Inspiratory vs expiratory limb of the pressure–volume curve for the positive end-expiratory pressure setting in acute lung injury

Background

It is not clear whether the lower inflection point (LIP) on the inspiratory limb or the point of maximum curvature (PMC) on the deflation limb of the pressure–volume (PV) curve should be used for the positive end-expiratory pressure (PEEP) setting in acute lung injury (ALI).

Objective

To compare two ventilatory strategies using LIP or PMC as the PEEP level.

Methods

Patients with early ALI were prospectively studied. PV curves were traced using the continuous positive airway pressure method. LIP and PMC were calculated using a sigmoid fitting. Patients were ventilated at the computed tomography (CT) scan ward with FiO₂ 1, tidal volume 6 ml/kg and rate according to PaCO₂ and absence of autoPEEP. Two PEEP levels (LIP + 2 cmH₂O and PMC) were tested consecutively. For each PEEP level, three CT scan slices were acquired in end-inspiration and end-expiration. Lung mechanics and volume of hyperinflated, normal, poorly-aerated and nonaerated lung at each slice were compared using a repeated-measures analysis of variance. P < 0.05 was considered significant.

Results

Five patients were studied (age 64.6 ± 13 years, APACHE-II 24.4 ± 5.3, lung injury score 3.2 ± 0.4, PaO₂/FiO₂ 154 ± 30). When compared with LIP, pressures at PMC were always higher (22.4 ± 4.5 vs 14.6 ± 3 cmH₂O, P < 0.05), and so as plateau pressures (34.5 ± 4.3 vs 25.6 ± 3.5 cmH₂O, P < 0.05), with no changes in compliance (39.6 + 12.4 vs 34 + 6.3 ml/cmH₂O, P = 0.26). Lung volumes (ml per slice) are presented in Table 1.

Table 1

Conclusions

Ventilation above the PMC improves recruitment without increasing hyperinflation. The long-term benefits of this approach remain to be determined.