LAUSR

Dear Editor, In a recent letter, the relative strength of association of mechanical power and driving pressure (ΔP) with mortality was assessed [1]. Both the mechanical power concept and the driving pressure approach rely on airway driving pressure (ΔPAW), i.e., driving pressure of the respiratory system, but it is the transpulmonary driving pressure (ΔPTP), which is the pressure that “hits” the lung and therefore one of the major factors of ventilator-induced lung injury (VILI). The Gattinoni group has shown in patients with acute respiratory distress syndrome (ARDS) subjected to a PEEP trial, that the overall ΔPTP/ΔPAW was 0.71, but ranged from 0.36 at PEEP 5 cmH2O in patients with extrapulmonary ARDS, to 0.98 at PEEP 15 cmH2O in patients with pulmonary ARDS [2]. In ten ARDS patients, ΔPTP/ΔPAW ranged from 0.28 to 0.76 during a PEEP trial with PEEP steps 0–4–8–12–16 cmH2O [3](Fig. 1). This indicates that for an individual patient, it is important to use transpulmonary driving pressure, rather than airway driving pressure as a rational basis for protective ventilation [4]. There is a notion that this requires esophageal pressure measurements, which are neither easy, nor simple, and would never be possible to use in all patients. However, we have shown, that the change in end-expiratory lung volume (EELV) following a PEEP increase is determined by the size of the PEEP step and the elastic properties of the lung only (EL), ΔPEEP/EL [3, 5]. Consequently, lung elastance can be determined as ΔPEEP/ ΔEELV, if ΔEELV is measured by the ventilator during a simple one-PEEP-step up and down procedure from baseline clinical PEEP, where the procedure is finalized by setting the tidal volume equal to ΔEELV [3, 5]. During such a measurement procedure, end-expiratory transpulmonary pressure (PTPEE) increases as much as PEEP is increased, i.e., ΔPTPEE = ΔPEEP, which means that endexpiratory transpulmonary pressure is equal to PEEP, PTPEE = PEEP. Transpulmonary driving pressure of the tidal volume equal to the change in EELV is equal to the change in PEEP, ΔPTP = ΔPEEP [3, 5], and end-inspiratory transpulmonary pressure is equal to PEEP + ΔPEEP. Thus, the one-PEEP-step up and down procedure provides both transpulmonary driving pressure and endinspiratory transpulmonary plateau pressure without esophageal pressure measurements. If the measurement procedure is extended to a twoPEEP-step procedure, a best-fit lung P/V curve can be calculated from end-expiration at baseline PEEP to end-inspiration of the highest PEEP level, covering the transpulmonary pressure range that is acceptable from a VILI point of view. The tidal lung P/V curve, of any tidal volume, moves up and down this lung P/V curve, which makes it possible to determine the PEEP level, where the transpulmonary driving pressure is lowest, irrespective of the size of the tidal volume, from the best-fit lung P/V curve [5] (for details, see e-supplement). The measurement procedure can easily be implemented and automatized in any ventilator’s software and used in trials where the effect of reduced mechanical power is compared with PEEP setting where transpulmonary driving pressure is lowest (least injurious).