Ventilator-related causes of lung injury : the mechanical power

Purpose: We hypothesized that the ventilator-related causes of lung injury may be unified in a single variable: the mechanical power. We assessed whether the mechanical power measured by the pressure–volume loops can be computed from its components: tidal volume (TV)/driving pressure (∆Paw), flow, positive end-expiratory pressure (PEEP), and respiratory rate (RR). If so, the relative contributions of each variable to the mechanical power can be estimated. Methods: We computed the mechanical power by multiplying each component of the equation of motion by the variation of volume and RR: Powerrs=RR·{ΔV2·[12·ELrs+RR·(1+I:E)60·I:E·Raw]+ΔV·PEEP},where ∆V is the tidal volume, ELrs is the elastance of the respiratory system, I:E is the inspiratory-to-expiratory time ratio, and Raw is the airway resistance. In 30 patients with normal lungs and in 50 ARDS patients, mechanical power was computed via the power equation and measured from the dynamic pressure–volume curve at 5 and 15 cmH2O PEEP and 6, 8, 10, and 12 ml/kg TV. We then computed the effects of the individual component variables on the mechanical power. Results: Computed and measured mechanical powers were similar at 5 and 15 cmH2O PEEP both in normal subjects and in ARDS patients (slopes = 0.96, 1.06, 1.01, 1.12 respectively, R2 > 0.96 and p < 0.0001 for all). The mechanical power increases exponentially with TV, ∆Paw, and flow (exponent = 2) as well as with RR (exponent = 1.4) and linearly with PEEP. Conclusions: The mechanical power equation may help estimate the contribution of the different ventilator-related causes of lung injury and of their variations. The equation can be easily implemented in every ventilator’s software.