It has been suggested that the airway pressure used in the mechanical ventilation of patients with ALI/ARDS should be in-between the low and upper inflection points of the pressure-volume (P-V) curve. During constant inspiratory flow, analysis of the airway pressure-time profile (P-t) can replace the static P-V curve [87] . From the mathematical point of view, the airway pressure can be described as a function of inspiratory time by the following equation: airway pressure = at b ? c, where the coefficient b, called the stress index, describes the shape of the P-t curve during a tidal breath. If b \ 1, it means that the compliance increases, suggesting tidal recruitment; if b = 1, it suggests that the compliance does not change, while if b [ 1, it indicates compliance decreases, suggesting tidal overinflation [88] . Formenti et al. [89] tested the predictive ability of the stress index compared with quantitative lung CT scan in paralysed, mechanically ventilated pigs. The tests were conducted at PEEP values of 1 and 10 cm H 2 O in the absence and presence of pleural effusion by fluid instillation in pleural space. The investigators demonstrated a dissociation between the stress index, which suggested lung overinflation, and the CT scan, which showed lung recruitment. This observation suggests that the ability of the stress index to predict lung recruitment and overdistension was significantly reduced in the pathophysiological conditions. However, the authors did not measure the transpulmonary pressure in their study. An editorial [90] suggests that computation of the stress index as usually obtained from the airway pressure curve should be replaced by analysis of the transpulmonary pressure curve profile. In the heterogeneous airspace environment of ALI/ARDS, a reliable tool at the bedside to set desirable PEEP and VT is critical [91] . Traditional methods of monitoring lung mechanics using the pressure-volume curve, static compliance and the stress index have shown drawbacks, especially when the compliance of the chest wall is altered. Computed tomography (CT) scan is a powerful tool for mapping the distribution of the gas volume, but it is yet to be a part of bedside technology. The forced oscillation technique (FOT) may offer several potential advantages over traditional methods because it applies very small volume displacement, it minimises artifacts resulting from nonlinearity of the respiratory system, and it does not require deep sedation and muscle paralysis. Using FOT in a surfactant depletion model of ALI, Dellaca and colleagues [92] performed incremental and decremental PEEP trials, and measured both conventional and FOTderived respiratory system compliance at each step of PEEP changes. CT scan analysis was performed as a gold standard for monitoring lung compartments at end expiration. FOT was able to detect the minimal PEEP value needed to keep the lung open, and this was in good agreement with CT scan analysis with respect to sensitivity and specificity, suggesting that FOT may have potential value in defining adequate levels of PEEP to prevent/reduce the occurrence of de-recruitment. However, the validation of FOT to detect the extent of recruitment/de-recruitment and hyperinflation remains to be determined in more complicated clinical situations [93] . Variable ventilation is a new mode delivering volume-controlled ventilation with variation of the tidal volume around an average value. Pillow et al. [94] found that variable ventilation increased dynamic compliance but not static compliance or oxygenation in preterm lambs. They also found decreased PaCO 2 and concluded that variable ventilation was more efficient than conventional ventilation. The authors speculated that the lack of improvement in oxygenation between the groups was due to right to left shunt. However, one may argue that the pulmonary vascular resistance might have decreased with the observed improvement in compliance and reduced CO 2 levels, thus resulting in better pulmonary blood flow and oxygenation. Further investigations are needed to address the effects of variable ventilation in physiological and pathological conditions.