B. Christian Renne, MD
In 2015, Amato et al released a groundbreaking article in NEJM that posited driving pressure, or ΔP, as the major determinant of morbidity/mortality in Acute Respiratory Distress Syndrome (ARDS) patients . This was eye-opening because traditional teachings focus on PEEP and Tidal Volume as the major players in ARDS management. Driving pressure (ΔP) is best thought of as the difference between Plateau Pressure (PPLAT) and Positive End Expiratory Pressure (PEEP). It is also determined by tidal volume (VT) and compliance of a respiratory system (CRS). ΔP is best summarized as:
ΔP = PPLAT - PEEP or ΔP = VT / CRS
Knowing this, what did Amato and colleagues find that was so revolutionary? His group performed a multilevel mediation analysis on 3562 ARDS patients across nine randomized control trials in which they evaluated random ventilator settings, the corresponding dependent variables (most notably ΔP) and then patient survival. They adjusted for the effect of underlying lung disease. They found that, of all ventilation variables, ΔP was most strongly associated with survival — strongly enough that a 1 standard deviation increase in ΔP (7 cmH2O) had a 1.41 relative risk of mortality! (P < 0.001) even in patients with otherwise “protective” plateau pressures and tidal volumes. That is to say, even patients with VT < 7 ml/kg and PPLAT < 30 had significantly increased mortality once their driving pressure rose above 14; when their PEEP was increased and thus their driving pressure decreased (ΔP = PPLAT - PEEP), their survival improved.
Why does ΔP matter? What makes it such a good marker of mortality amidst many other variables? The authors suggest that it is a surrogate for cyclic lung strain that is the most accessible and easiest to calculate. This cyclic lung strain is the very basis for ARDS — it is how ARDS rears its ugly pathophysiologic head and propagates alveolar damage. From this article, it appears that ΔP is the best marker of this cyclic lung strain and a more dynamic property than simply VT alone.
This article should is probably not the singular fulcrum upon which we should change his practice, but it does suggest we might pay more attention to ΔP than we currently do. These data are admittedly preliminary, but seem to suggest that our goal should be to minimize ΔP as much as possible: in fact, lowering a patient’s ΔP to 3-4 cmH2O reduces mortality by 5-10%! To that end, we should at least start thinking about and calculating ΔP on ARDS patients, knowing that it is - at least in this article - the major variable that emerged useful in predicting survival consistently. Meanwhile, there may be no single ideal VT, PEEP, or PPLAT setting, but instead it is a question of what these variables are doing to the ΔP.
Amato MB, Meade MO, Slutsky AS, Brochard L, Costa EL, Schoenfeld DA, et al. Driving pressure and survival in the acute respiratory distress syndrome. N Engl J Med. 2015;372(8):747–55.