Trusts may stop carrying out difficult heart operations if mortality league tables are used to measure performance, the Society of Cardiothoracic Surgeons claimed following the publication in The Times of heart bypass mortality rates this week.

Compiled from Department of Health data, it found the worst performer was Walsgrave Hospital, run by the zero-star rated University Hospitals Coventry and Warwickshire trust.

With the national average standardised at 100, the survey gave Walsgrave a score of 182, four times worse than United Bristol Healthcare trust - heavily criticised in the Kennedy report - which was the best performer at 48.

The SCS said that it welcomed the result because it also confirmed its own findings that heart surgery was improving and coronary bypass surgery in England is 'as good as anywhere in the world'.

But it said: 'The way tables have been constructed is the best that can be expected from the data available, but many other conditions such as underlying heart function, lung function, smoking history, diabetes, obesity, high blood pressure, kidney function and other vascular conditions all have an impact on the risk of a heart operation.

'These factors must all be taken into account when calculating surgical risk, particularly if meaningful comparisons between units or surgeons are to be made.

'Those units that feel misrepresented by improperly weighted data may find an easy solution by simply avoiding high-risk cases, as has happened in the US.'

The SCS pointed that, ironically, although turning down high-risk patients is an easy way to improve positions in the league table, the best surgeons and institutions often have higher mortalities because such patients are referred to them and they operate on the highest-risk patients.