Heart surgery patients at the Bristol Heart Institute who are at risk of bleeding to death after operations are taking part in a new clinical trial to help find better ways of treating them.
Blood has to be thinned using medication for cardiac operations and, if it does not start clotting properly afterwards, then problems arise.
Some 15% of patients having open-heart operations are at risk of potentially life-threatening bleeding afterwards.
When this happens they have to be given several units of blood - which in itself can cause heart and kidney damage.
During the surgery patients are put on a heart bypass machine.
For this to work properly patients have to be given blood-thinning medication.
The main danger occurs when patients come off the machine as the risk is that their blood may not clot properly.
"Coagulation and platelet function testing in cardiac surgery" (COPTIC) recognises that bleeding and blood transfusions in heart surgery are often the consequence of abnormal blood-clotting.
The research aims to find how abnormalities in the blood-clotting mechanism are linked to excessive bleeding during heart surgery and it hopes to find the best laboratory tests so patients can be offered the correct treatments to reduce bleeding.
About five teaspoons of blood are taken from the patient - one just before the anaesthetic - and the second just before the patient comes round.
The samples are analysed and the results compared with information about the patient's operation that are recorded routinely in his or her NHS clinical records.
Cardiac surgeon Gavin Murphy, who is leading the research, says bleeding is a "massive problem" that affects almost every patient.
"[This work] will tell us which tests we should be using to diagnose the cause for this bleeding and this will have an impact in every cardiac unit in the world," Mr Murphy said.
The trial has been given £1.4m by the National Institute for Health Research, a government body, which now targets larger grants to a smaller number of "big-impact" clinical trials.
Nearly 2,500 patients who have surgery at the Bristol Heart Institute are taking part in the study.
The trial - thought to be the largest in the world - started in April 2010 and the results are expected by March 2012.