Practical assessment of risk of VILI from ventilating power: a conceptual model

the bedside, assessing the risk of ventilator-induced lung injury (VILI) requires parameters readily measured by the clinician. For this purpose, driving pressure ( DP ) and end-inspiratory static ‘plateau’ pressure ( P_s ) of the tidal cycle are unquestionably useful but lack key information relating to associated volume changes and cumulative strain. ‘Mechanical power’, a clinical term which incorporates all dissipated (‘non-elastic’) and conserved (‘elastic’) energy components of inflation, has drawn considerable interest as a comprehensive ‘umbrella’ variable that accounts for the influence of ventilating frequency per minute as well as the energy cost per tidal cycle. Yet, like the raw values of DP and P_s , the absolute levels of energy and power by themselves may not carry sufficiently precise information to guide safe ventilatory practice. In previous work we introduced the concept of ‘damaging energy per cycle’. Here we describe how—if only in concept—the bedside clinician might gauge the theoretical hazard of delivered energy using easily observed static circuit pressures ( P_s and positive end expiratory pressure) and an estimate of the maximally tolerated (threshold) non-dissipated (‘elastic’) airway pressure that reflects the pressure component applied to the alveolar tissues. Because its core inputs are already in use and familiar in daily practice, the simplified mathematical model we propose here for damaging energy and power may promote deeper comprehension of the key factors in play to improve lung protective ventilation.