An electrocardiogram (ECG or EKG, abbreviated from the German Elektrokardiogramm) is a noninvasive transthoracic graphic produced by an electrocardiograph, which records the electrical activity of the heart over time. Its name is made of different parts: electro, because it is related to electrical activity, cardio, Greek for heart, gram, a Greek root meaning "to write". In the US, the abbreviation "EKG" is often preferred over "ECG", while "ECG" is used universally in the UK and many other countries. It is preferred as "EKG" in the US because doctor's handwriting of "ECG" can often be confused as "EEG" when transcribing orders or with echocardiography which is also abbreviated "ECG".
Sympathetic electrical impulses in the heart originate in the sinoatrial node and travel through the heart muscle where they impart electrical initiation of systole or contraction of the heart. The electrical waves can be measured at selectively placed electrodes (electrical contacts) on the skin. Electrodes on different sides of the heart measure the activity of different parts of the heart muscle. An EKG displays the voltage between pairs of these electrodes, and the muscle activity that they measure, from different directions, also understood as vectors. This display indicates the overall rhythm of the heart and weaknesses in different parts of the heart muscle. It is the best way to measure and diagnose abnormal rhythms of the heart, particularly abnormal rhythms caused by damage to the conductive tissue that carries electrical signals, or abnormal rhythms caused by levels of dissolved salts (electrolytes), such as potassium, that are too high or low. In myocardial infarction (MI), the ECG can identify damaged heart muscle. But it can only identify damage to muscle in certain areas, so it can't rule out damage in other areas. The ECG cannot reliably measure the pumping ability of the heart; for which ultrasound-based (echocardiography) or nuclear medicine tests are used.
