Lengthy heart operations increase the risk of subtle long-term brain problems caused by tiny fat particles lodging in the brain, experts have found.

The research team has measured for the first time the precise number of these tiny particles which break free, travel in the blood, then stick in the brain.

And they found that patients who spent the longest time attached to heart bypass machines had many more than those whose operations were completed more quickly.

For every extra hour spent on the bypass machine, the amount of "microemboli", as the particles are called, almost doubled.

Surgeons have long known that patients are at risk of stroke or minor brain damage during heart operations - and that this risk increases with the length of the operation.

In most cases, the patients recover over the course of the next few weeks, but some are left permanently damaged.

However, these results, published in the journal Stroke, reveal exactly how the risk multiplies with time.

The microemboli harm the brain by blocking tiny blood vessels leading to brain tissue.

Without their supply of blood, and the oxygen it carries, small amounts of brain cells die.

Although occasionally a larger lump can block a bigger blood vessel, causing a more damaging stroke, the brain damage caused by these microemboli is generally mild, resulting in slightly impaired memory or other intellectual abilities, and occasionally subtle behavioural changes.

During heart bypass surgery, the bypass machine, or heart/lung machine is used to pump blood around the body, so the heart can be stopped while the surgeon grafts new blood vessels to replace those clogged up by heart disease.

Preserved brains

The team used 36 brains taken from patients who had died within three weeks of bypass surgery.

By using special dyes, and examining the brains closely, they were able to spot the tiny blockages in the blood vessels.

The team wrote: "Current estimates indicate that more than 50% of patients who undergo bypass grafting have neurologic or neuropsychological deficits during the first week after surgery.

"Ten to 30% have long-term or permanent deficits and between 1% and 5% suffer severe disability or death."

Professor Ken Taylor, British Heart Foundation professor of cardiac surgery at the Hammersmith Hospital in London, said that surgeons were well aware of the need to not spend too long on heart surgery.

He said: "You can't rush. There has to be a balance between doing a good job for the patient and the risk of damage.

"Often the effects of this damage are not even noticeable to the patient, but perhaps to the patient's relatives."

The latest methods to try to monitor and prevent damage include using ultrasound scans to spot potential problems, and even filtering the blood to remove as many of the particles as possible.