Understanding the Graphs

All graphs included on this website are based on UK data from the National Adult Cardiac Surgery Audit. 

These pages do not display data for congenital, paediatric, or thoracic surgery. For more information about the National Adult Cardiac Surgery Audit please click here.

Mortality Rates

The mortality rates on these pages show the percentage of patients who died before they were discharged from the hospital they had their heart operation at. Some patients and types of procedures are excluded from the data before we calculate these mortality rates:

  • Patients aged under 18 years
  • Heart transplants
  • Trauma
  • Insertions of artificial mechanican hearts
  • Emergency procedures, where a patient's condition is serious enough for them to be taken immediately to the operating theatre. This type of operation is not common, and because the patients having them are often very sick, it cannot be taken properly into account by risk adjustment

The charts that have been used to show mortality rates are called 'funnel plots'. These funnel plots show how risk adjusted in hospital mortality rate of particular hospitals/surgeons compare to the national average, which is the standard that heart surgeons work towards.

Risk adjustment explained

Some hospitals and consultants do more complicated surgery on patients who are more sick, whilst others do fairly routine surgery. So that we can make fair comparisons between them, the mortality rates have been ‘risk adjusted’ to take into account the difficulty of each operation.

Risk adjustment is done using complex mathematical methods, to effectively show what the mortality would have been had all operations been of 'average' difficulty.

The risk adjustment method that has been used for adult cardiac surgery is an adaptation of the 'EuroSCORE' model. This model has been adapted to make sure that it is an accurate predictor of mortality overall. This means that it disciminates well between patients with higher and lower risk in general. However, no risk model is perfect, and there are some surgeons or hospitals who carry out specialist operations or take on very high risk patients. Because it is difficult for risk adjustment to fully account for these specialist practices, we should be very careful about drawing conclusions based on their risk adjusted mortality rates.

Funnel plots

Mortality rates are expected to be closer to the national average when more procedures have been carried out. This is because when there are more procedures a death affects the overall morality rate less. 

When fewer procedures have been done, even a single death (which could be due to chance), affects the overall mortality rate much more.

There will always be some variation between hospital and consultant mortality rates because of the differences between patients and operation types. Using only a national average as the standard can make it difficult to tell whether a mortality rate that sits above the national average is higher than we would expect it to be or not. 

For this reason, the funnel plots also show 'control limits'; the curved lines on the charts that give them the 'funnel' shape. Control limits show the highest mortality rate we would expect, based on the number of operations performed and their difficulty. If a mortality rate is below the control limit near the top of the graph, it is an 'expected' mortality rate, and any variation above or below the national average can be put down to chance alone. If the mortality rate is above the control limit, it is higher than expected. This may mean a number of things, including problems with the quality of the data submitted to the audit, specialist practice that can't be properly risk adjusted, chance, or poor quality of care. 

It is important to remember that the consultant surgeons whose mortality rates are included on this site work as part of a larger clinical team. These teams include anaesthetists, junior medical staff, nurses, perfusionists, pharmacists, and physiotherapists. All of these team members, along with hospital facilities, may affect patient outcomes.

Managing higher mortality rates

The Society for Cardiothoracic Surgery in Great Britain and Ireland is a membership organisation, which exists because of the enthusiasm and financial contributions of our members, who are mainly cardiac and thoracic surgeons. The executive committee of the society and the database project are both accountable to our membership.

We have analysed all data in the National Adult Cardiac Surgery Audit database. As part of our governance process we have looked for mortality rates that are higher than expected and fed that data back to all hospitals and surgeons in line with the process described in our publication 'maintaining patients' trust'. On this site we have published data for all hospitals that carry out adult cardiac surgery. Whilst some hospitals outside of England may opt out of publication of mortality rates for individual consultant surgeons, they cannot opt out of our process for identifying and managing higher mortality rates. A small number of hospitals have taken up this option. 

Questions and feedback

If you have any questions about the information shown on these pages, or would like to provide feedback, please email

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