Better understanding of the physiopathology of ventilatory mechanisms associated with ARDS and the recent re evaluation of the iatrogenic potential of mechanical ventilation (MV) brings us closer to the best suited ventilatory mode for these patients. In severely ill ARDS patients, only a small lung volume is ventilated, and remains available for the totality of the gas exchanges (baby lung concept). The goal of MV is to restore and maintain an optimal exchange volume while limiting mechanical agression of the lung tissue. Analysis of the ARDS related pressure-volume relationship (P/V) is helpful in specifying the tolerable limits of the ventilatory pressure regimen. The lower limit (end expiratory pressure) must be kept above the lower inflexion point of the curve, in order to increase the ventilated lung volume and avoid distal airway collapse. Under this limit, gas exchanges are altered by the shunt effect, and shear stress lesions result from the repeated opening and closing of the distal airways. The upper limit (end inspiratory pressure) must be situated below the upper inflexion point of the curve, in order to avoid lesions resulting from surdistension of the alveolocapillary membranes and barotraumatisms. The only way to position MV in such a narrow pressure window, is to greatly reduce the tidal volume (VT). Though CO2 retention would inevitably occur under conventional MV conditions, high frequency ventilation (HFV) seems better adapted to these theoretical objectives; small VT's are injected under a limited amplitude pressure regimes and a satisfactory CO2 clearance is maintained. This ventilatory mode, existing since more than 15 years, has recently benifited from many technical improvements as well as the concept of oscillating the ventilation around a selected mean pressure in the central region of the PN curve. In the past, HFV was applied using low pressure regimens, situated below the lower inflexion point of the curve. The resulting failures are, a posteriori, explained by insufficient lung volumes, unable to maintain adequate gas exchanges. Current work is aimed at re-evaluating HFV, using higher mean airway pressure levels. Combined HFV is another advance towards the theoretical goal of restoring and maintaining optimally ventilated lung volumes. Though HFV alone ran maintain lung volumes oscillating around a mean value, it cannot re-expand atelectatic areas. The small VT's used are insufficient to overcome these area's elevated opening pressures. Volume recruitement by periodic hyperinflations, or sighs, though generally considered useless in conventional MV, have been shown to be of great benefit in HFV. The atelectatic areas opening pressures are reached with the sighs, and ventilation of the newly recruited territory is maintained by the immediate succession of HFV. Combined HFV exploits the hysteresis between the expiratory and inspiratory limbs of the PN curve, as well as the lung's visco-elastic properties. The pressure necessary to maintain an airway open once recruited is, in fact, lower than the required opening pressure. As well, the lung's visco-elastic properties introduce a notion of time dependence : as the pulmonary stress and strain are out of phase, the rapid succcession of the HFV cycles does not provide the time necessary for the unstable alveoli to collapse. As such, combined HFV establishes a double airway pressure regimen; a high frequency component produced by a high frequency jet ventilator and low frequency component produced by a conventional ventilator. This relatively complex mode of ventilation requires heavy sedation and administration of muscle relaxants, and strict surveillance by a trained staff. A simplified procedure for setting and adjusting the respiratory parameters is described. Though further studies are necessary to confirm the interest of HFV, the first evaluations on small patient populations are quite encouraging. In the future, this technique might well find its place between the most sophisticated conventional ventilatory modes and the extra-pulmonary gas exchange techniques which represent the ultimate treatment of gas exchange failure.
