Transpulmonary pressure targets for open lung and protective ventilation: one size does not fit all

Dear Editor,

More than 20 years ago Dreyfuss et al. and Hernandez et al. [1, 2] showed that ventilator-induced lung injury is better linked to transpulmonary pressure (P _L) than to airway pressure (P _AW). With this concept in mind Grasso et al. recently reported that using a P _L target, as opposed to a P _AW target, allowed them to use unconventionally high PEEP and end-inspiratory plateau pressure of the respiratory system (PPLAT_RS) in a subgroup of patients with ARDS associated with influenza A (H1N1) who were candidates for extracorporeal membrane oxygenation (ECMO) due to life-threatening refractory hypoxemia [3]. In patients with high chest wall elastance (E _CW), the application of higher PEEP and PPLAT_RS allowed by this approach provided sufficient lung recruitment to avert the need for ECMO. This proof-of-concept report compellingly shows the limitations of P _AW to gauge ventilatory settings in the context of high E _CW.

One intriguing aspect of this communication, though, is the selection of 25 cmH₂O as the upper physiological limit of P _L to which end-inspiratory plateau pressure of the lung (PPLAT_L) was titrated [3]. No direct data support the safety of this value. Terragni et al. [4] described a subpopulation of ARDS patients with tidal hyperinflation and lung inflammation at a mean PPLAT_RS of 28.9 cmH₂O. This less-protected subpopulation seemed to fit with Rouby’s focal ARDS tomographic lung morphology [5]. Likewise, in patients with focal-ARDS, Grasso et al. [6] showed overdistension and systemic inflammation at a mean PPLAT_RS of about 28.7 cmH₂O that was relieved through PEEP reduction at a mean PPLAT_RS of about 21.6 cmH₂O. Pursuing a PPLAT_L of 25 cmH₂O with PEEP in patients with focal ARDS would therefore likely lead to overdistension. Grasso et al. [7] also showed PEEP-induced overdistension at a mean PPLAT_RS of 28.6 cmH₂O in nonrecruiter ARDS patients. From the partitioned respiratory system mechanics data provided in these studies it can be inferred that overdistension occurred at a mean PPLAT_L of about 23.8 and 23.4 cmH₂O, respectively [6, 7].

ARDS associated with influenza A (H1N1) seems to fit with the diffuse and highly recruitable lung morphology [8]. In fact increasing PEEP and PPLAT_L led to an impressive oxygenation improvement in the no-ECMO group [3]. It is not that clear if overdistension was absent as the mean PEEP increase of 4.4 cmH₂O led to a mean PPLAT_L rise of 8.7 cmH₂O and a 40 % higher lung elastance. Ventilatory efficiency cannot be used as an index of overdistension, as four of seven patients in this group required low-flow extracorporeal CO₂ removal [3]. The benefit of recruitment seemed to outweigh any potential overdistension in this group of patients [3].

In conclusion, it seems reasonable to explore the effects of higher-than-usual PEEP and PPLAT_RS in patients with catastrophic ARDS and high E _CW. However, in the same way we have been warned to be cautious before adopting standardized tidal volumes [4], PPLAT_RS limits [4, 6] and FiO₂-based PEEP tables [6, 7], we should be careful before selecting transpulmonary pressure targets. Conceivably, tomographic lung morphology could help to discriminate safe pressure limits for ARDS patients with specific attenuation patterns [9].