Trauma patients may be affected by several conditions predisposing them to acute lung injury (ALI) and frequently fulfill all criteria for ALI proposed by the American-European Consensus Conference on Acute Respiratory Distress Syndrome (AECC) 1 . However, concerns have been raised that these ALI criteria (acute onset, presence of a typical risk factor, arterial partial pressure of oxygen to fraction of inspired oxygen ratio (PaO₂/FiO₂) less than 300 mmHg, absence of heart failure and bilateral infiltrates visualized on chest X-rays) capture a heterogeneous group of patients and may be nonspecific, particularly in trauma patients 2 3 4 . The appropriateness of ventilatory management of trauma patients based solely on these criteria has also been questioned 4 5 .

Computed tomography (CT) has a higher sensitivity than radiographs for detecting lung parenchymal changes 6 7 . Nevertheless, the visual confirmation of bilateral pulmonary infiltrates by CT instead of chest X-rays is not supported by the current ALI definition and carries the risk of detecting pulmonary opacifications with limited clinical relevance 1 6 . Despite this limitation, quantitative CT (qCT) analysis enables the unique noninvasive assessment of total lung weight (M_lung) and can be used to distinguish different causes of early posttraumatic pulmonary opacification and thus different populations of ALI patients 2 8 9 10 11 12 13 14 .

If a patient has pulmonary opacifications on qCT but has a normal M_lung, atelectasis due to hypoventilation, the use of anesthetics and high inspiratory oxygen concentrations would be the most likely explanation for impaired oxygenation 15 . If a significantly increased M_lung suggests consolidation from a more significant lung injury (for example, hemorrhage, contusion or edema from capillary leakage) 10 11 12 13 , a focus on the prevention of secondary lung injury, such as by performing damage control surgery and implementing lung-protective mechanical ventilation, would appear appropriate 3 4 16 17 18 19 . Atelectasis mimicking ALI instead may warrant more aggressive ventilatory management and early definitive surgical management 4 5 20 21 22 23 24 .

In this study, we aimed to use qCT to (1) establish a reference interval for M_lung of mechanically ventilated trauma patients with morphologically and functionally normal lungs and (2) study M_lung in trauma patients who fulfilled the ALI criteria. We hypothesized that qCT would identify atelectasis as a frequent mimic of early posttraumatic ALI. In the future, this information could aid in managing patients with early posttraumatic lung dysfunction.

Data for this prospective observational study were collected during routine clinical management at the University Hospital Leipzig. The study was approved by the ethics committee of the University of Leipzig (approval numbers 202/2003 and 311/2007). The need for informed consent was waived because no interventions or additional patient manipulations were required.

Our study consisted of two parts (Figure 1). First, we analyzed the M_lung of trauma patients with normal lungs to establish a reference interval (reference group). Second, M_lung values were assessed in patients with early posttraumatic ALI. A small subset of qCT data used in the present study were analyzed in a previous noninterventional study 25 .

We found that atelectasis was the most likely cause of lung dysfunction in more than half of patients who fulfilled the clinical criteria for ALI and showed lung opacifications on admission CT early after trauma.

Comparison of M_lung values derived from qCT with a reference interval for normal M_lung could help to assess the etiology of ALI and improve the definition of different populations of ALI patients 2 8 10 11 12 13 14 42 . A group of mechanically ventilated, volume-loaded trauma patients with morphologically and functionally normal lungs offered us the opportunity to confirm the normal range of M_lung obtained in previous analyses of diagnostic CT in healthy, spontaneously breathing volunteers 10 11 . The M_lung values measured in our reference groups are in good agreement with the M_lung values from these previous qCT analyses and M_lung of normal lungs at autopsy 10 11 40 . Thus, our results suggest that moderate positive intrathoracic pressure potentially affecting pulmonary blood and/or lymph flow and moderate intravenous volume loading have limited effect on M_lung.

Calculation of M_lung and parameters such as the excess lung tissue or weight by performing qCT can help to distinguish atelectasis from consolidation due to more significant lung damage 10 11 12 13 43 . It could be argued that atelectasis may also be distinguished visually from contusion or edema on the basis of typical topographical distributions. Analysis of qCT, however, can still assess M_lung in the presence of pleural fluid or when atelectasis is obscuring edema or pulmonary contusions 16 22 . When lung aeration is impaired without a concomitant increase in M_lung, atelectasis is the most likely explanation 11 13 . Accordingly, atelectasis was the most plausible cause of lung dysfunction in 59% of our ALI patients (Table 3). Interestingly, atelectasis patients also had significantly lower V_lung values than consolidation patients (Table 3). Although V_lung was not controlled in our study, the latter observation is compatible with the concept of atelectasis: V_lung is reduced by collapse, while consolidation of the lung does not necessarily decrease V_lung 44 . The identification of trauma patients in whom atelectasis mimics ALI could be helpful in decision making and individualization of care (that is, early definitive stabilization rather than damage control surgery). Atelectasis may persist into the posttraumatic period, promote bacterial growth and nosocomial pneumonia and affect patient outcome 3 23 45 46 47 48 49 50 . Therefore, more aggressive ventilatory management, early definitive surgical treatment and timely weaning from mechanical ventilation could shorten the ICU treatment and reduce the incidence of infections in patients with atelectasis 4 20 21 22 23 24 49 .

Thirty-two ALI patients (41%) had increased M_lung. In only 17 patients (22%) was M_lung increased to the range previously reported for ALI patients, suggesting consolidation from more significant lung injury due to contusion, hemorrhage, aspiration or edema resulting from pulmonary and/or systemic inflammation with capillary leakage 10 11 12 13 . Although fluid overload may also play a role 3 , we did not find significantly higher infusion volumes in consolidation patients, and all five patients who received more than four liters of infusions had M_lung values within the reference interval (Table 3). The association of severe head injury with increased M_lung further underlines the fact that multiple factors, such as neurogenic pulmonary edema, may be involved in the development of posttraumatic lung dysfunction 41 . Even if the precise etiology of posttraumatic lung dysfunction remains unclear in some patients, information on preexisting lung damage could help clinicians to judge the individual patient's tolerance for further aggressive shock resuscitation and definitive surgical repair 20 24 . It could also guide clinicians in choosing treatment concepts such as lung-protective mechanical ventilation or damage control surgery, which are focused on the prevention of "second hits" to lungs which have already been primed by shock and pulmonary or systemic injuries. Among such "second hits" are surgical trauma, ongoing intraoperative blood loss and transfusion, fat embolism following intramedullary nailing or injurious mechanical ventilation 3 17 18 19 20 51 .

Parameters such as ISS or PaO₂/FiO₂, which have previously been used for the prediction and further characterization of posttraumatic ALI, failed to distinguish atelectasis from consolidation patients 3 52 53 . In contrast, age as well as LIS, GCS and qCT results differed statistically significantly between these groups. Interestingly, atelectasis patients spent fewer days on mechanical ventilation and in the ICU than consolidation patients (Table 3). However, given the fact that all patients fulfilling the ALI criteria early after trauma have been managed according to the damage control concept in our institution, the latter differences should be considered hypothesis-generating rather than hypothesis-confirming. The variable reliability of clinical parameters and scores for characterizing posttraumatic ALI supports the potential clinical usefulness of qCT, which is the only available in vivo method to directly and reliably quantify M_lung and the amount of nonaerated lung tissue, which both characterize the severity of lung injury 10 11 12 52 .

Some aspects of our methodology warrant discussion. (1) We studied ALI patients within 24 hours after trauma (Table 1) because it was our aim to study the etiology of early posttraumatic respiratory failure, which may differ significantly from respiratory problems developing later 3 4 49 54 . (2) All whole-body CT scans performed in our emergency trauma patients routinely involved the clinically indicated application of contrast material 21 31 . A possible effect of contrast material on the normal M_lung was the reason why we included a reference group and did not refer only to existing data 10 11 40 55 . The normal M_lung found in our reference patients matched that in previous reports, which supports the lack of an effect of contrast material on the qCT assessment of M_lung in patients with normal lungs 55 . Patients with atelectasis should also remain unaffected by a possible contrast material-associated increase in M_lung. In contrast, the leakage of contrast material into the pulmonary interstitium may artefactually increase M_lung calculated on the basis of qCT in patients with an injured alveolar-capillary barrier 55 . However, although desirable from a scientific perspective, contrast material administration appears unavoidable in emergency trauma patients, and a possible artefactual increase in M_lung must be taken into account. (3) Because varying segmentations result in inconsistent M_lung values, we used a threshold-based (-350 HU) segmentation technique in addition to manual segmentation to improve the highly subjective manual exclusion of partial volume effects at the boundaries of aerated lung regions. So far, no CT study in ALI patients has included such attempts, and thus this threshold was adopted from other thoracic qCT applications. (4) Because the manual interaction necessary for qCT analysis is time-consuming, it might still be considered unrealistic to introduce qCT-based information into clinical practice. The extrapolation method, which we described recently, offers significant time savings and could aid the clinical implementation of qCT 14 25 .

qCT can detect different etiologies of posttraumatic lung dysfunction. Atelectasis was the most likely cause of early posttraumatic lung dysfunction in more than half of our patients. Whether individualized care based on qCT actually offers an option to prevent secondary lung injury, reduce posttraumatic pulmonary complications and improve outcome remains to be studied.

• Diagnosis, management and further study of ALI in trauma patients may be hampered by uncertainties about the fulfillment of the criteria for ALI proposed by the AECC.

• Differentiation between atelectasis and consolidation of the lung by qCT may help to identify patients with different etiologies of posttraumatic lung dysfunction.

• In our study, atelectasis was the most likely cause of early posttraumatic lung dysfunction in more than half of patients, and only 20% of patients had M_lung values in the range previously reported for ALI.

• Trauma patients with atelectasis may require shorter periods of mechanical ventilation and treatment in the ICU.

• In the future, information from qCT could aid in managing patients with early posttraumatic lung dysfunction.

AECC: American-European Consensus Conference on Acute Respiratory Distress Syndrome; AIS-T: Abbreviated Injury Scale of the Thorax; ALI: acute lung injury; ANOVA: analysis of variance; ARDS: acute respiratory distress syndrome; 95% CI: 95% confidence interval; CT: computed tomography; FiO₂: fraction of inspired oxygen; GCS: Glasgow Coma Scale; HU: Hounsfield units; ICU: intensive care unit; IQR: interquartile range; ISS: Injury Severity Score; LIS: Lung Injury Score; M_lung: lung weight; PaO₂: arterial partial pressure of oxygen; PEEP: positive end-expiratory pressure; qCT: quantitative analysis of computed tomography; TTSS: Thoracic Trauma Severity Score; V_lung: lung volume.

The authors declare that they have no competing interests.

AWR and APR contributed equally to this work. AWR, APR, DS, MS, CJ and MBPA planned the study. AWR, APR, DS, MS, HB, and UG were responsible for the data acquisition. AWR, APR, TH, AR, MS, HB, SB and UG performed the quantitative CT analysis. AWR, PMS, HW, MGA and MBPA undertook the statistical analysis. All authors contributed to the preparation of the manuscript. The principal investigators, AWR and APR, had full access to the data analyzed in the study and take full responsibility for the integrity of all of the data and the accuracy of the data analysis.