Supporting Low Tidal Volume Ventilation for Everyone
In early 2013, I wrote a column for Medscape titled, "Low-TV Ventilation: Is It the Rule for All ICU Patients?" In it, I discussed whether or not all patients who are mechanically ventilated, not just those with the acute respiratory distress syndrome (ARDS), should have their tidal volume (TV) set at 6 mL/kg of ideal body weight (IBW). Limited data indicate that they should.
A study published in the mid-January issue of the American Journal of Respiratory and Critical Care Medicine provides more justification for using 6 mL/kg of IBW for everyone.
The investigators examined TV settings serially over time for patients with ARDS. They found that patients who started out with volumes > 6.5 mL/kg of IBW had a higher subsequent mortality, and each increase in initial TV of 1 mL/kg of IBW was associated with a 23% increase in intensive care unit (ICU) mortality.
How does this apply to patients without ARDS? The investigators postulated that many of the physicians caring for patients in their study simply didn't recognize the onset of ARDS right away. The study used data to support this hypothesis, which showed that patients with an initial TV > 6.5 mL/kg of IBW had at least one subsequent TV decrease during their hospitalization. This was inferred to mean that physicians did adjust their settings once they recognized ARDS, but there was a delay in recognition.
Early recognition is probably a product of multiple factors, including (but not limited to) ICU census, ICU care delivery model, and overall patient acuity. In addition, lung injury almost certainly occurs before the appearance of bilateral infiltrates on chest radiography, which is required for diagnosis.
If early recognition can be difficult and variable, why not start with 6 mL/kg of IBW for TV for all patients, so that you don't get caught with your pants down?
In my earlier commentary, I cited a pro/con debate published in Chest in 2011, in which one of the debaters argued that 6 mL/kg of IBW should not be the default setting for all patients.
Although the author accurately identified the limits to using a standardized setting based on IBW alone, he failed to identify a more practical approach. By the end, he said that if your ICU doesn't routinely measure lung volume and transpulmonary pressure, you're best off using 6 mL/kg of IBW. I've yet to work in an ICU that measures those values, so I'm sticking with low TV for all.