Abdominal aortic aneurysm: International studies show a relationship between the volume of operations and the quality of outcomes

According to the studies published to date, mortality associated with open surgery on unruptured abdominal aortic aneurysms decreases as the volume of operations increases. However, it remains unclear whether the quality also improves in respect to other endpoints, such as complications or follow-up operations. In any case, the studies published to date have only investigated the relationship between the volume of procedures and the quality of outcomes for very short periods of time and therefore do not provide sufficient evidence on long-term outcomes. This is the main finding of a final report of the German Institute for Quality and Efficiency in Health Care (IQWiG) published on 30 June 2006.

On 16 November 2004, the Federal Joint Committee ( Gemeinsamer Bundesausschuss, G-BA) commissioned IQWiG to evaluate the literature on the minimum volume of operations for abdominal aortic aneurysms and to present the results in the form of an evidence report.

All studies included by the Institute in its statistical evaluation indicate that mortality within a 30-day post-surgery period decreases as the volume of operations performed per surgeon increases. This relationship was also demonstrable between mortality and the volume of operations per hospital. However, none of the original studies included in the analysis were designed in such a way that this relationship could be assessed for a longer period, e.g. one year. The studies also failed to provide sufficient results on other endpoints apart from mortality, e.g. post-operative complications or the necessity for further surgery.

It is not possible to establish a concrete threshold value for the minimum volume of operations from the available studies that would provide a suitable basis for improving the quality of treatment.

Lack of data from intervention studies

The studies evaluated by IQWiG were register studies that mainly used administrative data sources. The data were originally collected for other purposes, e.g. billing. However, while concomitant diseases were recorded, clinical details, for example on their severity, were not. This is a substantial disadvantage of administrative data. Only studies that fulfilled certain minimum criteria for the consideration of the distribution of risk characteristics (risk adjustment) were included in the IQWiG report. Data from intervention studies are needed in order to be able to evaluate whether and how patient-relevant factors are influenced by the specification of minimum volumes for surgical procedures.

The evidential value of the available study data is also limited by the fact that the data sources of some of the studies overlap considerably. Thus, while 11 studies were used to assess the volume of inpatient surgical procedures, the data analysed were drawn from only 7 sources. Moreover, in most cases the quality of the statistical analyses and their documentation was limited. Each study showed at least one deficit in the quality of reporting or of the study itself.

Not a single study explicitly investigated the relationship between the volume of procedures and the quality of outcomes for endovascular surgery. This indicates that all results referred to open surgery. The results of the present report are therefore subject to some uncertainty.

Since none of the studies analysed data on German patients, it is questionable whether the results are applicable to the German health care system. In order to be able to determine the relationship between the volume of surgical procedures and the quality of outcomes in Germany, either new data should be collected (e.g. by inclusion in the benefits catalogue for inpatient quality assurance of the Federal Office of Quality Assurance), or data from existing quality registers of professional societies should be drawn upon.

Contact: 49 - 221/35685-0, info@iqwig.de