More than a million people a year have open-heart surgery to unblockor bypass clogged arteries, or repair damaged valves. The heart is usually stopped for about one hour while they are under the knife because it is impossible to operate on a moving target. But the standard method of achieving this- flooding the heart with potassium ions- can cause permanent damage.
Now Geoffrey Dobson and Michael Jones of James Cook University in Townsville, Australia, have developed an alternative. It has only been tested on animals so far, but experts say the results, due to appear in the Journal of Thoracic and Cardiovascular Surgery, look promising. "By keeping potassium levels normal they are preventing injuries to the muscle cells and to the vessels that carry blood to the heart," says Peter Macdonald, a cardiologist at St Vincent's Hospital in Sydney.
In resting muscle cells, the distribution of ions across the cell membrane generates an electric potential. The waves of electrical activity that sweep across the heart, telling muscle cells to contract, are generated when channels in the membrane open, allowing ions to flow across the membrane and depolarise the cells. Flooding the heart with potassium ions freezes its cells in this depolarised state.
Dobson and Jones instead freeze the heart in the resting state using two drugs: adenosine, which opens some of the channels that transport potassium ions in and out of the cell, and lignocaine (known as lidocainein the US), which blocks sodium ion channels.
"There are two ways to stop a heart cell. Let it run out of gas- that's the potassium infusion-or to not even turn it on. That's what Dobson has done," says Jakob Vinten-Johansen, a cardiovascular physiologist at Emory University in Atlanta, Georgia, who now collaborates with Dobson.
When rat hearts were perfused with potassium for two hours, only half started beating again afterward. Yet when Dobson and Jones perfused rat hearts with adenosine and lignocaine for four hours, they all started again, although the hearts retained only around 70 per cent of their previous pumping ability.
Dobson and Vinten-Johansen found that a single five-minute infusion of the drugs was all it took to stop the heart of an anaesthetised dog for an hour. It started beating again spontaneously, and appeared to work just as well as before.
What's more, the heart was kept at body temperature, whereas in open heart surgery the body is usually cooled to help prevent damage. "The injection seemed to put the heart in a state of suspended animation, something you'd never see with potassium,"says Vinten-Johansen. But even if the technique is adopted by surgeons, it will not prevent all the side effects of open-heart surgery.
The heart-lung machines used while the heart is stopped cause a variety of problems, such as damaging blood cells. In some cases, surgeons now use" off-pump" surgery, in which the heart is kept beating but a small area is mechanically immobilised. But this is not yet suitable for most operations.
Author: Rachel Nowak, Melbourne
New Scientist issue: 20 September 2003
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