American Journal of Respiratory and Critical Care Medicine

To the Editor:

We read with great interest the article by Chiumello and colleagues (1) on alveolar recruitment. We have some questions for the authors regarding their computed tomography (CT) method, as well as comments.

First, when the authors describe the effects of changing the CT thresholds for measuring recruitment (from −100 to −500 Hounsfield units [HU]), in their Table 3 and Table E6, they show that in the patients with the lowest tertile of PaO2/FiO2 (median, 126 mm Hg), the amount of recruited tissue decreases when the authors move this threshold from −100 to −500 HU. We find this counterintuitive. Does it mean that the change in radiodensity with positive end-expiratory pressure (PEEP) in the sickest patients is essentially a result of an increase in poorly inflated tissue with increasing PEEP (i.e., a derecruitment possibly caused by local compression atelectasis [2, 3])? This is hard to believe at a PEEP of only 15 cm H2O, and if there were to be a derecruitment, we would expect it to be very minimal. We therefore suspect another possibility, which is that a portion of the noninflated tissue was reaerated with PEEP but remained below −500 HU and was then counted as poorly inflated tissue in their analysis. In that case, although the authors count it as less recruitment (greater poorly inflated tissue at higher PEEP), others could decide to count this as more recruitment in terms of reaeration entering the sick lung. We find the latter approach more intuitive.

In addition, we would like to point out that the title of the authors’ Table E6 indicates a threshold of −300 HU, whereas the authors reached a threshold of −500 HU.

Second, as the authors mentioned in the Discussion, method B (the voxel-by-voxel analysis with a threshold of −500 HU) was different from Rouby’s method (4). Using method B, a change of radiodensity from −100 to −499 HU for a given lung region would not be counted as recruitment. Again, as clinicians, we find this definition counterintuitive, as the gas component increasing from 10 to 49.9% suggests the aeration is improved in the poorly inflated tissue and can be considered as recruitment. With our limited knowledge, we think it may be too early to conclude that this reaeration (within the poorly inflated tissue) has no benefit in terms of reducing ventilator-induced lung injury.

Third, another source of confusion about the recruitment defined by the authors comes from the fact that the weight of lung tissue is computed by the average CT number of a given amount of voxels. For example, assuming the poorly inflated compartment is made of 105 voxels (each voxel being 0.002625 ml in this study), it represents 262.5 ml; a change of the average CT number of this compartment from −100 to −499 HU should be measured as a remarkable reduction in the weight of poorly inflated tissue, according to the following equation: recruitment in PI (g) = PI5 − PI15 = 90% × 262.5 ml × 1 g/ml − 50.1% × 262.5 ml ×1 g/ml = 105 g, where PI5 and PI15 are grams of poorly inflated tissue at 5 and 15 cm H2O PEEP, 90 and 50.1% are the tissue components, 262.5 ml is the volume of compartment, and 1 g/ml is the approximate density of tissue. The authors, however, mention in the discussion that their calculation would give zero recruitment.

In summary, it is very difficult, despite our great interest in this article, to be convinced that what the authors call recruitment is what, as clinicians, we have in mind. We hope the authors can help to clarify this point.

1. Chiumello D, Marino A, Brioni M, Cigada I, Menga F, Colombo A, Crimella F, Algieri I, Cressoni M, Carlesso E, et al. Lung recruitment assessed by respiratory mechanics and by computed tomography in patients with acute respiratory distress syndrome: what is the relationship? Am J Respir Crit Care Med 2016;193:12541263.
2. Gattinoni L, Pelosi P, Crotti S, Valenza F. Effects of positive end-expiratory pressure on regional distribution of tidal volume and recruitment in adult respiratory distress syndrome. Am J Respir Crit Care Med 1995;151:18071814.
3. Gattinoni L, Caironi P, Cressoni M, Chiumello D, Ranieri VM, Quintel M, Russo S, Patroniti N, Cornejo R, Bugedo G. Lung recruitment in patients with the acute respiratory distress syndrome. N Engl J Med 2006;354:17751786.
4. Malbouisson LM, Muller JC, Constantin JM, Lu Q, Puybasset L, Rouby JJ; CT Scan ARDS Study Group. Computed tomography assessment of positive end-expiratory pressure-induced alveolar recruitment in patients with acute respiratory distress syndrome. Am J Respir Crit Care Med 2001;163:14441450.

Author disclosures are available with the text of this letter at


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American Journal of Respiratory and Critical Care Medicine

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