All in the critical care community know 6 ml/kg (recently changed to 4 ml/kg) tidal volume guidelines for ventilating ARDS patients.
Many mistakenly interpret ARDS Net trials as a recommendation for 6 ml/kg, while the only logical conclusion should have been that using 12 ml/kg is dangerous. Assuming “the smaller, the better,” logic does not consider basic physiological principles.
In practice , extremely small tidal volumes cause significant dyspnea directly and via inducing hypercarbia. Additionally, short inspiratory times associated with low tidal volumes cause double triggering. As a result, considerable dyssynchrony ensues, which promotes lung injury requiring deep sedation and paralysis, which carry their own risks.
In theory, normalizing tidal volume to patients’ body weight is flawed since the relationship is only valid if the lungs are healthy. Lungs are injured by various degrees in different patients, and the size of the functional aerated lung has little relationship to the body weight.
Multiple studies demonstrated no correlation between mortality and tidal volume size (outside of 12 ml/kg). Plateau pressure, PEEP, and driving pressure (plateau pressure minus PEEP) were suggested as possible targets.
In his clever study from Brazil published in NEJM in 2015, Marcelo Amato et al. used data from existing ARDS trials and regrouped patients so that one of the variables was kept constant. He then observed the relationship between the other variable that changed and mortality. The only variable strongly correlated with mortality was DRIVING PRESSURE, as below.
