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The surface electrocardiogram (ECG), introduced more than 100 years ago by Willem Einthoven, is the most common technique for the study of heart diseases. It is very useful for the evaluation of acute chest pain, palpitations, syncope, and acute dyspnea and the gold standard for the diagnosis of cardiac arrhythmias, conduction disturbances, pre-excitation syndromes, channelopathies, and some aspects of acute ischemic heart disease (IHD). Additionally, it is a fundamental tool to assess the evolution of heart diseases, in particular ischemic diseases, as well as in situations such as electrolytic disorders and drug therapy. It is also useful for epidemiologic studies, screening, checkup of athletes, and other situations.

Despite its invaluable usefulness if used correctly, the interpretation of an ECG recording of normal appearance must be performed with caution. We should bear in mind that a relatively high percentage of patients with coronary heart disease, in the absence of chest pain, show a normal ECG recording. In approximately 5% to 10% of acute coronary syndromes (ACSs), ECG is normal or borderline, especially in the early phases. Furthermore, ECG may appear normal after a myocardial infarction. Therefore, a normal ECG is not completely reassuring, because a patient may die from cardiac causes even on the same day a normal recording was taken. However, in the absence of any clinical symptoms or signs or family history of sudden death, the possibility of this occurring is very remote.

On the other hand, some subtle ECG abnormalities without evidence of heart disease may be observed occasionally. Notwithstanding, one must be cautious in such circumstances, and IHD, channelopathies, and pre-excitation syndromes should be ruled out before considering this as a nonspecific abnormality. Therefore, it is necessary to read the ECG recordings while keeping in mind the clinical setting (eg, family history, chest pain, syncope) and, if necessary, taking sequential recordings.

In addition, normal variants may be observed in the ECG recording that are related to constitutional habits, chest malformations, age, or other factors. Even transient abnormalities may be detected due to a number of causes (eg, hyperventilation, hypothermia, glucose or alcohol intake, ionic abnormalities, effect of certain drugs).

Today, the importance of ECG goes beyond its established role of diagnosing abnormal patterns. It has become a tool to determine prognosis and perform risk stratification in many clinical situations, because it can provide insight into basic electrophysiology by ascertaining abnormalities at the molecular level, such as in channelopathies. Therefore, as has been recently stated,1 it could be said that we face a new renaissance of ECG.

These facts should be borne in mind before starting to learn a technique such as ECG, because the ECG assessment has to be done within the context of a given clinical condition. For example, the occurrence of a small ST-segment depression, in the presence of precordial pain, may be due to ischemia (ACS), or in the absence of pain, it may be ...

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