Is there a positive volume-outcome relationship in peripheral vascular surgery? Results of a systematic review

Objectives: to examine the evidence for the existence, or otherwise, of a positive volume-outcome relationship in the area of peripheral vascular surgery.

Design: systematic overview of prospective or retrospective volume-outcome studies.

Data sources: seven bibliographic databases were searched for English-language articles published between 1986 and 1998.

Study selection: thirty-six articles published in peer-reviewed journals; excluding editorials, letters or abstracts; and addressing volume and outcome in peripheral vascular surgery. Criteria were applied and agreed by consensus between two of the authors.

Data extraction: the articles identified were independently assessed by two of the authors. Studies were categorised into three distinct areas - carotid endarterectomy- (17 studies), abdominal aortic aneurysm repair (16 studies) and other vascular interventions (four studies). Within each category studies were further classified according to full adjustment, partial adjustment or no adjustment for case mix. Where discrepancies arose, decisions were referred to a third author for arbitration.

Data synthesis: findings for carotid endarterectomy identified a positive volume-outcome relationship for both mortality and stroke at the physician level. There was less support for a positive relationship for mortality at the hospital level, and no evidence of benefits for stroke in higher volume hospitals. If only studies making a full adjustment for case mix are included, there is no clear support from statistically significant evidence for or against a positive volume-outcome relationship. For repair of unruptured abdominal aortic aneurysms there is evidence of a positive volume-outcome relationship at both the physician and hospital level, with evidence being particularly strong at the level of the hospital. For ruptured aneurysms the evidence is suggestive of there not being a positive volume-outcome relationship at the hospital level, while for physicians the evidence is more balanced with no clear support either way. For other vascular interventions there were insufficient studies (n=4) from which to draw meaningful conclusions.

Conclusions: our results show that evidence of a relationship between volume and outcome in peripheral vascular surgery may be attributable to factors such as lack of adjustment for case-mix, different definitions of volume and poor quality of studies, especially those of retrospective design. Future studies should address these deficiencies by making full adjustment for case mix and by being prospective in design.