Low-tidal-volume ventilation should be implemented at an initial tidal volume of 6 ml per kilogram of predicted, not actual, body weight, as in the ARDSNet trial (NEJM, 2000). The predicted body weight (PBW) is calculated as follows:
- for men, PBW = 50.0 + 0.91 (height in centimeters - 152.4); and
- for women, PBW = 45.5 + 0.91 (height in centimeters - 152.4).
Lung size has been shown to depend most strongly on height and sex. A given person should not receive a higher lung volume just because of weight gain as the lungs are essentially the same size irrespective of weight gain.
Figure 2. Conventional Ventilation as Compared with Protective Ventilation.
NEJM: This example of ventilation of a 70-kg patient with ARDS shows that conventional ventilation at a tidal volume of 12 ml per kilogram of body weight and an end-expiratory pressure of 0 cm of water (Panel A) can lead to alveolar overdistention (at peak inflation) and collapse (at the end of exhalation). Protective ventilation at a tidal volume of 6 ml per kilogram (Panel B) limits overinflation and end-expiratory collapse by providing a low tidal volume and an adequate positive end-expiratory pressure. Adapted from Tobin.