The Role of Hospital Volume in Coronary Artery Bypass Grafting: Is More Always Better?

Abstract

This study sought to determine whether regionalization of non-emergent coronary artery bypass grafting (CABG) from low to high volume hospitals would improve surgical outcomes. Recognizing that regionalization could reduce patient access and choice, the authors hoped to determine whether “targeted” regionalization of specific subgroups of patients at different levels of surgical risk could identify those patients most likely to benefit from care at high volume hospitals.

The authors assessed outcomes of CABG at 56 US hospitals, using 1997 administrative and clinical data from Solucient EXPLORE, a national outcomes benchmarking database. Predicted in-hospital mortality rates were calculated using a logistic regression model, and subjects were classified into five risk categories: minimal (< .5%), low (.5% to 2%), moderate (2% to 5%), high (5% to 20%), and severe (> 20%). In each of the five risk groups, differences between low and high volume facilities were assessed in terms of hospital costs, lengths of stay, and in-hospital mortality.

Two thousand and twenty nine patients underwent CABG at 25 low volume hospitals (those that performed < 200 CABG procedures annually), and 11,615 had CABG at 31 high volume hospitals. There were significant differences in in-hospital mortality between low and high volume hospitals for patients at moderate (5.3% vs 2.2%; P = .007) and high risk (22.6% vs 11.9%; P = .0026), but not in patients at minimal, low, or severe risk. Hospital costs and lengths of stay were similar for all five risk groups. Based on these results, the authors concluded that “targeted” regionalization to high-volume hospitals of patients at moderate or high risk would have resulted in an estimated 370 transfers and avoided 16 deaths; in contrast, full regionalization would have led to 2,029 transfers and avoided 20 deaths. They conclude that “targeted” regionalization achieved most of the benefits with far fewer transfers, and therefore might be a feasible strategy for balancing the clinical benefits of regionalization with patients’ desires for choice and access. Comment

Aside from its clinical findings, this study demonstrates that elaborate demographic studies continue to be published in major, peer-reviewed journals even though the studies depend upon risk-adjusted mortality calculations that are probably inaccurate because they are not derived from the STS National Database for Cardiac Surgery. The risk model in this study from the Solucient EXPLORE database predicted risk based upon age, gender, surgical priority, and “severity of illness” which was derived from the ICD-9-CM (International Classification of Diseases, 9th Edition) discharge diagnoses and procedures. Considering the difficulty of deriving an accurate risk adjustment model even from the numerous surgical and medical variables evaluated for the STS Database, one must question a risk-adjustment model that ignores all surgical variables, such as cardiac function, coronary anatomy, reoperation, use of IMA grafts, etc., all of which are proven to affect surgical risk. Nevertheless, the authors subdivided the patients into five risk categories, and further subcategorized their recommendations based on these presumed differences in risk-adjusted mortality.

Neither the article, nor an accompanying editorial, adequately considered the inadequacy of the data to support any arbitrary numerical demarcation between “low” and “high” volume. The present study used an annual volume of 200 non-emergent cases as the threshold for distinguishing high and low volume centers. The authors infer, but do not state unambiguously, that the results were similar when the threshold was varied from 100 to 300 cases, but if so, it only reinforces the difficulty of making such arbitrary distinctions. Fortunately, an editorial accompanying this article did point out that it is important to “move beyond” a simple focus on patient volume, since some low-volume facilities may provide excellent care for all of their patients.

Finally, the authors do not contemplate the fact that such a transfer policy will further deplete patients from low-volume and borderline-volume hospitals, resulting in the reclassification of some hospitals from high volume to low volume status. If, as this article presumes, low volume status is associated with higher risk, the creation of additional low volume hospitals will likely cause some increase in deaths, which will counteract the benefits of a regionalization policy. This is an important consequence of any proposal for regionalization, and must be included in calculations of benefit.

Nallamothu BK, Saint S, Ramsey SD, et al
J Am Coll Cardiol 2001;38:1923-30