An ECG may also be recommended by a doctor for persons who are experiencing symptoms such as chest discomfort, shortness of breath, dizziness, fainting, or rapid or irregular heartbeats. The ECG is a non-invasive, safe treatment with no known dangers. It is important to note that excessive heat or cold can cause abnormalities in the heart's electrical system that can be interpreted as damage; therefore, patients should avoid overexertion and exposure to extreme temperatures during testing. An ECG measures the voltage across the skin's surface, from one arm to the other, or from the front to the back, in order to detect changes that may occur due to disease or injury. These changes can indicate problems such as heart attack, heart failure, abnormal rhythms, myocardial infarction (MI), and stroke. The ECG machine produces small electrodes that are attached to the skin's surface at specific locations. These locations are chosen so that different areas of the body will produce different results. For example, placing the electrodes on the upper body will show us how the heart functions, while placing them on the lower body will show us how it malfunctions. Only certain parts of the body are tested, never the whole body. Test results are displayed immediately after each other in separate graphs or charts. A physician reviews these results with you to determine if there is any reason why your heart should not function normally.