Dozens of patients had appointments for medication or surgery changed after an investigation revealed mistakes in the way clinicians have been carrying out and interpreting heart scans.

Some 3,420 cases were re-examined in the review following concerns about the quality of scans undertaken and interpreted by four members of the echocardiogram team at the West Herts Hospitals NHS Trust.

It emerged this week 108 patients needed to be re-scanned and, of these, 74 had to have their care changed - but hospital chiefs say no "long term harm" was done.

The four clinicians - three of whom were locums and worked for the trust on a temporary basis - no longer work for the trust and all have been referred to their professional body, The British Society of Echo-cardiography, the health trust said.

Mariane Covington, spokesman for the trust, said 3,312 patients had been written to "as we wanted to be open and transparent with them about what had happened and to let them know their scan had been reviewed. These patients have no need to be concerned.

"A further 108 patients have returned to the trust to be re-scanned and/or reassessed by our doctors, as we had concerns about the quality of their scan and/or the way it had been interpreted. "Of these, 74 had their care changed. This included amending their medication or bringing forward the date of routine surgery. However, it is important to note that there is no current evidence to suggest any of the 74 patients have suffered long term harm."

As a result of the review, the trust, which looks after Watford, St Albans and Hemel Hempstead hospitals, is increasing staffing within the department.

Other changes include enhancing check systems for the way scans are undertaken and interpreted, and introducing a new assessment process to check the competency of staff who undertake scans.

Dr Mike Van der Watt, trust medical director, said: "It is important patients’ scans are undertaken and reported to the highest standards.

"Our system identified concerns about the scans undertaken and reported by four members of the echocardiogram team and, as a result, we reviewed all of the scans they had conducted and interpreted.

"I would like to apologise to all the patients affected by this review, particularly those whose care or medication had to be amended. However, it’s important to note there is no current evidence to suggest any of these patients have suffered long term harm.

"We have made changes to help prevent this happening again."