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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 it is the gold standard for the diagnosis of cardiac arrhythmias, conduction disturbances, preexcitation 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 electrolyte disorders and drug therapy. It is also useful for epidemiologic studies and for screening and evaluation of athletes.

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), the ECG is normal or borderline, especially in the early phases. Furthermore, the ECG may appear normal after a myocardial infarction (MI). 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 preexcitation syndromes should be ruled out before relegating minor changes to nonspecific changes that are not meaningful. 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, or effect of certain drugs).

Today, the importance of the 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. Furthermore, it is sometimes the key technique to inform the need for such invasive devices as permanent pacemaker or cardiac resynchronization therapy. 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 ...

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