We thank Drs. Dongelmans and Schultz for their letter expressing interest in our recent publication on adaptive support ventilation (ASV).1They correctly describe how ASV works but indicate that the ability of ASV to vary tidal volume in response to a changing clinical presentation is of concern especially if the tidal volume is allowed to exceed 6 ml/kg.
First, it is important to remember that the tidal volumes used by the Acute Respiratory Distress Syndrome Network2in its landmark study varied between 4 and 8 ml/kg. Indeed, as we showed in our study, even though the average tidal volume delivered to patients in the low-tidal volume arm was about 6 ml/kg, tidal volume did vary between 4 and 8 ml/kg in many patients.
We believe that allowing tidal volume to increase while keeping plateau pressure at a minimum setting (<28 cm H2O in our study) is the major concern of Drs. Dongelmans and Schultz, and they reference Hager et al. 3to demonstrate their point. However, they failed to acknowledge the subsequent letter from Shiu and Rozen4who determined from Hager's data that no significant change in mortality was observed regardless of tidal volume once plateau pressure was less than 28 cm H2O. Drs. Dongelmans and Schultz also referred to the article by Gajic et al .,5which was a retrospective review with plateau pressures available on only a few patients to illustrate the potential of large tidal volumes causing acute lung injury. The tidal volume range applied by ASV is essentially within the range of the lowest risk group (≤9 ml/kg) in the article by Gajic et al .5In addition, there are numerous articles in the surgical literature that indicate that at least short-term application of large tidal volume does not result in lung injury6–10in patients without existing lung injury. As we noted in our discussion, the upper and lower limits on ASV may need to be adjusted, and we believe that the upper limit should be set for patients with acute lung injury or acute respiratory distress syndrome at 8 ml/kg. However, the concept of ASV is sound because if practitioners are left on their own to adjust tidal volume, even centers who participated in the Acute Respiratory Distress Syndrome Network trial do not always appropriately select low tidal volumes and plateau pressures.11
Where we believe the concept of ASV is most critical is in the patient where in spite of tidal volume being set at 6 ml/kg, plateau pressure exceeds 28 cm H2O. It is very clear that in these patients, the risk of increased mortality is real.3,12ASV does in these patients what the clinician should do and that is to reduce the tidal volume to avoid overdistension. ASV may not have the absolute limits correct, but the concept of closed loop control of ventilation is the future!
*Massachusetts General Hospital and Harvard Medical School, Boston, Massachusetts. email@example.com