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Caffeine and Irregular Heart Beats
200mg of caffeine does not change the myocardial blood flow at rest, but during exercise it significantly reduces it<ref name="Namdar-2009"/><ref name="Namdar-2006"/>. This decrease in myocardial blood flow is worse at altitude<ref name="Namdar-2006"/> and in people with Coronary artery disease<ref name="Namdar-2009"/>. This change in myocardial blood flow is not a risk factor for healthy individuals<ref name="Namdar-2009"/><ref name="Namdar-2006"/>, but for those with Coronary artery disease the combination of caffeine and exercise may exacerbate their condition.
==Caffeine and Irregular Heart Beats==
[ Cardiac arrhythmias] (irregular heartbeats) are a group of conditions where the heart does not beat normally and can be a life threatening emergency. There is a common belief that caffeine is linked to some types of arrhythmias, but this does not seem scientifically supported for <ref name="RashidHines2006"/><ref name="Katan-2005"/><ref name="Pelchovitz-2011"/>. In fact, an animal study showed that moderate caffeine reduces the risk of atrial fibrillation<ref name="RashidHines2006"/>. A study giving recent heart attacks 450mg of caffeine showed no change in the risk of arrythmia<ref name="Myers-1990"/>. Giving 200mg of caffeine before a bicycle stress test to patients who have malignant ventricular arrhythmias made no difference<ref name="Graboys-1989"/>. However, there are anecdotal reports of people having arrhythmia triggered by caffeine. Note that a caffeine overdoes can result in heart problems<ref name="Pelchovitz-2011"/>.
==Evaluating the risks==

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