Bypass blood-thinning could harm

Thousands of bypasses are carried out in the UK each year

A technique used to protect the kidneys during major surgery could actually be doing more harm than good, say researchers.

Doctors often thin the blood of bypass patients using saline solution because it is believed to decrease the risk of kidney damage during heart bypass.

But research from Duke University Medical Center in the US suggests the reverse may be true.

Overweight patients were at particular risk, claimed the research team.

The report was published in a journal called the Annals of Thoracic Surgery.

Heart-lung machine

During major operations to the heart - such as bypass grafts - in the vast majority of cases the patient is connected to a "heart-lung machine", which takes over the pumping of oxygen-rich blood around the body while the heart is stopped for delicate surgery.

In order to "prime" this pump, saline solution is added to it, which has the effect of diluting the blood once the machine is switched on.

Doctors believe this is actually beneficial, as it makes the blood less "sticky" - and therefore less likely to form clots in vital blood vessels, which could be dangerous or damaging for the patient.

Normally, blood is made up of between 36% and 40% red blood cells, but after dilution, this can fall to between 22% and 26%.

It is commonly thought that because the body is often cooled during the operation, there is a high enough concentration of red blood cells remaining to satisfy the reduced demand for oxygen.

High toll

But approximately one in 12 bypass patients will experience kidney damage as a result of the operation - and 2% will need dialysis as a result.

The Duke researchers looked at the impact of this diluting process on the efficiency of the kidneys.

In a study of more than 1,400 bypass patients, they compared the percentage concentration of red blood cells after dilution - which they called "haematocrit" - against the levels of a chemical called creatinine in the blood.

The higher the levels of creatinine, the less likely it is that the kidney is functioning normally, as it is the kidney's job to remove this chemical from the bloodstream and add it to urine.

The researchers found the lower the concentration of red cells, the higher the creatinine levels.

In overweight patients, the link between the two was particularly striking.

Dr Mark Stafford-Smith, who led the research, said: "This is the first report highlighting the association of haemodilution during bypass surgery with acute injury to the kidneys.

"Our findings question the wisdom of tolerating the lowest levels of haematocrit during bypass surgery."

Using donated blood instead of saline has also been shown to be potentially harmful, so one solution offered by the team was to redesign the heart-lung equipment so that less liquid was needed to fill it.