 The risk of excessive breath-stacking leading to barotrauma and pneumothorax is low but not zero.In HFJV exhalation is passive (depends on passive lung and chest-wall recoil) whereas in HFOV gas movement is caused by in-and-out movement of the “loudspeaker” oscillator membrane.Thus in HFOV both inspiration and expiration are actively caused by the oscillator, and passive exhalation is not allowed.