An arrhythmia is an abnormal heart rhythm such as atrial fibrillation (irregular heart beat) The cause of a fast heartbeat is difficult to say without tests, anaemia (low haemoglobin - oxygen carrier) can cause palpitations on exertion as the body isnt getting enough oxygen. Is she known to be anaemic? it would usually be accompanied with shortness of breath as well and generally feeling tired. A blood test at her doctors should clarify this. There are also a number of other causes for palpitations/fast heart rate...(you dont say whether it is regular or not), such an electrical malfunction in the heart, stress and anxiety or even too much caffeine or alcohol!. I would recommend that she see a doctor as soon as possible, persistant tachycardia (fast heart rate) can be life threatening in a very small number of cases, it is therefore important that they are able to identify if it is an arrhythmia so that if required it can be treated. The GP will most likely take some blood tests to make sure it isn t being caused by anaemia or a chemicle imbalance (unlikely as it would occurr more frequently), an ECG to assess her normal rhthym, dependant on her age and medical history they may decide to watch and wait or send her for further tests such as a 24 hr ECG. I would suggest that the next time this happens, and persists for more than a few minutes, and she feels unwell with it, she should call an ambulance, they carry a basic heart monitor which will hopefully catch the heart rhythm and will be able to assess how this is affecting her body ie blood pressure etc. In many cases such as these, stress/anxiety/panic attacks are the culprit rather than a long term physical problem, which the doctor will also be able to provide advice for.