On each occasion, the researchers measured volunteers' heart rate and blood pressure and collected a blood sample before they used the e-cigarettes, then 15 minutes after use and again 60 minutes after use. Researchers also carried out tests to measure any impact on the circulation of blood through the body's tiny blood vessels, before volunteers used e-cigarettes and 30 minutes afterward. These tests use a laser to visualize how well blood vessels in the skin are able to dilate and therefore regulate the supply of blood around the body.

Comparing the results of the tests, researchers found that using e-cigarettes containing nicotine created a set of immediate short-term changes in the volunteers. Dr Lyytinen and his team discovered an average 23% increase in blood clots after 15 minutes that returned to normal levels after 60 minutes. There were also increases in volunteers' heart rates (from an average of 66 beats per minute/bpm to an average of 73bpm) and blood pressure (from an average of 108 millimetres of mercury/mmHg to an average of 117mmHg). Researchers found that the volunteers' blood vessels became temporarily narrower after they used nicotine-containing e-cigarettes.
These effects were not seen after volunteers used e-cigarettes that did not contain nicotine. Nicotine is known to increase levels of hormones such as adrenaline in the body, which in turn can increase the formation of blood clots.

Dr Lyytinen said: "Our results suggest that using e-cigarettes that contain nicotine have similar impacts on the body as smoking traditional cigarettes. This effect on blood clots is important because we know that in the long-term this can lead to clogged up and narrower blood vessels, and that of course puts people at risk of heart attacks and strokes."

Jonathan Grigg, who was not involved in the research, is Chair of the European Respiratory Society Tobacco Control Committee and Professor of Paediatric Respiratory and Environmental Medicine at Queen Mary University of London, UK. He said: "The damage caused by smoking traditional cigarettes, including the effects of nicotine on the body, are well-known. E-cigarettes are relatively new, so we know much less about what they do to the body.

""This study suggests that e-cigarettes containing nicotine can make clots form in users' blood and make their small blood vessels less adaptable. This is a small study, so we'd like to see more research looking at these effects.""

Jonathan Grigg, Chair, European Respiratory Society Tobacco Control Committee

"Some people may use e-cigarettes when attempting to quit smoking because they are marketed as being safe, but this study adds to the growing evidence on the harmful effects of e-cigarettes. Other aids to quitting smoking which are evidenced-based and recommended by ERS, such as patches or gum, do not result in the lungs being exposed to high concentrations of potentially toxic compounds."