Bedside Estimate of Mechanical Power (Part 4): Power Intensity Thresholds and Mortality Risk in ARDS

Background: Mechanical power (W) is ventilator energy transferred to the respiratory system (ie, WRS) over time (J/min). WRS exceeding 12, 15, and 25 J/min are considered ventilator-induced lung injury (VILI) thresholds of increasing severity.1 This study examined mortality risk associated with five WRS thresholds. Methods: 948 ARDS subjects were studied who: 1) met the Berlin definition, 2) were managed with ARDSNet protocols initiated ≤ 24 h after ARDS onset, and 3) survived ≥ 24 h after protocol initiation. A simplified WRS calculation was used: WRS = 0.098 x frequency x VT x (PEEP + PDR).2 Mortality risk was assessed at WRS thresholds of > 12, > 15, > 20 > 25, and > 30 J/min using Fisher-exact tests. Alpha was set at 0.05. Results: Mortality steadily increased both below and above each threshold, with cumulative mortality increases 8% vs. 14% respectively: the relative increases being the same for each cohort (138%). Over 40% of cases had WRS levels exceeding those associated with severe VILI risk. Conclusions: Mild and severe ARDS mortality (27%, and 45% respectively)3 approximates that found between cohorts across the 25 J/min threshold (27% and 47% respectively): a threshold associated with severe ventilator-induced lung injury.1 Given relative inexactness of the simplified clinical equation used, this particular finding only cautiously suggests a relatively high tolerance of WRS ≤ 25 J/min.1. Silva PL et al. Anesthesiology 2020;132:949. 2. Chiumello D et al. Crit Care 2020;24:417. 3. Berlin Taskforce. JAMA 2012;307:2526.Table 1Wrs ThresholdMortality (at or below)Odds Ratio (95%CI)Mortality (above)Odds Ratio (95%CI)% of Sample Above Threshold12 J/m21%0.45 (0.22-0.88)37%2.2 (1.14-4.27) †94%15 J/m23%0.49 (0.32-0.75)38%2.03 (1.34-3.12) ‡85%20 J/m24%0.43 (0.32-0.58)42%2.33 (1.72-3.16) ‡65%25 J/m27%0.43 (0.33-0.56)47%2.34 (1.77-3.07) ‡43%30 J/m29%0.40 (0.30-0.54)51%2.51 (1.87-3.33) ‡29%† P=0.019, ‡ P < 0.001