 different experimental conditions. In the second experiment, driving pressure was progressively increased from 5 to 45 psig, and PEEP, respiratory rate, and injector size were changed in 32 experimental conditions. Mean airway pressure, tidal volume, driving pressure, thoracic aortic mean pressure, and abdominal aortic mean pressure were the variables measured. Tidal volume linearly increased with driving pressure, while airway pressure only increased when tidal volume exceeded 25 ml/kg. Blood pressure was inversely related to mean airway pressure. Tidal volume was twice as high with the 1.62 mm injector, as compared to the 1.06 mm injector, although resistances are 6 times higher with the smaller injector. The difference is related to the higher entrainment, which is observed when jet flow velocity increases, as is the case when the injector cannula is smaller. The experiments confirmed that HFJV follows the physical principles of jet mixing and entrainment.