To the Editor: In the observational study of ECMO treatment of patients with 2009 influenza A(H1N1) by Dr Davies and colleagues,1 we noted that tidal volume delivered before ECMO was described in mL/kg of body mass. We would like to know if these values were derived from the actual body mass of each patient or from the use of an ideal body mass.
We would question the use of actual body mass to determine lung volumes because physiological and anatomical lung volumes correspond with the height-derived “ideal” rather than the actual body mass.2 Tidal volumes in the original ARDSnet study were determined using ideal body mass.3 In any population, description of tidal volumes in terms of actual body mass would cause concern. In a disease with a possible predilection for patients with obesity, and in this group of patients with a median body mass index (BMI, calculated as weight in kilograms divided by height in meters squared) of 29, the actual body mass would be significantly higher than the ideal body mass and would systematically understate tidal volumes expressed as mL/kg of ideal body mass.
Freebairn R, McHugh G, Hickling K. Extracorporeal Membrane Oxygenation for ARDS Due to 2009 Influenza A(H1N1). JAMA. 2010;303(10):941-942. doi:10.1001/jama.2010.201