Tuesday November 27 2007
Transfusion aims to increase oxygen level in the blood
Transfusions of red blood cells after heart surgery increase the risk of strokes and heart attacks, report BBC News and other news sources. The stories say half of all patients having heart surgery in the UK are given a blood transfusion because they have low levels of oxygen-carrying red blood cells and not because they have lost too much blood. Contrary to the orthodox belief that transfusions improve the circulation of oxygen around the body, “patients who received a transfusion had a three-fold increase in complications linked to lack of oxygen”, BBC News said.
The news story is based on a study of 8,598 people who had heart surgery over a period of eight years. The study results suggest that giving a transfusion may not always produce the expected benefits in terms of a reduced risk of heart disease-related complications. It seems that in some cases, the minimal benefit is outweighed by the risks inherent in all blood transfusions. This research may mean transfusion guidelines will have to be changed and suggests doctors should make sure a blood transfusion is really necessary for each patient.
Where did the story come from?
This research was carried out by Gavin Murphy and colleagues of Bristol Heart Institute. The study was funded by the British Heart Foundation and was published in the peer-reviewed medical journal: Circulation.
What kind of scientific study was this?
This was a retrospective cohort study in which the researchers looked back over patient records after heart surgery with the aim of examining the links between blood transfusion and patient outcomes and hospital costs.
The researchers obtained data for 8,598 patients between 1996 and end-December 2003, from the database established by the Bristol Royal Infirmary, which has collected the pre- and post-operative information for all adult patients undergoing heart surgery since 1996. Information is held about the anaesthetic used, details about the operation, length of time spent in the high dependency unit, and nursing details. The researchers linked this with data from the haematology and blood bank databases containing information about all blood results and blood products issued. They looked at the number of units of blood transfused and the haematocrit level of blood samples taken. Haematocrit levels provide a percentage measure of the proportion of total blood volume that is made up of oxygen-carrying red blood cells.
The researchers identified those patients at higher risk of complications who were, for example, older or had defective heart valves, or kidney or lung problems. The main outcomes that they considered after surgery were a combined outcome of infection (wound, chest or blood infection combined), or one of ischaemia (complications due to lack of oxygen to target organs, such as heart attack, stroke, or kidney failure). They looked at the risk of these outcomes occurring in patients who were not transfused compared with those who did receive transfusions.
What were the results of the study?
Of the total sample of patients undergoing heart surgery, the researchers found that 9% had infection, and 10% had ischaemia. They found that patients who received blood transfusions had a three-fold increase in the odds of either ischemia or infection compared with non-transfused patients. They found that the more units of blood the patients had received, the higher the risk. They also found that operations in which the patients were transfused were associated with an increased risk of longer hospital stay, death from any cause and increased hospital costs.
What interpretations did the researchers draw from these results?
The authors conclude that red blood cell transfusions following heart surgery are associated with an increased risk of infection, ischemia, lengthy hospitalisation, hospital costs, and overall death rate. They say that their finding of an increased risk of ischemia suggests that transfusions are ineffective in improving blood oxygenation and may “at worst cause tissue ischemia and organ dysfunction”. It is suggested that the decision to transfuse should in future be based upon low volumes of blood pumped by the heart (low cardiac output) and an objective measure of how much oxygen there is in the tissues, rather than the current approach of setting a threshold of the proportion of red blood cells at which to transfuse, based upon patient age and comorbidity.
What does the NHS Knowledge Service make of this study?
This is a reliable study with important findings that may lead to a re-evaluation of transfusion guidelines and the way hospitals routinely carry out blood transfusions. However, it is important to note that these findings do not mean that the transfusion itself is the cause of infection or heart disease-related problems. Although patients who received blood transfusions had an increased risk of infection, cardiovascular problems and mortality, the fact that they were given a blood transfusion after heart surgery suggests that they were more likely to be unwell than those patients who did not need a transfusion.
Patients who received transfusions were more likely to have had heart-related complications such as severe heart failure, kidney problems, to have had previous heart surgery and to have triple vessel heart disease, so were generally more unwell. Although the researchers attempted to adjust for factors that may have had an influence on patient outcome, it is impossible to be certain that all the possible factors that may have biased the results were taken into account.
This study has combined a large amount of information and the details of each individual case cannot be examined in context. There could have been some misclassification of outcomes in the database which might introduce errors. In addition, some of the patients died in hospital and therefore did not have data available for analysis and some had missing haematocrit data.
More research into the positive and negative outcomes of blood transfusion after surgery is likely to carried out as a result of this study. For now, as always, it is important that doctors continue to carefully consider the decision of whether a patient requires blood transfusion or not based upon the clinical situation and whether the benefits of transfusing outweigh any possible harms or risks.
Sir Muir Gray adds...
Always avoid transfusion if you can; even of your blood.