Bedside Estimate of Mechanical Power (Part 1): Mortality Risk in ARDS Comparing Power Applied to the Respiratory System and Its Lung and Chest Wall Components

Abstract

Background: Mechanical power (W) represents ventilator energy transferred to the lungs and chest wall (ie, W_RS) over time. W_RS is linked to both ventilator-induced lung injury and ARDS mortality. Estimating W_RS is complex and unwieldy as it encompasses minute ventilation, peak airway pressure, PEEP, and inspiratory flow.¹ A simplified, clinical equation for pressure control ventilation (PCV): W_RS = 0.098 x frequency x V_T x (PEEP + P_DR),¹ was used in this retrospective study to assess mortality risk, without distinguishing CMV modes (ie, P_DR = Pplat-PEEP vs. PIP-PEEP, and excluding inspiratory flow).

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 (allowing lung-protective ventilation optimization and censoring moribund cases). W_L and W_CW were estimated using subcomponents of ARDS elastance data:² E_L/E_RS (0.72), E_CW:/E_RS (0.28). Other calculations included ratio of measured W_RS to normal W_RS (~4 J/min) and ΔW_RS ~24 h after protocol initiation. Mortality risk was assessed by univariate logistic regression. Alpha was set at 0.05.

Results: Hospital mortality was 35.6%. All W estimates predicted mortality risk (Table), having identical area under the receiver operating curve and P-values: AUC (95%CI) 0.65 (0.61-0.69), P < .001. W_RS was ~ 6-fold higher than normal: median (IQR) 5.8 (4.4-7.8). W_CW, was associated with a higher mortality risk than W_L.

Conclusions: A simplified W_RS estimate of ARDS mortality risk suggests complex W equations may not be clinically necessary. Higher mortality risk with increasing W_CW might reflect the presence of intra-abdominal hypertension that may portend worse clinical outcomes in ARDS. 1. Chiumello D et al. Crit Care 2020;24:417. 2. Kallet RH et al. Respir Care 2007;52(8):989.