High frequency jet ventilation: the influence of different methods of injection on respiratory parameters.

Fourteen critically ill patients with ventilatory failure were ventilated with HFJV using a constant driving pressure of 3 atm and a constant I:E ratio of 0.43. In six (group I) HFJV, using a frequency of 100 b.p.m., was delivered via a proximal injector cannula. Four injector cannulae (i.d. 1.8, 1.6, 1.4 and 1.2 mm, respectively) were used at random. In eight patients (group II), HFJV using three frequencies (100, 300 and 600 b.p.m.) was administered either via a 1.8-mm i.d. proximal injector cannula (proximal injection), for via a Hi-Lo jet tracheal tube (distal injection). The following indices were measured in vivo: mean airway pressure (Paw), mean pulmonary volume above apnoeic FRC (delta V), mean alveolar pressure (clamps method) and arterial blood-gas tensions. Injected volume (Vinj), entrainment (E) and tidal volume (VT) were measured in vitro using a water-sealed spirometer. Operating pressure (the pressure in the connecting tube) was measured directly using a high pressure calibrated transducer. In group I, Paw, delta V, PaO2, Vinj and VT significantly decreased whereas operating pressure and E significantly increased when the internal diameter of the injector cannula was reduced. In group II, Paw, delta V, E, VT and operating pressure significantly decreased, whereas Vinj increased and PaO2 and PaCO2 remained unchanged, when proximal injection was switched to distal injection. In both groups, a significant relationship was found between operating pressure and E. At a constant driving pressure, operating pressure increased with narrow injector cannulae and increased frequency, and decreased when proximal injection was switched for distal injection.