Pressure Control Surrogate Formula for Estimating Mechanical Power in ARDS is Associated with Mortality

Abstract

BACKGROUND: Mechanical power applied to the respiratory system (MP_RS) is associated with ventilator-induced lung injury (VILI) and ARDS mortality. Absent automated ventilator MP_RS measurements, the alternative is clinically unwieldy equations. However, simplified surrogate formulas are now available and accurately reflect values produced by airway pressure-volume curves. This retrospective, observational study examined whether the surrogate pressure-control equation alone could accurately assess mortality risk in ARDS subjects managed almost exclusively with volume-control ventilation.

METHODS: 948 subjects were studied in whom invasive mechanical ventilation and implementation of ARDSNet ventilator protocols commenced ≤ 24hr after ARDS onset, and who survived > 24hr. MP_RS was calculated as 0.098 x respiratory frequency x VT x (PEEP + driving pressure [P_DR]). MP_RS was assessed as a risk factor for hospital mortality, and compared between non-survivors and survivors across Berlin Definition classifications. In addition, mortality was compared across 4 MP_RS thresholds associated with VILI or mortality (ie. 15, 20, 25 and 30 J/m).

RESULTS: MP_RS was associated with increased mortality risk: Odds Ratio (95% CI) of 1.06 (1.04-1.07) per J/m, P<0.001). Median MP_RS differentiated non-survivors from survivors in Mild (24.7 vs. 18.5 J/m, respectively, P==0.034); Moderate (25.7 vs. 21.3 J/m, P<0.001); and Severe ARDS (28.7 vs. 23.5 J/m, P<0.001). Across 4 MP_RS thresholds mortality increased from 23–29% when MP_RS was < threshold vs. 38–51% when MP_RS was > threshold (P<0.001). In the > cohort the Odds Ratio (95%CI) increased from 2.03 (1.34-3.12) to 2.51 (1.87-3.33).

CONCLUSION: The pressure control surrogate formula is sufficiently accurate to assess mortality in ARDS, even when using volume control ventilation. In our subjects when MP_RS exceeds established cut-off values for VILI or mortality risk, we found mortality risk consistently increased by a factor of > 2.0.