One variable involved with mechanical ventilation – driving pressure – may predict mortality in adult respiratory distress syndrome, according to a report published online Feb. 19 in the New England Journal of Medicine.
Research suggests that scaling ventilator tidal volumes (VT) to patients’ body weight will minimize ventilator-induced lung injury. But patients with ARDS have a marked decrease in the proportion of lung available for ventilation, as is indicated by their lower respiratory-system compliance (CRS), which is not related to their body weight. “Therefore, we hypothesized that normalizing VT to CRS and using the ratio as an index indicating the ‘functional’ size of the lung would provide a better predictor of outcomes in patients with ARDS than VT alone,” said Dr. Marcelo B. P. Amato of the cardiopulmonary department, University of Sao Paulo (Brazil) Heart Institute, and his associates.
This ratio, also known as the driving pressure, is easily calculated at the bedside. The investigators explored whether driving pressure or other variables related to mechanical ventilation, including variables set by the ventilator operator, could be statistically linked to survival outcomes and therefore serve as a risk predictor.
They first devised a survival-prediction model using data from a cohort of 336 ARDS patients participating in four randomized clinical trials examining different ventilation strategies. They then tested their findings using a validation cohort of 861 patients from a single large trial, then tested them again in a more recent validation cohort of 2,365 patients participating in four more randomized trials comparing different ventilation strategies.
Driving pressure was the only ventilation variable found to be strongly associated with survival. Higher driving pressures strongly predicted higher mortality: Every 1-SD increase in driving pressure was related to increased mortality, with a relative risk of 1.41. Even in patients receiving lung-protective plateau pressures and low tidal volumes, higher driving pressure was associated with increased mortality, with a relative risk of 1.36, Dr. Amato and his associates said (N. Engl. J. Med. 2015 Feb. 19 [doi:10.1056/NEJMsa1410639]).These findings can only suggest that driving pressure is a critical mediator of various ventilator strategies, since they are derived from a post hoc observational statistical analysis and cannot establish causality. Now, prospective clinical trials are needed to determine whether adjusting ventilator settings to lower driving pressure will improve survival in ARDS; in other words, “whether our observations can be translated into changes that may be implemented at the bedside,” the investigators noted.
