Isocapnic high frequency jet ventilation: dead space depends on frequency, inspiratory time and entrainment.

Twelve healthy pigs were ventilated with high frequency jet ventilation via a Mallinckrodt HiLo jet tube. The expired gas was led to a conventional ventilator and CO2 analyzer which were used to measure CO2 elimination. There was no bias flow, so that the jet entrained only expired gas, i.e. rebreathing occurred. Frequency was varied between 2 and 11 Hz and the duration of inspiration, as a fraction of the ventilatory cycle (Ti/Ttot), from 5 to 20%. The minute ventilation, Vjet, delivered by the jet ventilator was adjusted to maintain a constant PaCO2. At 2 Hz and a Ti/Ttot of 5%, Vjet was of the same magnitude as ventilation during conventional intermittent positive pressure ventilation, and the total dead space fraction, VD/VT was 0.32. Both increasing frequency at a constant Ti/Ttot, and increasing Ti/Ttot at a constant frequency, increased VD/VT which was maximal (0.8) at 11 Hz and a Ti/Ttot of 20%. When entrainment was blocked, tidal jet volume had to be greatly increased. The continuous measurement of CO2 elimination was found to be useful for maintaining isocapnia when the jet ventilator setting was changed.