Size and How You Measure It Matters

For many years, a practice known as vanity sizing has been increasing the physical dimensions of women’s clothing relative to its nominal size, resulting from the lack of an industry standard that is adhered to.1 Vanity sizing impedes the reliable selection of properly fitting garments without donning them since similarly labeled clothing may be proportioned using different scales and vary significantly in size. An analogous predicament faces clinicians and researchers when they attempt to properly tailor tidal volume for a patient with ARDS who is mechanically ventilated. Unlike fashion where actual weight determines fit, tidal volumes are best indexed to predicted, not actual, body weight; being overweight does not grow lung and being underweight does not shrink lung. Yet, even knowing the major determinants of predicted body weight (height, age, and sex), there is no accepted standard for predicted body weight calculation and, in the past, researchers have used several different equations.2-5 In this issue of CHEST (see page 73), Linares-Perdomo and colleagues6 concisely illustrate how poorly correlated and widely discrepant tidal volumes can be when calculated using several different published prediction equations despite identical input variables. The authors compare recommended tidal volumes across a broad range of clinically relevant heights and ages for both sexes using three equations used in clinical trials.2-5 They found that the equation used by the National Institutes of Health/National Heart, Lung, and Blood Institute’s ARDS Network overall yielded the lowest predicted tidal volumes.3 Furthermore, at the edges of the age and height envelope, differences among methods ranged from 15% to 30% and were greatest for older, shorter subjects, especially women. It is to be expected that predicted body weights and the corresponding tidal volumes would differ when using different equations, but the magnitude of the variance is impressive, especially considering that the variance is not uniformly distributed but is rather disproportionately biased toward certain phenotypes such as the diminutive older patient. Two questions are likely to be asked about this report. First, are these calculated differences really relevant? And second, might the authors’ findings serve to explain why one clinical trial of lower tidal volume in ARDS might be successful and another not? The latter question cannot be answered with certainty, but use of an overall lower tidal volume is one plausible explanation for the lower mortality and shorter time on ventilation seen in the ARDS Network trial but not in all studies of lung protective ventilation. One skeptical of this possibility could argue that if the tidal volume derived from a given equation were too large it would be reduced when the resulting plateau pressure exceeded the recommended target. Unfortunately, in practice, plateau pressures are not always measured or acted upon, and lower tidal volumes might offer benefits even if the plateau pressure did not exceed a recommended threshold. The answer to the first question, however, is almost certainly yes; tidal volume differences of 30% could amount to > 100 mL of inflation for some patients and almost certainly could contribute to volutrauma in vulnerable individuals. Fortunately, lower, or perhaps more accurately described as normal, tidal volumes determined using predicted body weight have now become widely accepted for care of many patients who are ventilated with and without ARDS.7,8 But the question remains: Which equation should be used? Undoubtedly, the authors’ recommendation that the predicted body weight equations used by the ARDS Network become the “industry standard” will generate controversy among some readers who will claim that we do not know those equations are the “best.” While it is clear that we do not know if the equations used by the ARDS Network will be the best choice forever, it is clear that for now, they are the best we know.