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Tidal Volume in Mechanical Ventilation: The Importance of Considering Predicted Body Weight

We agree with Drs. Günther and Taut, in their comments on our article (1), that the best way to express tidal volume would have been as ml/kg of predicted body weight (BW), but unfortunately, in the first study of mechanical ventilation in 1998 (2), we did not record height, and were therefore unable to calculate predicted BW. For this reason, the comparison between the two international studies was done with tidal volume expressed as ml/kg of actual BW. The comparison provided in the abstract was of mean tidal volume (ml/kg actual BW) in the first week following a diagnosis of acute respiratory distress syndrome (ARDS). In the second international study of mechanical ventilation, we were able to calculate the predicted BW and, as surmised, the tidal volume expressed in this fashion was higher. Among the 4,968 patients included in the 2004 study, median (interquartile range [IQR]) recorded actual BW was 75 kg (64–85), while predicted BW (using the ARDSNet formula) was 64 kg (55–71). In 2004, the highest tidal volume recorded in the first week after the diagnosis of ARDS was a median of 11 ml/kg predicted BW (IQR, 9.5–12) and the lowest tidal volume was a median of 8 ml/kg predicted BW (IQR, 6.5–9).

Regardless of the availability of predicted BW, we believe that our results do show a convincing decline in administered tidal volume between 1998 and 2004. It is possible that outcomes would have improved further in 2004 with a greater tidal volume reduction, but this remains speculative. While many would agree that large tidal volumes should be avoided in ARDS, as these authors are no doubt aware, strict adherence to a tidal volume of 6 ml/kg remains controversial. In addition, whether it is the tidal volume or the plateau pressure limitation that is most important in limiting ventilator-induced lung injury also remains unresolved. As we outlined in the DISCUSSION of our article (1), there are numerous reasons why our study did not demonstrate a statistically significant improvement in mortality for patients with ARDS, only one of which is the delivered tidal volume in 2004.

Andrés Esteban and Fernando Frutos-Vivar

Hospital Universitario de Getafe
Madrid, Spain

Niall D. Ferguson

University Health Network
University of Toronto
Toronto, Canada

Antonio Anzueto

South Texas Veterans Health Care System
and
University of Texas Health Science Center
San Antonio, Texas

FOOTNOTES

Conflict of Interest Statement: None of the authors has a financial relationship with a commercial entity that has an interest in the subject of this manuscript.

REFERENCES

1. Esteban A, Ferguson ND, Meade MO, Frutos-Vivar F, Apezteguia C, Brochard L, Raymondos K, Nin N, Hurtado J, Tomicic V, et al.; for the VENTILA Group. Evolution of mechanical ventilation in response to clinical research. Am J Respir Crit Care Med 2008;177:170–177.[Abstract/Free Full Text]
2. Esteban A, Anzueto A, Frutos F, Alía I, Brochard L, Stewart TE, Benito S, Epstein SK, Apezteguía C, Nightingale P, et al. Characteristics and outcomes in adult patients receiving mechanical ventilation: a 28-day international study. JAMA 2002;287:345–355.[Abstract/Free Full Text]

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