1858 DETERMINANTS OF TIDAL VOLUME (VT) DURING HIGH'S-FREQUENCY JET VENTILATION(HFJV)

Abstract

The very short inspiratory time (TI) during HFJV may compromise the ability to deliver adequate VT. To document the airway pressures needed to maintain a constant VT (2.5 to 3.0cc/kg) (approx.dead space), TI was varied by changing frequency (f) over 120,240 and 480/min at I:E ratios of 1:1, 1:3, 1:5 and 1:9. A 16g cath. was inserted in a 3.0mm ET tube in 9 adult rabbits and used to monitor aΔP (peak minus end expiratory pressure). VT was measured via a pneumotachometer placed on the expiratory arm of the ventilator and validated with a body plethysmograph. The integrated flow signal was reset by the jet solenoid at onset and end of inspiration. At f of 240 and 480/min there was a significant inverse relationship between TI and the ΔP required to deliver constant VT (ANOVA p<.05). At TI <50ms there was at least a 3-fold increase in ΔP. Despite the longer TI (62ms) at f=480/min and I:E=1:1, ΔP was still increased because the shortened expiratory time produced marked air trapping. At a rate of 120/min, TI did not significantly influence ΔP. Net air flow at the pneumotach during the on phase of the jet cycle was in the outward direction indicating lack of gas entrainment. In summary: 1) as TI was shortened during HFJV higher ΔP was necessary to maintain VT and 2) in this model, gas entrainment did not contribute to the delivered VT. Duration of TI is therefore critical for optimal VT delivery if barotrauma is to be minimized. Furthermore, the delivery of humidification and O2 during HFJV cannot be dependent on gas entrainment. Supp. ALA-Ohio, ALAND