The way that even low levels of carbon monoxide can be fatal, by disrupting the heart's rhythm, has been unravelled by researchers in Leeds.They found that levels common in heavy traffic could affect the way the heart resets itself after every beat.Their study in the American Journal of Respiratory and Critical Care Medicine showed a common angina drug may reverse the effect.The British Heart Foundation said the research was a promising start.Carbon monoxide is produced by faulty boilers, cigarettes and car exhausts.It is deadly at high levels as it "shoulder-barges" oxygen out of the blood, meaning less is transported around the body. Carbon monoxide poisoning kills more than 50 people in the UK each year and many more around the world.However, studies have suggested that even low levels, such as that found in built-up cities with lots of traffic, may also damage the heart.The University of Leeds research team found that the gas kept sodium channels, which are important for controlling the heartbeat, open for longer.Disrupting the sodium channels can disrupt the heart's rhythm, leading to cardiac arrhythmia, which can be fatal.SolutionIn collaboration with researchers in France they tested an angina drug - which also affects the sodium channels - on rats.Prof Chris Peers, from the University of Leeds, told the BBC: "It was very exciting for us. When we monitored rats exposed to levels of carbon monoxide similar to heavy pollution, they had the same heart problems and we could reverse them."At the moment no one knows how to treat this. We're saying look there's a drug on the shelf that might be able to help."Of course it needs clinical trials, but we believe it is a great start."Dr Helene Wilson, a research advisor at the British Heart Foundation, said: "This study is a good example of research being used to better understand the underlying causes of an abnormal heart rhythm and in this case it has uncovered the ability of an old drug to perform a new trick."Carbon monoxide poisoning is tragically common but hopefully these promising results can be replicated in people so that it saves lives in the future."