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Ann Thorac Surg 2005;79:217-218
© 2005 The Society of Thoracic Surgeons
University of Kentucky, C-208, UKMC, 800 Rose St, Lexington, KY 40536
The article by Dimick and coworkers addresses an important subject that has gained much national lay-press notoriety in recent weeks, that is, the issue of provider volume affecting patient outcomes [1]. This article engendered divergent opinions from reviewers with some reviewers strongly opposed to publication whereas others were not. I suppose this reflects the current uncertainty about the volume–outcome relationship in surgical patients and the article by Dimick and coauthors deserves some comment [1].
These authors use a claims database from the Agency for Health Research and Quality to identify trends in hospital mortality and length of stay for patients undergoing esophagectomy between 1988 and 2000. The authors conclude that improvements in operative mortality at high-volume hospitals are at least partly responsible for an improving trend in outcomes for esophagectomy. There is a trap inherent in using administrative claims databases to reach conclusions that have clinical implications. Because these data are generated for the collection of bills, their clinical accuracy is inadequate, and it is likely that these databases overestimate complications for billing purposes and underestimate the effects of comorbid illness [2–5]. The quality of databases used to generate comparisons cannot be overemphasized.
Mortality after coronary artery bypass grafting (CABG) is the prototypical procedure used to explore the volume outcome relationship. At least 10 large studies address the notion that hospitals performing small numbers of CABG operations have higher operative mortality. Seven of these 10 studies found increased operative mortality in low-volume providers. In three other large studies there was no such association. The Institute of Medicine summarized the relationship between higher volume and better outcome (see http://www.nap.edu/catalog/10005.html) and concluded that procedure or patient volume is an imprecise indicator of quality even though a majority of the studies reviewed showed some association of higher volume and better outcome [6]. The dilemma is that some low-volume providers have excellent outcomes, whereas some high-volume providers have poor outcomes. It is simplistic to suggest that hospital volume is a principle surrogate of outcome and much more sophistication is required to sort out this relationship. Nonetheless, decisions about utilization of healthcare resources will undoubtedly be made based on the presumed association between high volume and good outcome.
Provider (either hospital or physician) volume is a structural variable. Structural variables are those that reflect the setting or system in which care is delivered. Structural variables are different than process variables that describe the care that patients actually receive. The literature assessing structural variables is incomplete. The disadvantages of using structural variables as indicators of quality include (1) inability to evaluate outcomes by randomized trials, (2) other confounding variables are more important in determining outcomes, and (3) structural variables are not readily actionable (eg, a small hospital cannot readily increase its volume). The advantages of using structural variables include (1) expediency, which is easily measured, (2) structural variables, which are inexpensive to measure, and (3) association exists between hospital volume and outcomes in most reports [7]. In my view, the authors have failed the readers of their article by not adequately identifying the process of care variables and risk factors that predict mortality in patients having esophagectomy. In fact, provider volume is not an independent predictor of mortality in the authors' multivariate analysis. The explanation of this negative finding may be the inadequacy of the database used or the failure to account for important risk factors or process variables that determine outcome. Adequate risk adjustment requires a database that contains as many relevant clinical variables as possible. Variables (eg, stage of disease, prior abdominal operations, and significant weight loss) are not part of the Agency for Health Research and Quality database. It is likely that for operative mortality after esophagectomy there are better clinical predictors than the ones found in the database used by the authors. This shortcoming results in a failure to really analyze the causes of mortality in patients having esophagectomies, something that readers would like to know.
The authors claim that mortality has only declined in high-volume hospitals over the study period. I wonder about defining a high-volume provider as one that does six or more esophagectomies per year. The authors make important assumptions to arrive at their conclusions. It is necessary to spell out these assumptions in order to test their robustness. For example, continuous variables such as hospital volume and the time periods studied are divided into arbitrary groups without much justification for doing so. The authors do not identify the effect of dichotomizing these variables on their conclusions. In statistical terms there is no sensitivity analysis to see what effect the authors' assumptions have on their conclusions. What if high volume providers were defined as those providers who performed > 10 operations per year? Would the results and conclusions be the same? By making continuous variables into discrete variables the authors may waste information that is available and obtain results that are insensitive to changes in predictor variables. A more complete analysis of the consequences of the assumptions used would help convince the reader that, for example, an institution that does > 6 esophagectomies per year should be considered a high-volume provider.
Having listed mostly my negative biases about this article, I have to admit that hospital volume is the easiest, least costly surrogate of outcome that can be measured. In the United States the Leapfrog Group and other groups of health care payers recognize this fact. They are concerned about the spiraling increase in health care costs, and ultimately the cost that corporations must pay to provide health care to their beneficiaries. By default, and mostly for expediency, these consortiums have chosen provider volume as a surrogate for quality. To me, it seems so obvious that volume is a poor indicator of quality, and there are better indicators of quality available (eg, risk adjusted mortality ratios). Nonetheless, the consortiums that worry about health care costs are at a loss to understand and implement a better indicator of quality than hospital (or provider) volume. Until we as a profession can provide a better indicator of quality of care, we are stuck with the consequences of applying volume as a quality indicator, namely the potential for forced regionalization and possible disenfranchisement of low-volume providers.
References
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