Is Mechanical Ventilation Protective Just Because We Say So?

In a recent issue of the American Journal of Critical Care, Chian and colleagues presented a case report describing the management of a patient who developed acute respiratory distress syndrome (ARDS) following inhalation of gas from a smoke bomb.1Much of the report focused on the use of extracorporeal life support to overcome recalcitrant hypoxemia. I would like to comment briefly on the mechanical ventilation strategies that were employed.The authors described numerous ventilator modes used in the effort to find optimal oxygenation response and patient-ventilator synchrony. Initial ventilator settings were a volume-controlled mode with a tidal volume of 500 mL. On day 16, the ventilator was changed to pressure-controlled ventilation that was programmed to produce a plateau pressure of 35 cm H2O. What is remarkable is that the authors contend that “protective ventilator strategies were used.” I would argue that they were not.The ARDS Network showed that a tidal volume of 6 mL/kg ideal body weight reduced mortality compared with larger tidal volumes.2 I do not know the height of the patient in this case, but this patient would have to be 190.5 cm tall (significantly taller than the 170.2 cm average male height in Taiwan) for a 500 mL tidal volume to be considered protective.Moreover, even if 500 mL were 6 mL/kg for this patient, plateau pressures of 35 cm H2O are anything but protective. In fact, these pressures are almost identical to the plateau pressures of the control group in the ARDS Network study (mean 34 cm H2O).1 Also, a plateau pressure of 35 is only 2 cm H2O lower than the mean plateau pressure of the control group in the study by Amato et al3 that the authors cite in their discussion as a example of protective ventilation.The beauty of hard science in medicine is the discovery and promulgation of objective data that minimize the subjective ideas and bias of clinicians. In terms of tidal volume selection for the management of patients with ARDS, the evidence-based recipe for protective ventilation is very straightforward: ≤6 mL/kg and plateau pressure <30 cm H2O.