To the Editor:
Adaptive support ventilation (ASV) allows clinicians to set a maximum plateau pressure (PP) and a desired minute ventilation. Thus, ASV automatically determines the respiratory rate and tidal volume (VT) based on its algorithms and hereto adjusts VTto keep PP below the set maximum. In a lung model with varying mechanics, all mimicking acute respiratory distress syndrome (ARDS), Sulemanji et al. 1compared ASV with conventional mechanical ventilation with a fixed VTof 6 ml/kg. Maximum airway pressure limit was 28 cm H2O in ASV. The major finding was that ASV “sacrifices” VTand minute ventilation to maintain PP in some scenarios (i.e. , VTwas <6 ml/kg, and minute ventilation was lower than desired). As such, ASV seems a safe mode of mechanical ventilation. However, their results also suggest that ASV may be unsafe in other scenarios. Indeed, although median-delivered VTwas similar with ASV compared with conventional mechanical ventilation with a fixed VTof 6 ml/kg (6.27 vs. 6.08 ml/kg in the 60-kg group and 5.24 vs. 6.13 ml/kg in the 80-kg group), in certain scenarios, maximum-delivered VTcould be as high as 9.0 and 8.3 ml/kg in the 60-kg group and the 80-kg group, respectively. Such large VTcan and should never be seen as safe.
The commonly held view that large VTventilation may be tolerated as long as the PP remains at less than 30–35 cm H2O has been questioned in a secondary analysis of the landmark study on lung-protective lower VTventilation by the ARDS Network.2To assess for independent effects of VTreduction on mortality, Hager et al. 3constructed a multivariable logistic regression model. For this, the study groups were stratified by quartiles of PP. Hager et al. identified groups of patients who would have had similar PP had they been randomized to the same VTstrategy. The lower VTstrategy was associated with a lower mortality than the traditional VTstrategy in all PP quartiles. From this, we conclude that the beneficial effect of VTreduction from 12 to 6 ml/kg is independent of PP.
The same may apply for patients at risk for ARDS. Gajic et al. 4reported significant variability in the initial VTsettings in mechanically ventilated patients without acute lung injury or ARDS at the onset of mechanical ventilation. Of the patients ventilated for more than 5 days, 25% developed lung injury within 5 days of mechanical ventilation. In this study, the main risk factors associated with the development of lung injury were the use of large VT, next to transfusion of blood products, acidemia, and a history of restrictive lung disease. The odds ratio of developing lung injury was 1.3 for each milliliter of VTabove 6 ml/kg.
In this context, we would like to stress that the terminology chosen for lung-protective mechanical ventilation (using lower VT) is wrong and maybe even misleading. Instead of “lower” VT, we should use the term “normal” or “normally sized” VT. Let us compare “traffic speeding” with lung-injurious forms of mechanical ventilation: traffic speeding (using too high VT) during “rush hours” (ARDS) is dangerous, but traffic speeding (using too high VT) may always be dangerous, even when there are not so many other cars on the road (no ARDS); therefore, regulations (guidelines) mandate that we should drive not faster than the speed limit (6 ml/kg). “Sacrificing” lower VTwith mechanical ventilation may be dangerous.
*Academic Medical Center, University of Amsterdam, Amsterdam, The Netherlands. firstname.lastname@example.org