Ventilation inhomogeneity is one criterion among many in multidimensional PEEP titration

We appreciate the commentary from Dr Costa and Dr Amato [1] on our recent study [2], in which we proposed that ventilation inhomogeneity should be regarded as an additional prospective index along with blood gases, lung mechanics and hemodynamics in a multifactorial method to optimize positive end-expiratory pressure (PEEP) at the bedside.

We agree with Costa and Amato that some ventilation heterogeneity may be good [1]. Especially in patients with normal lungs, inhomogeneity of lung perfusion and ventilation along the gravity axis match each other - a match that is essential to optimize gas exchange. Little attention was paid, however, to isogravitational inhomogeneity of pulmonary perfusion and ventilation. When isogravitational inhomogeneity is included in the estimation, the influence of gravity is no longer dominant [3, 4]. Minimizing the ventilation inhomogeneity during PEEP titration is not intended to eliminate inhomogeneity at all (which is impossible by PEEP alone), but rather to find a balance between overdistension and atelectasis. Physiological hetero geneity (good heterogeneity) may be preserved at the selected PEEP level.

The global inhomogeneity index [2] can be independently combined with any region-of-interest-definition method, if appropriate. Given that no perfect method exists currently for identification of collapsed lung areas by electrical impedance tomography, our approach [5] guarantees to include as much of the collapsed lung regions as is detectable in the analysis of ventilation inhomogeneity and provides satisfactory results, as shown in a preliminary study by comparison with computed tomography.

The titration of PEEP actually requires a multidimensional approach. A single ideal PEEP level, satisfying all clinical objectives and situations, may even not exist. A weighted combination of lung mechanics, blood gas analysis and imaging techniques to titrate PEEP therefore seems appealing and promising.