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Editorial |
1 From the Department of Radiology, Columbia-Presbyterian Medical Center, 622 W 168th St, New York, NY 10032-3270. Received February 4, 1999; accepted February 5. Address reprint requests to the author. (e-mail: jha3@columbia.edu).
Index terms: Editorials • Emphysema, pulmonary, 60.751 • Lung, radiography, 60.11 • Lung, surgery • Lung, ventilation, 60.1295
Now in its 7th year, modern lung volume reduction surgery for treatment of severe pulmonary emphysema has achieved results that appear to be impressive: The results of many studies (1–13) have shown symptomatic and functional improvement after this procedure. However, on review of Medicare data, the U.S. Health Care Financing Administration (HCFA) found that 215 (30%) of 722 patients who underwent lung volume reduction surgery in 1995–1996 died within 18 months of the procedure (14). The disparity between the HCFA data and that in the peer-reviewed literature led to the current National Emphysema Treatment Trial (NETT).
When lung volume reduction surgery does work, what does it do? Downsizing the thorax to a normal or nearly normal configuration probably reduces dyspnea by unloading thoracic stretch receptors and allowing respiratory muscles to again contract at optimal points along their length-tension curves (2,6,15). Removing nonfunctional and minimally functional lung tissue improves many aspects of lung function. Results include decreased air trapping, increased elastic recoil, and increased maximum expiratory airflow (1–13).
What is the role of imaging in lung volume reduction surgery? The answer is simple: Imaging is central. Clearly, the assessment of severe pulmonary emphysema in vivo depends on modern imaging, but the roles of chest radiography, computed tomography (CT), and other imaging modalities in confirming the diagnosis, finding target sites for resection, and excluding coexistent morbidity are timely issues that need careful investigation in patients who are candidates for lung volume reduction surgery.
Is the distribution of emphysema a predictor of success after lung volume reduction surgery? Virtually every day, radiologists see chest radiographs that show opacification of upper lung zone vessels, which is the most common chest radiographic sign of the characteristic upper zone distribution of centrilobular pulmonary emphysema in tobacco abusers. The path-setting work of Cooper et al (1) suggested that this population, whose lower lung regions are mostly intact, would be the obvious group to benefit from lung volume reduction surgery. Support for this concept came in 1997 from three separate series, in which CT and radiographic data showed the best outcomes after lung volume reduction surgery in patients with predominantly heterogeneously distributed emphysema, which was predominantly in the upper lung zones and associated with both compressed lung and sizable zones of normal or mildly emphysematous lung tissue (7–9). Although the more heterogeneous the distribution of emphysema, the better the results of lung volume reduction surgery, some patients with homogeneous disease do show some decreased dyspnea and some improvement in pulmonary function after lung volume reduction surgery (9,13).
Are chest radiographic findings alone a satisfactory predictor of outcome after lung volume reduction surgery? The data of Slone et al (8) in 1997 suggested this possibility, which is now supported by the results of an outcome study by Maki et al (16), from the Hospital of the University of Pennsylvania, Philadelphia, which is published in this issue of Radiology. The results of Maki et al appear to be compelling: High levels of heterogeneity of emphysema and unequivocal compression of lung parenchyma at inspiratory chest radiography both were 100% predictive of a 30% or greater forced expiratory volume in 1 second (FEV1). Their definition of heterogeneity is the difference between the maximal upper lung zone score and minimal lower lung zone score—a measure of predominant upper zone disease. On the other hand, they found that low heterogeneity and a lack of lung compression strongly (94% and 92%, respectively) indicated that the postoperative FEV1 would not increase by at least 30%.
What is "heterogeneous" distribution of pulmonary emphysema? Investigators have been struggling with its definition. While Maki et al (16) compared scores in one zone with those in another, others have compared differences within zones—for example, severely destroyed "target areas" in regions of mild emphysema (8,17) have been described—or they have compared differences in the extent of emphysema among adjacent segments (9,13). To compound matters, investigators' subjective rating systems vary, using three- to 10-point scales (8,9,13,16,17). Investigative radiologists are still groping for a workable measure to assess nonhomogeneous disease. Will quantitative CT assessment be needed (10,18)? The work of Slone et al (8) and now that of Maki et al (16) suggest that fairly simple chest radiographic assessments may suffice.
A methodologic analogy to the pneumoconioses may be useful to consider. The analytic scales that Maki et al used were short: a two-point scale (all or nothing) for compression and a four-point scale for heterogeneity. However, only after the scale for chest radiographic evaluation of pneumoconiosis was expanded from four points (absent, mild, moderate, severe) to 10 points (absent, possibly mild, probably mild, mild, possibly moderate, etc) did the radiologic data correlate positively with nonradiologic variables (19). It might be advisable to consider a comparable analysis of the conventional radiographic findings of compression and heterogeneity in candidates for lung volume reduction surgery.
What about hyperinflation? The study results of Slone et al (8) and Maki et al (16) failed to show hyperinflation as a predictor of outcome after lung volume reduction surgery. However, nearly all patients with severe pulmonary emphysema have hyperinflated lungs. As both groups of authors appropriately suggest, demonstration of hyperinflation with imaging appears to be too nonspecific to be helpful. Rather, the aspect of hyperinflation that appears to be most important is the functional aspect: The higher the ratio of residual volume to total lung capacity, the better the functional results after lung volume reduction surgery, as measured by using vital capacity and the residual volume–to–total lung capacity ratio (13).
What about exercise capability? The two major clinical tests of breathlessness are the 6-minute walk test and dyspnea index (20). Unfortunately, Maki et al (16) found weak correlations between 6-minute walk test results and both compression and heterogeneity; they did not analyze the dyspnea index. The FEV1, as Maki et al (16) appropriately state, is an exceedingly well established test for patients with pulmonary emphysema. However, as they also indicate, other tests, such as the dyspnea index, ultimately may become accepted as the most valuable tests in assessing the outcome of lung volume reduction surgery (21).
What about chest radiographs obtained in inspiration and expiration? Expiratory chest radiographs to assess the variables of respiratory motion before and after lung volume reduction surgery were not included in the study of Maki et al. The inspiratory chest radiographs obtained by Takasugi et al (11) before and after bilateral lung volume reduction surgery showed decreased heights of each lung postoperatively. However, a diaphragmatic excursion of less than 3 cm before lung volume reduction surgery, which is a finding that one might expect to be a predictor of favorable outcome, was not reported as a predictor in a major series in which upper lobe heterogeneous distribution was a favorable predictor (7).
What about CT? Certainly, chest CT is an appropriate preoperative test in this population. The diagnosis of emphysema needs to be confirmed, bronchiectasis and other airway diseases need to be excluded (22), and in 2%–5% of patients, CT may help to detect incidental stage I peripheral "early" lung cancer, which is appropriate for excision during lung volume reduction surgery (23,24). CT also has investigative uses. By using inspiration and expiration CT, Becker et al (10) assessed each lung separately in patients who had undergone unilateral and bilateral lung volume reduction surgery and found that the magnitude of the response to lung volume reduction surgery was comparable in all operated lungs—that is, "a lung's response to LVRS [lung volume reduction surgery] was independent from that of the contralateral lung."
The results of quantitative CT analysis have shown that lung volume reduction surgery is associated with increases in airflow, improved 6-minute walk test results, and decreases in lung volumes and imaging indexes of emphysema (10,25,26). Another use of CT findings as predictors may be to combine them with the results of tests of airway obstruction. For example, in a small series, Hunsaker et al (26) described a combination of CT assessment of emphysema and measurements of inspiratory resistance; the high-resistance cohort fared poorly after lung volume reduction surgery.
What about magnetic resonance (MR) imaging? MR imaging is an excellent tool for comparing thoracic dimensions in inspiration and expiration; however, its uses in lung volume reduction surgery patients are likely to be only investigational. Gierada et al (27) reported that MR images showed decreases in pulmonary dimensions in all three planes after lung volume reduction surgery, especially in expiration.
What about lung scanning? Perfusion scintigraphy "may help to identify target areas for resection" (13), but in that same series, assessment of heterogeneity with CT was found to be superior to that with scintigraphy in helping to predict the outcome after lung volume reduction surgery. Wang et al (17) found that preoperative perfusion scintigraphy "can provide modest prognostic information" and noted, as a scintigraphic antecedent for the study of Maki et al (16), that upper lobe predominance of emphysema and heterogeneous distribution of emphysema were major predictors of postoperative increases in FEV1, albeit with a fairly wide scatter in their data. As for ventilation-perfusion scanning, which would seem like an excellent tool to help identify abnormal regions of the lung for resection in a disease of abnormal ventilatory function, to my knowledge, no series has yet been published.
What will happen with the NETT? Currently under way in 17 centers in the United States, the NETT was designed as a prospective randomized trial to compare lung volume reduction surgery and contemporary nonsurgical management of pulmonary emphysema (14,28). Lung volume reduction surgery does have major implications as a source of health care expenditures (29). Its role in nonheterogeneous emphysema is especially uncertain (3,13). Cooper and Lefrak (5) have questioned the ethical appropriateness of a trial that assigns half of the subjects to a nonsurgical arm. Yang (28) has called the ethical issue of randomization "unsettling." Other investigators (14,30), however, believe that a randomized trial is the best way to clarify the issues. The NETT is designed to continue for a number of years. How the NETT data will place in the debate between the peer-reviewed literature data and HCFA data will be important for patients and health care providers.
What is the take-home message? Long-term data are not yet in, but lung volume reduction surgery has certainly improved the lives of many patients. Although we are clearly still on a learning curve in predicting the outcome after lung volume reduction surgery, because of the relative simplicity of the old-fashioned chest radiograph, it may yet turn out to be as important a tool as the CT scan.
Footnotes
See also the article by Maki et al (pp 49–55 ) in this issue.
References
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