Understanding the Graphs

All graphs included on this website are based on UK data from the National Adult Cardiac Surgery Audit (NACSA). The data is collected and analysed by an independent organisation – the National Institute for Cardiovascular Outcomes Research (NICOR).
We only publish graphs for surgeons who have performed at least 100 operations in the time period. For less than 100 operations the statistics used to calculate risk adjusted survival rate are unreliable.
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.

Survival Rates

The survival rates on these pages show the percentage of patients discharged alive from the hospital where they had their heart operation. Some patients and types of procedures are excluded from the data before we calculate these survival rates. These are:

  • Pre-operative ventilation
  • Patients aged under 18 years
  • Heart transplant
  • Trauma
  • Insertion of artificial mechanical hearts
  • Pericardectomy
  • Emergency cardiac operations

Two types of charts have been used to show the survival rates. One is called a ‘funnel plots’. The funnel plots show how the risk adjusted in-hospital survival rate of particular hospitals/surgeons compares to that predicted by the risk score of the patients that they have operated on. The other chart shows the risk adjusted in-hospital survival for an individual surgeon or unit compared with all other surgeons or units.

Risk adjustment explained

Some patients are more unwell than others and some need more complex operations. Hence, hospitals and consultants perform a range of cardiac operations and the type of patients they operate on can differ. So that we can make fair comparisons between them, the survival rates have been ‘risk adjusted’ to take into account how ill the patient was and the complexity of the operation.
Risk adjustment is done using complex mathematical methods, to effectively show what the survival would have been had all patients been similar and 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 survival overall. This means that it discriminates 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 risk adjusted survival rates alone.

Funnel plots

Survival 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 survival rate less.
When fewer procedures have been done, even a single death (which could be due to chance), affects the overall survival rate much more.
There will always be some variation between hospital and consultant survival 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 survival 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. The horizontal line in the middle of the funnel represents the predicted survival for that hospital or surgeon’s case mix. Control limits show the lowest survival rate we would expect, based on the number of operations performed and their difficulty. If a survival rate is between the two control limits near the top and bottom of the graph, it is an ‘expected’ survival rate, and any variation above or below the national average can be put down to chance alone. If the survival rate is below the bottom control limit, it is lower 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. If the survival rate is above the top control limit it is better than expected.
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, affect patient outcomes.

Managing survival rates that are lower than expected

NICOR have analysed all data in the National Adult Cardiac Surgery Audit database. As part of our governance process all data is fed back to all hospitals and surgeons in line with the process described in our publication ‘Maintaining Patients’ Trust’. If a hospital or surgeon is found to have risk-adjusted survival that is lower than expected, a set process of investigation is triggered to examine their practice in more detail to make sure that patients are appropriately protected.

Questions and feedback

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