Measuring Mortality in ARDS: Mechanical Power vs Ventilatory Variables

In patients with acute respiratory distress syndrome (ARDS) using mechanical ventilation, the energy transfer rate (mechanical power) from ventilator to patient is linked to mortality during controlled mechanical ventilation; however, a simpler model suggests that only driving pressure and respiratory rate are independently linked to survival and, together, can also predict mortality, according to study results published in the American Journal of Respiratory and Critical Care Medicine.

An international team of investigators aimed to determine the effect of mechanical power on mortality in patients with ARDS compared with primary ventilatory variables including driving pressure, tidal volume, and respiratory rate by using data from a large observational cohort and 6 randomized clinical trials of protective mechanical ventilation. The primary outcome measured morality at 28 or 60 days.

A total of 4549 patients (mean age, 56 years; 38% women) were included in the analysis. The average mechanical power was 0.32±0.14 J/min/kg predicted body weight, the driving pressure was 15.0±5.8 cmH₂0, respiratory rate was 25.7±7.4 breaths per minute, and the overall mortality rate was 38%.

In adjusted analyses, the driving pressure, respiratory rate, and mechanical power were significant predictors of mortality, and the effect of driving pressure on morality was 4 times greater than that of respiratory rate.

The driving pressure and respiratory rate were independently linked to survival, but were comparable with mechanical power in predicting mortality; however, its dynamic-elastic component was mainly why mechanical power was independently associated with mortality. In addition, when compared with respiratory rate, a greater effect of dynamic pressure may have practical implications such as ventilatory strategies with higher respiratory rates, a lower tidal volume, and can be beneficial for severely ill patients.

Despite mechanical power being associated with mortality in patients with ARDS, driving pressure and respiratory rate were easier to assess and as informative.

“Whether a ventilation strategy based on these variables improves outcomes needs to be tested in randomized controlled studies,” the investigators concluded.

Reference

Costa ELV, Slutsky A, Brochard LJ, et al. Ventilatory variables and mechanical power in patients with acute respiratory distress syndrome. Am J Respir Crit Care Med. Published online March 30, 2021. doi:10.1164/rccm.202009-3467OC