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From Fellrnr.com, Running tips
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200mg A moderate amount of caffeine does not change the myocardial blood flow at rest, but during exercise it the caffeine significantly reduces itthe flow<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 (These changes were seen with 200mg of caffeine, but the studies did not try other amounts to see of the effect varies with dose.) The 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 Myocardial Blood Flow
The heart not only pumps blood but, like any muscle, it requires its own supply of blood as well. If this blood supply, called myocardial blood flow, is interrupted the result is a [http://en.wikipedia.org/wiki/Myocardial_infarction Myocardial infarction] or heart attack. Coronary artery disease, which reduces the myocardial blood flow, is the most common cause of sudden death worldwide.
==Caffeine and Myocardial Blood Flow ==
==Caffeine and Irregular Heart Beats==
[http://en.wikipedia.org/wiki/Cardiac_arrhythmia 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"/>.
