Higher volumes have been found to produce lower mortality rates in vascular surgery, explain Professor Ross Naylor, Michael Wyatt and David Mitchell.

Evolving medical specialties and an increasing body of evidence – derived from nationally led clinical audits – are creating new opportunities and challenges for NHS commissioners. Vascular surgery is on the verge of becoming the first new surgical specialty in more than a decade, and a national audit of mortality rates following elective infra-renal abdominal aortic aneurysm (AAA) surgery supports the move towards concentrating major arterial procedures within larger-volume units.

With the NHS coming under increasing financial pressure, decisions about the organisation of secondary care have never been more immediate or more important. It has traditionally been very difficult for the NHS (and politicians) to make strategic decisions on how to deliver specialist care, especially where that may mean fewer hospitals acting as providers. When NHS funding was increasing, this was an issue that could be conveniently deferred or ignored completely. However, the squeeze on NHS finances and much greater public and political expectations about safety and quality of care have forced reconfiguration of services higher up the agenda.

In the field of surgery, we know there are many specialties where larger volume units deliver better results for patients, and vascular surgery is a good example. Last week, the Vascular Society published the first National Vascular Database audit of mortality rates following more than 8,000 elective infra-renal AAA operations performed between October 2008 and September 2010. For the first time, this will, provide commissioners, GPs and the public with information on outcomes in their region compared with those in other parts of the UK. The results establish a clear link between the number of patients a unit treats and the outcomes achieved. The mortality rate was 2.4 per cent (down from nearly 8 per cent in 2008), compared with a Europe wide average of 3.5 per cent. However, the average mortality in the lowest volume hospitals was 4.4 per cent compared with 1.9 per cent in the highest – more than double the risk.

Safety in numbers

The relationship between volume and outcome is particularly vital to vascular surgery and is not just limited to outcomes after elective AAA surgery. Evidence suggests larger volume units have a lower turndown rate for treating patients with ruptured AAA, they have lower complication rates after carotid surgery (an operation to prevent stroke) and higher rates of revascularisation in patients with limb threatening ischaemia (reduced blood supply). Understanding the drivers underlying these differences in outcome cannot be achieved by mortality rates alone. However, gathering and reporting them is a vital first step towards demonstrating the urgency and the opportunity to save lives.

As a starting point, the Vascular Society currently advises commissioners that AAA repair should only be undertaken in hospitals that perform at least 100 elective procedures over any three-year period. In these higher volume hospitals, vascular units are able to routinely offer a level of structural support and surgical experience that is economically impossible in lower volume units, especially in the modern endovascular era. As the  database becomes larger and more sophisticated, we will also be looking at the effect that factors, such as staffing and intensive care bed ratios or co-location with other services, have on mortality rates. This will ensure commissioners can ask more critical questions of its vascular services in the future.

Vascular surgery used to be a component of the general surgeon’s overall workload. But specialisation in vascular surgery has helped the UK catch up with the rest of Europe, just as it has in other medical fields.

The shift towards moving major arterial reconstructions into larger volume hospitals will mean some patients will have to travel further to undergo their surgery, but evidence suggests they would prefer this if it offers better outcomes.

Nevertheless, the regional reconfigurations of practice have recognised the need to keep outpatient clinics, investigations and rehabilitation as close as possible to the patient’s home. And that will not change.

AAA Hospital Mortality stratified by Patient Volume (2008-10)

Hospital abdominal aortic aneurysm patient volumesTotal number of AAA surgery patientsAverage mortality rate
Low volume hospitals3404.4%
Low/medium volume1,2453.2%
Medium/high volume2,1222.5%
High volume hospitals4,7621.9%