Thorax 1985;40:161-165 Editorial

Mechanical ventilation at higher than normal fre-quencies (HFV) enables gas exchange to be achieved with low tidal volumes and peak airway pressures, thus offering the possibility of reducing barotrauma in patients with severe, acute lung dis-ease. The technique was first used experimentally in the late 1960s ' to minimise fluctuations in intra-arterial pressure associated with mechanical ventila-tion and has now been used for respiratory support in patients with normal and abnormal lungs. The range of frequencies used is 1-40 Hz (60-2400 bpm), the lower range (1-5 Hz) being used predo-minantly with high frequency positive pressure ven-tilation (HFPPV), or high frequency jet ventilation (HFJV), and the higher range (5-40 Hz) with high frequency oscillation (HFO). Although this rather crude categorisation ignores many of the subtle dif-ferences in the design of the ventilator and gas delivery systems which may affect their perfor-mance, it highlights the essential differences be-tween the three techniques. In HFPPV an intermittent flow of gas is delivered via a pneumatic or mechanical valve system to an endotracheal tube or insufflation catheter placed in the trachea. Entrainment of additional gas is mini-mal and expiration is passive. In HFJV the gas is delivered at high velocity from a jet. Entrainment may be significant but is very variable, and expira-tion is again passive. In both these techniques minute volume is determined by the inspiratory flow rate and the duration of inspiration, so that an increase in frequency results in a reduction in tidal volume. Usually, the inspiratory:expiratory time ratio (Ti/Ttot) can be varied. This is an important consideration since the time for expiration is limited at high frequencies and large increases in lung vol-ume may occur.2 In HFO, a sinusoidal pattern of flow is generated by a piston pump, metal bellows or loudspeaker cone, so that inspiration and expiration are usually of equal duration and the pattern of inspiratory and expiratory flow is actively control-led. Since stroke volume is unaffected by frequency, minute volume is increased when frequency is increased