Abdominal aortic aneurysm (AAA) is a swelling of the lower part of the major blood vessel that supplies blood to the body. AAA is a health risk in older people. It is treated with a type of keyhole surgery - endovascular abdominal aortic aneurysm repair (EVAR). The surgery places a stent-graft within the aneurysm to reduce the chance of the aneurysm bursting. The stent-graft is placed into the artery in the leg near the groin (femoral artery).
3000 people have EVAR surgery in the UK every year. After EVAR surgery, patients are monitored to be sure that there are no complications such as endoleaks (leaks into the swelling sac) or the stent-graft moving.
Monitoring patients after EVAR surgery
Patients are usually monitored using Computed Tomography Angiography (CTA) and X-rays. CTA is accurate but carries the risk of repeated exposure to radiation and of contrast-agent toxicity. Ultrasound, either using a colour duplex ultrasound (CDU) or a contrast-enhanced ultrasound (CEU), has been suggested as a possible safer alternative but it is not used in all hospitals in the UK.
At present, it is not clear which imaging technique should be used or how often patients should be scanned. The National Institute for Health Research, through the Health Technology Assessment Programme, commissioned the University of Aberdeen and collaborators from NHS Grampian and Royal Liverpool University Hospital to assess the clinical and cost-effectiveness evidence of ways to monitor patients who have had an EVAR surgery.
Previous studies do not provide clear answers
We found 27 studies that had looked at different ways to monitor patients after aneurysm surgery. The studies were mostly of poor or moderate quality. It was difficult to draw firm conclusions from these studies as they were all very different. They used a variety of scan types, the patients in the studies had a different number of scans per year and were followed for different lengths of time. Based on the results of these studies, CDU may be a safe alternative to CTA, and CTA could be used to investigate patients who have abnormal or inconclusive CDU scans.
We find CDU was the best value for money
We built a Markov model to bring together the costs and consequences of different ways of following up patients after EVAR surgery. We found that CDU was the best value for money for the NHS for people with a normal level of risk of developing complications. There was a 63% probability of CDU being cost-effective at a £30,000 cost-effectiveness threshold. CTA was the next-best value and CEU was the least-best value for money.
We concluded that CDU might be a suitable alternative to CTA for the long-term follow-up of some patients after aneurysm surgery. However, more research is needed to understand how patients should be scanned based on their own risk of developing complications.
More details on our Markov model
Markov models are a good way to describe issues that can repeat themselves over time and chronic diseases. Markov models have Markov states. These represent health states in which people spend a period of time named a ‘cycle’. At the end of each cycle, the people can remain in their current health state or move to another state. People accumulate costs and benefits (years of life) depending on the amount of time they spend in each state and the interventions and/or events modelled within each Markov state. They allow patients to move between surveillance, further diagnosis and treatment. In our case, the model states reflected the underlying health problem, which is post-EVAR surgery with known or unknown complications. The model also reflects the treatment decision, such as reintervention after EVAR.
For more details about the study see:
Brazzelli, M., Hernández, R., Sharma, P., Robertson, C., Shimonovich, M., MacLennan, G., Fraser, C., Jamieson, R. and Vallabhaneni, S.R. (2018) 'Contrast-enhanced ultrasound and/or colour duplex ultrasound for surveillance after endovascular abdominal aortic aneurysm repair: a systematic review and economic evaluation', Health Technology Assessment, 22(72).
There is a project page on the Health Services Research Unit (HSRU) site, and there is a PROSPERO record for the study.
Dr Rodolfo Hernández led the health economics aspects of this study. Dr Hernández is a Research Fellow in the Assessment of Technology theme. His research focuses on the economic evaluation of healthcare interventions. These evaluations are conducted using decision analytic models (such as Markov models) or as part of randomised controlled clinical trials.
HERU is supported by the Chief Scientist Office (CSO) of the Scottish Government Health and Social Care Directorates (SGHSC). The views expressed here are those of the Unit and not necessarily those of the CSO.
