Transfusion of donated blood more than three weeks old may result in impaired blood vessel function, a new study of hospital patients has found.

Although blood banks consider six weeks to be the maximum permitted storage time of blood for use in transfusion, but recent studies have suggested transfusing blood stored for more than a few weeks has adverse effects in patients undergoing cardiac surgery or critical care.

The new finding suggests a mechanism explaining why older blood might be detrimental to patient health: a deficiency in nitric oxide, a short-lived chemical messenger that relaxes blood vessels.

In the study, 43 patients at Emory University Hospital were set to receive cross-matched red blood cells for clinical indications.

Members of the group were in hospital for various reasons, such as cancer treatment and surgery. They were randomly chosen to receive either fresh (less than ten days old) or aged (more than three weeks old) red blood cells. On average, they received the equivalent of two units. A unit is 450 millilitres of blood, Science Daily reported.

Robert Neuman, cardiovascular research fellow from the Emory Clinical Cardiovascular Research Institute and his colleagues tested blood vessel function by measuring flow-mediated dilation (FMD).

By ultrasound, they tested how much a blood vessel in the arm opens up after a blood pressure cuff is first tightened then removed. Flow-mediated dilation is an indicator of the health of the endothelial lining of the blood vessels and is a process that is dependent on nitric oxide.

Healthy, younger individuals can have flow-mediated dilation of up to 10 per cent — the average for the hospitalised group was 5 per cent.

Patients receiving aged blood saw their FMD halved to 2.4 per cent 24 hours after the transfusion, while patients receiving fresh blood saw no significant change in FMD.

This effect of older blood on blood vessel function is similar in size to that of eating a fatty meal, or the long-term effects of a cardiovascular disease risk factor such as smoking or diabetes.

Healthy flow-mediated dilation reflects sufficient production of nitric oxide, which is generated by the blood vessels’ endothelial lining and causes them to relax. Nitric oxide is also important for delivery of oxygen by haemoglobin.

Red blood cells carry nitric oxide bound to haemoglobin, and play a critical role in recycling the nitric oxide. Over time in storage, the nitric oxide is lost. Transfused red blood cells last a couple months in the patient. The Emory team did not measure FMD beyond 24 hours.

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