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(Radiology. 2001;218:689-693.)
© RSNA, 2001

Thoracic Imaging

Acute Respiratory Distress Syndrome Caused by Pulmonary and Extrapulmonary Injury: A Comparative CT Study1

Sujal R. Desai, MD, MRCP, FRCR, Athol U. Wells, MD, FRACP, Ganesh Suntharalingam, BA, MB, BChir, FRCA, Michael B. Rubens, DMRD, FRCR, Timothy W. Evans, BSc, MD, PhD, DSc, FRCP and David M. Hansell, MD, FRCP, FRCR

1 From the Department of Radiology (S.R.D., M.B.R., D.M.H.), the Interstitial Lung Disease Unit (A.U.W.), and the Intensive Care Unit (G.S., T.W.E.), Royal Brompton Hospital, Sydney St, London SW3 6NP, England. Received May 11, 2000; revision requested June 19; revision received August 16; accepted September 12. Address correspondence to D.M.H. (e-mail: d.hansell@rbh.nthames.nhs.uk).


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
PURPOSE: To determine computed tomographic (CT) differences between acute respiratory distress syndrome (ARDS) due to pulmonary injury (ARDSp) and extrapulmonary injury (ARDSex).

MATERIALS AND METHODS: CT appearances in 41 patients (27 male, 14 female; mean age, 47.1 years ± 17.1 [SD]; age range, 17–79 years; those with ARDSp, n = 16; those with ARDSex, n = 25) were categorized as typical or atypical of ARDS by two observers. The extent of individual CT patterns was also quantified.

RESULTS: Typical CT appearances were more frequent in ARDSex than ARDSp (18 [72%] of 25 vs five [31%] of 16 patients, respectively; P < .01). Sensitivity, specificity, and accuracy of a typical CT pattern for the diagnosis of ARDSex were 72%, 69%, and 71%, respectively. Atypical appearances were characterized by more extensive nondependent intense parenchymal opacification (IPO) (P = .03) and cysts (P = .05), whereas typical CT appearances had more extensive dependent IPO (P = .01). Typical appearances at CT were independently related to the cause of ARDS (odds ratio, 8.9; 95% CI: 1.8, 44.2; P < .01) but were independent of the time from intubation. Foci of nondependent IPO were more extensive in ARDSp (P = .05) than ARDSex, but this finding was ascribable to differences in time to CT (after intubation) between ARDSp and ARDSex.

CONCLUSION: The differentiation between ARDSp and ARDSex can, with some caveats, be based on whether the CT appearances are typical or atypical of ARDS but not on any individual CT pattern in isolation.

Index terms: Computed tomography (CT), electron beam, 60.12111, 60.12118 • Lung, CT, 60.12111, 60.12118 • Respiratory distress syndrome, adult (ARDS), 60.413


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
The acute respiratory distress syndrome (ARDS) is a nonspecific response of the lung to injury that can occur after a pulmonary or extrapulmonary insult. Examples of ARDS due to pulmonary injury (ARDSp) include pneumonia, aspiration, and toxic fume inhalation, while systemic sepsis and nonthoracic trauma result in ARDS due to extrapulmonary injury (ARDSex) (1–3). Histopathologic changes are common to both ARDSp and ARDSex (4–7). However, important physiologic differences between ARDSp and ARDSex have recently been described (8). At zero end-expiratory pressure, lung elastance is higher in patients with ARDSp than in those with ARDSex. Furthermore, there are differences in response to ventilation between ARDSp and ARDSex that may be attributable to more extensive consolidation in the former (8).

Computed tomography (CT) has been used in the detection of complications in ARDS (9). Furthermore, the ability of CT to accurately depict parenchymal abnormalities has resulted in several pathophysiologic insights in patients with ARDS (10–12). Goodman and colleagues (13) have recently reported differences at CT in patients with ARDS caused by pulmonary and extrapulmonary injury. The aim of the present study was to determine the morphologic abnormalities at CT in the two groups and to ascertain whether CT features can be used to distinguish ARDSp and ARDSex.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 REFERENCES
 
Ninety-six patients fulfilling American-European Consensus Conference criteria for the diagnosis of ARDS (3) were admitted to the intensive care unit at our institution between October 1993 and August 1998. All CT scans were obtained as part of the routine clinical treatment protocol of patients with ARDS at our institution. Patients were excluded from the study if (a) CT scanning was not possible because of unstable clinical status (n = 10); (b) CT scanning had been delayed more than 2 weeks from the time of intubation (n = 23); (c) there were large pneumothoraces, pleural effusions, lobar collapse, or extensive preexisting lung disease, such as emphysema, which would limit the precision with which parenchymal abnormalities could be scored (n = 7); and (d) the cause of ARDS could not be defined or was multifactorial (n = 15).

Forty-one patients (27 male, 14 female; mean age, 47.1 years ± 17.1 [SD]; age range, 17–79 years) were examined; a subset of this group (n = 19) has previously been described in a study (14) in which CT changes in the acute phase were compared with those of survivors of ARDS. Case records were reviewed to determine the cause of ARDS in each patient (Table 1). Patients were categorized as having ARDSp or ARDSex on the basis of the American-European Consensus Conference criteria (3); there were 16 patients with ARDSp and 25 patients with ARDSex. Patients who developed ARDS following lobectomy or pneumonectomy for lung cancer (n = 11) were included in the ARDSex group; in these patients, CT evaluation was confined to the lung not operated on.


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  		TABLE 1. Causes of ARDS
 
CT studies were performed with an electron-beam scanner (Imatron, San Francisco, Calif) with 3-mm collimation (at 10-mm intervals) to allow the shortest acquisition time of 100 msec; the mean duration between intubation and CT scanning was 6.0 days ± 3.9. Images were reconstructed with a high-spatial–frequency reconstruction algorithm and photographed at window settings appropriate for viewing the lung parenchyma (window center, -550 HU; window width, 1,500 HU).

CT Scoring
CT images were reviewed independently by two observers (M.B.R., D.M.H.). Scans were scored at five levels as follows: 1, aortic arch; 2, carina; 3, pulmonary venous confluence; 4, between levels 3 and 5; and 5, 1 cm above the dome of the right hemidiaphragm. Levels 1–3 were considered the upper zone; levels 4 and 5, the lower zone. In each patient, the first observer identified the five image sections to be scored and at each level drew a horizontal line across the image to demarcate anterior and posterior quadrants to ensure that identical CT areas were scored by the observers.

In each quadrant, the observers independently quantified (to the nearest 5%) the extent of the following CT patterns in accordance with the standard CT definitions (15): (a) ground-glass opacification, defined as a hazy increase in lung attenuation with preservation of bronchovascular markings; (b) intense parenchymal opacification (IPO), defined as a homogeneous increase in lung attenuation with obscuration of the bronchovascular structures in which an air bronchogram may be present (observers separately recorded the extent of dependent IPO [IPOd] and nondependent IPO [IPOnd]); (c) septal lines, defined as thin linear opacities corresponding to thickened interlobular septa; and (d) cysts, defined as thin-walled air spaces.

The global extents of individual CT patterns were derived from the mean percentages of all scored sections. The mean scores of the two observers were used in analysis. Observers also recorded a consensus statement of whether the distribution of morphologic abormalities at CT were typical of ARDS (ie, dependent regions of IPO merging with widespread nondependent ground-glass opacification and normally aerated lung [12,16]).

Statistical Analysis
Results are given as the mean ± SD (for normally distributed variables) or the median with a range (for nonnormally distributed data); a P value of less than .05 indicated a significant difference. Differences between subgroups were examined nonparametrically, by using the Wilcoxon rank sum test or the Kruskal-Wallis test, as appropriate (Stata data analysis software; Computing Resource Center, Santa Monica, Calif). Univariate correlations were examined by using the Spearman rank correlation test. Differences between proportions were tested by using the {chi}2 test or Fisher exact test as appropriate. To evaluate the possible confounding effect of time, logistic regression models were used to determine whether group differences were independent of the time between intubation and CT.


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 REFERENCES
 
The proportions of patients with individual CT patterns and extents are given in Table 2. Irrespective of cause, the most extensive CT patterns were ground-glass opacification and IPOd.


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  		TABLE 2. Proportion of Patients with Individual CT Patterns
 
Typical versus Atypical CT Appearances in Relationship to the Cause of ARDS
Among the whole group, 23 (56%) of 41 patients had CT appearances typical of ARDS (Fig 1). Typical CT findings were more strongly associated with ARDSex (18 [72%] of 25 patients) than with ARDSp (five [31%] of 16 patients) (P < .01). A typical pattern of ARDS at CT had a sensitivity of 72% (18 of 25 patients), a specificity of 69% (11 of 16 patients), and an accuracy of 71% (29 of 41 patients) for a diagnosis of ARDSex. As shown in Table 3, the typical CT pattern was characterized by more extensive IPOd (P = .01) but less extensive IPOnd (P = .03) and cysts (P = .05) than an atypical distribution (Fig 2). There was no difference in the total extent (ie, IPOd plus IPOnd) of IPO between patients with and without a typical CT pattern.



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  		Figure 1a. CT images in two patients with extrapulmonary injury demonstrate a typical pattern. (a) CT image (at level 3) obtained 5 days after intubation in a patient with ARDS following abdominal surgery shows that IPOd (x) merges with ground-glass opacification (o) in the nondependent lung. (b) CT image (at level 3) obtained in a patient with ARDS secondary to systemic sepsis (day 3 after intubation) shows less-intense parenchymal opacificaton in dependent lung. There is extensive ground-glass opacification and normally aerated lung parenchyma anteriorly.

 


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  		Figure 1b. CT images in two patients with extrapulmonary injury demonstrate a typical pattern. (a) CT image (at level 3) obtained 5 days after intubation in a patient with ARDS following abdominal surgery shows that IPOd (x) merges with ground-glass opacification (o) in the nondependent lung. (b) CT image (at level 3) obtained in a patient with ARDS secondary to systemic sepsis (day 3 after intubation) shows less-intense parenchymal opacificaton in dependent lung. There is extensive ground-glass opacification and normally aerated lung parenchyma anteriorly.

 

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  		TABLE 3. Association of CT Patterns with ARDS
 


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  		Figure 2a. Atypical CT pattern in two patients with ARDSp following bacterial pneumonia. (a) CT image (at level 3) obtained 6 days after intubation, shows patchy IPOnd (arrows) and ground-glass opacification (arrowhead). (b) CT image obtained 8 days following intubation shows that there are multiple bilateral cystic spaces (arrowheads) in the lower zones (level 5). Focal areas of IPO (arrow) and ground-glass opacification are also depicted.

 


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  		Figure 2b. Atypical CT pattern in two patients with ARDSp following bacterial pneumonia. (a) CT image (at level 3) obtained 6 days after intubation, shows patchy IPOnd (arrows) and ground-glass opacification (arrowhead). (b) CT image obtained 8 days following intubation shows that there are multiple bilateral cystic spaces (arrowheads) in the lower zones (level 5). Focal areas of IPO (arrow) and ground-glass opacification are also depicted.

 
The logistic regression showed a significant relationship between typical CT appearances and ARDSex (P < .01), with no independent relationship between CT appearance and the time between intubation and scanning; typical CT appearances were 8.9 times more likely in ARDSex than ARDSp (95% CI: 1.8, 44.2). Moreover, when analysis was restricted to 29 patients examined within 7 days of intubation, the association between typical CT appearances and ARDSex remained statistically significant (ARDSex vs ARDSp, 12 [75%] of 16 patients vs four [31%] of 13 patients, respectively; P = .02).

Comparison of CT Patterns: ARDSp versus ARDSex
In patients with ARDSp, IPOnd was more extensive than in patients with ARDSex (P = .05) (Table 3). However, the extents of other CT patterns did not differ between ARDSp and ARDSex. The more extensive IPOnd in ARDSp was evident both in the anterior (P = .01) and the posterior (P = .05) quadrants and in the lower (P < .01) but not upper zones.

The difference in the extent of IPOnd between ARDSp and ARDSex was partially ascribable to a shorter interval between intubation and CT in patients with ARDSp (median, 4 days; range, 0–12 days) than in patients with ARDSex (median, 7 days; range, 0–14 days) (P = .04). There was a negative correlation between the extent of IPOnd and the interval between intubation and CT (Spearman correlation coefficient, -0.35; P = .03). Logistic regression showed that, after controlling for time to CT, the trend toward more extensive IPOnd in ARDSp was no longer significant (P = .12). When analysis was confined to the 29 patients examined with CT in the 1st week after intubation, the extent of IPOnd did not differ between ARDSp and ARDSex.


   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
REFERENCES
 
In the early stages of ARDS, exudate accumulates in the interstitium and air spaces, and there is associated hyaline membrane formation and lung atelectasis (4,17). The conventional radiograph often shows diffuse and apparently homogeneous opacification of the lungs (7,18). However, at CT, it is clear that the parenchymal changes of ARDS are frequently heterogeneous; areas of IPO, ground-glass opacification, and normally aerated lung can coexist in varying proportions and distributions (10,19,20).

From several studies (10–12,16,19), it has been possible to define the typical CT distribution of parenchymal abnormalities in patients with ARDS; there is IPO in dependent lung that merges with ground-glass opacification and normally aerated parenchyma in the nondependent regions. The present study findings have shown that a typical CT distribution is seen more frequently in patients with extrapulmonary injury. In addition, our observations show that foci of IPOnd and parenchymal cysts are part of the atypical CT pattern; the extent of ground-glass opacification did not discriminate between typical and atypical CT appearances.

The sensitivity and specificity (both approximately 70%) of a typical CT pattern in the identification of extrapulmonary injury are surprisingly high in view of the nonspecific CT appearances of ARDS (21–23). However, in our study, seven of 25 patients with ARDSex had an atypical distribution, and five of 16 patients with ARDSp had typical CT features. Thus, the association between the gestalt observation of a typical CT pattern and ARDSex is not wholly consistent. This finding highlights a fundamental limitation of the present and previous studies of ARDS. In many patients, it is difficult to accurately assign a single cause, and this difficulty could mask potential differences between ARDSp and ARDSex. For example, a patient with ARDS following abdominal surgery might reasonably be categorized as having ARDSex (3). However, it is conceivable, in this specific situation, that aspiration of gastric contents (ARDSp) could have occurred during induction of anesthesia and might have been the inciting event. Moreover, because ARDS is multifactorial in many patients, it may not be possible to apply the apparently simple dichotomy of pulmonary and extrapulmonary injury (24). Our inclusion of patients who developed ARDS following thoracic surgery in the extrapulmonary group might be regarded as questionable, although nine (82%) of 11 patients had a typical CT distribution following lobectomy or pneumonectomy.

The potential therapeutic importance of confirming the cause of ARDS has recently been highlighted (8). In 21 patients with ARDS, lung elastance (the reciprocal of compliance) was higher in patients with ARDSp than in those with ARDSex. More important, there were striking differences in response to positive end-expiratory pressure ventilation; in patients with ARDSp, increasing positive end-expiratory pressure led to an increase in total respiratory system elastance, which was attributed mainly to the decreasing lung compliance. By contrast, increasing positive end-expiratory pressure in ARDSex was associated with a decrease in total elastance due to improved compliance of both the lung and chest wall. The authors postulated that a relatively high proportion of consolidation in ARDSp, as opposed to the predominance of interstitial edema and atelectasis in ARDSex, could account for the observed physiologic differences.

Recently, Goodman and colleagues (13) reviewed the CT abnormalities in patients with pulmonary and extrapulmonary injury. In ARDSex, ground-glass opacification was more extensive than consolidation. However, ground-glass opacification and consolidation were equally extensive in ARDSp. When the groups were compared, consolidation was more extensive in patients with ARDSp, and ground-glass opacification was more extensive in patients with ARDSex. In our study, there were no significant differences between the two groups for the total extent of IPO. There are several possible reasons for this difference, notably the time between intubation and CT. In the present study, IPOnd (the only CT feature that differed between pulmonary and nonpulmonary causes) was more extensive in ARDSp. However, patients with ARDSp underwent CT earlier than those with ARDSex, and there was a tendency for foci of nondependent consolidation to decrease in extent with time.

Theoretically, the difference in the extent of IPOnd may have reflected the earlier performance of CT in patients with ARDSp. This hypothesis is supported by the finding that, in patients examined within the 1st week, the extent of IPOnd did not differ between ARDSp and ARDSex. Therefore, we would agree with the hypothesis of Goodman et al (13) that the time to CT may have contributed to the observed differences between the two groups in their study. Additional factors that may explain the contrasting results between the two studies are the possibility of multiple causes of ARDS and the methods for quantifying CT abnormalities. In the present study, CT patterns were quantified on a continuous scale, but they were scored semiquantitatively in the study by Goodman et al (13).

It is likely that IPOnd has a different pathophysiologic meaning than IPOd: In dependent regions, intensely opacified lung is believed to reflect atelectasis that results from compression by the overlying edematous parenchyma (25–27). In contrast, it is probable that regions of IPOnd in ARDS reflect foci of consolidation. Severe pneumonia is a common cause of ARDSp (3,28,29) and, indeed, ARDS was a consequence of pulmonary infection in 13 (81%) of 16 patients in our study. In this context, it is important to emphasize that the time delay between intubation and CT may have a crucial bearing on the pattern of parenchymal opacification; nosocomial pneumonia is common in patients with ARDS (30,31), and the risk increases with prolonged mechanical ventilation (32), a problem compounded by the acknowledged difficulties in establishing a diagnosis of nosocomial infection (33–35).

In a recent study (36) of the CT features of ventilator-associated nosocomial pneumonia, the authors showed that nondependent opacities were more common in patients with ventilator-related pneumonia than in those without nosocomial pulmonary infection. However, a proportion of patients had developed ARDS secondary to pneumonia, and whether the underlying cause could have contributed to the CT appearances, in some cases, was not evaluated (36).

In summary, the differentiation of pulmonary from extrapulmonary ARDS, on the basis of CT features alone, is not straightforward; no single CT feature can be used in isolation to accurately predict whether ARDS is of the pulmonary or extrapulmonary type. However, differences do exist, and we have demonstrated that a typical CT pattern at CT, characterized by more extensive IPOd but less extensive nondependent consolidation and parenchymal cysts, is more frequently seen in patients with ARDSex.


   FOOTNOTES
 
Abbreviations: ARDS = acute respiratory distress syndrome, ARDSex = ARDS due to extrapulmonary injury, ARDSp = ARDS due to pulmonary injury, IPO = intense parenchymal opacification, IPOd = dependent IPO, IPOnd = nondependent IPO

Author contributions: Guarantor of integrity of entire study, D.M.H.; study concepts, and design, S.R.D., A.U.W., D.M.H.; definition of intellectual content, A.U.W., T.W.E., D.M.H.; literature research, S.R.D., G.S.; clinical studies, T.W.E., S.R.D., M.B.R., D.M.H.; data acquisition, S.R.D., G.S.; data analysis, M.B.R., D.M.H.; statistical analysis, A.U.W.; manuscript preparation, S.R.D.; manuscript editing, A.U.W., D.M.H.; manuscript review, T.W.E., D.M.H.; manuscript final version approval, D.M.H.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 

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