Questionable association between mechanical power and ventilator-induced lung injury in acute respiratory distress syndrome

To the Editor: To mitigate ventilation-induced lung injury (VILI) in acute ­respiratory distress syndrome (ARDS), restraint of total inspiratory power (W’insp), denoted mechanical power, transferred from ventilator to the respiratory system is proposed.1 W’insp equals the total inspiratory work transferred per breath (W’TOT, insp) times the respiratory rate (RR). Injurious work is dissipated in the process. Bates et al. reasoned that only the fraction of W’insp dissipated ­toward the lung periphery where VILI occurs (which we denote dissipated power [W’DIS, lung]) is relevant, and they called for a method to measure dissipated power and clinical data.2 We present such a method and data from a reanalysis of a ­published series of ARDS patients with the goal to evaluate the effect of a strategy favoring low mechanical power by assessing the effect on dissipated power. This method is not readily available today. A preliminary report has been presented previously.3 Fourteen consecutive adult patients with moderate to severe ARDS were retrospectively studied at low and high positive end-expiratory pressure (PEEP), 8 ± 2 and 16 ± 2 cm H2O, respectively.4 Patients’ characteristics included ventilation for 3 days, age 59 ± 13 years, Simplified Acute Physiology Score II (SAPS II) 57 ± 19, tidal volume (VT) 6 ± 1 mL/kg of predicted body weight, and respiratory rate (RR) 25 ± 3 breaths/minute. Ventilator flow and pressure signals were digitized. Integration of flow ­yielded ­volume. The study was approved by the Ethics Committee of Henri Mondor University Hospital, Créteil, France.