Explanatory Notes

The Vascular Society of Great Britain and Ireland (VSGBI) and the National Vascular Registry are pleased to publish outcomes for five common vascular procedures:

What data are used to produce these figures?

Clinical data on the patients having these vascular procedures are collected by vascular surgeons and other members of the vascular team. The NHS hospitals then submit this data to the National Vascular Registry using a secure web-based data collection system.

For all types of operation, the NVR captures information about the demographics of a patient (their age, sex, and region of residence), as well as where and when the patient was admitted to hospital. Further information is captured on the severity of a patient’s condition, the type and timing of surgery received, and the care prior to discharge from hospital.

Is the clinical data accurate?

The NVR uses a range of validation processes to ensure that the outcome information is based on data that are as accurate as possible. First, we provide hospitals with information about the expected number of procedures that should have been entered into the NVR. This is to ensure the data are complete. If surgeons or NHS trusts did not submit data on all their patients, the outcome information produced by the NVR may not be representative of their practice. We give information for each NHS trust on its case-ascertainment to show the level of completeness of an organisation’s data.

The number of procedures is surrounded by a coloured box, based on the rate of case ascertainment, and is accompanied by a number of shaded circles. The thresholds for case ascertainment are as follows:

  • 85% or greater – green box and three shaded dots
  • 70%-84% – amber box and two shaded dots
  • Below 70% – red box and 1 shaded dot
  • A rate that is too small to show any meaningful results – no coloured box and no shaded dots

Second, after an initial analysis of the data by vascular consultants, the results were sent to each individual specialist for review and checking. If there were any errors, consultants have had the opportunity to correct the data before the NVR undertook the final analysis.

Interpreting funnel plots

We have assessed whether there are systematic differences between NHS trust outcomes by using a funnel plot. This is a widely used graphical method for comparing the outcomes of surgeons or hospitals1. The report includes funnel plots of risk-adjusted outcomes for both elective AAA repairs and carotid endarterectomy.

  • RED dots show the outcomes for the named NHS trust
  • BLUE dots show the outcomes for the other NHS trusts

The outcome is shown on the vertical axis, with dots higher up the axis showing higher values. The horizontal axis shows the level of activity, with dots further to the right showing the NHS trusts that perform more vascular operations.

The benefit of funnel plots is that they show whether the outcomes NHS trusts differ from the national average by more than would be expected due to random variation. Random variation will always affect outcome information like survival rates, and its influence is greater among organisations that perform smaller numbers of procedures. This is shown by the curved “funnel limit”. The limit defines the region (green zone) within which we would expect the outcome figures to fall if their outcomes only differed from the national rate because of random variation.

An NHS trust whose dot is below this funnel limit (the pink zone) is called an outlier. The funnel limit is chosen so that only on 1 in 500 occasions would an NHS trust be below the funnel limit if the difference between their outcome and the national average was due to random variation. Outliers are managed according to the outlier policy of the Vascular Society, drawn up using guidance from the Department of Health.

1 Spiegelhalter DJ. Funnel plots for comparing institutional performance. Stat Med 2005; 24(8): 1185-202.

Glossary and abbreviations

  • Aneurysm: An aneurysm is the local expansion (eg, bulge) of an artery
  • CEA: Carotid Endarterectomy
  • EVAR: Endovascular Aneurysm Repair; a graft placed under x-ray guidance, usually via the groin arteries
  • NCEPOD: National Confidential Enquiry into Patient Outcome and Death; a national body whose purpose is to assist in maintaining and improving standards of care for adults and children for the benefit of the public by reviewing the management of patients, by undertaking confidential surveys and research, by maintaining and improving the quality of patient care and by publishing and generally making available the results of such activities.
  • NVR: National Vascular Registry
  • Open Repair: Open Aneurysm Repair (the traditional open surgical approach) in which the operation is performed through an opening in the abdomen.
  • Risk adjustment: A process that uses statistical techniques to predict how the outcome of treatment is influenced by different patient characteristics. The resulting model is used to remove the effect of these characteristics when the outcomes of health care providers are being compared.
  • Stenosis: Stenosis is a narrowing or constriction of the inner surface of an artery. This typically occurs due to a build-up of fatty substances and cholesterol deposits.
  • VSGBI: Vascular Society of Great Britain & Ireland