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  1. Lead systems
  2. Cardiac vector
  3. Monitor leads
  4. Cellular electrophysiology of the heart
  5. Cell depolarization and repolarization
  6. Normal electrocardiogram
  7. Normal segments and junctions
  8. Conclusions
  9. Bibliography

The electrocardiogram (ECG) is a recording of the electrical potentials produced by cardiac tissue. Formation of electrical impulses occurs within the conduction system of the heart. When excited, or depolarized, atrial and ventricular myocardial muscle fibers contract. The electrical currents produced by these electrical impulses spread through the body and are recorded from the body surface by applying electrodes at various body surface points and connecting them to a recording apparatus.

[...] The normal QRS axis lies between 0 and +90 degrees; superior axis deviation (between -45 and -90 degrees) and right axis deviation (between +90 and ±180 degrees) are considered abnormal. Leftward deviation of the mean frontal plane QRS axis can occur with advancing age in the absence of clinically overt heart disease. The normal frontal plane P wave and T wave axes usually correspond to the normal QRS axis and point in the same general direction. The unipolar precordial leads approximate the electrical potentials (vectors) in the horizontal plane. [...]

[...] The U wave is the (usually positive) deflection following the T wave and preceding the subsequent P wave; it is thought to be due to repolarization of the intraventricular (Purkinje) conduction system and often is accentuated in left ventricular hypertrophy. In some circumstances, such as hypokalemia and hypomagnesemia, the U wave is thought to represent an oscillatory membrane potential, called an afterdepolarization. Negative U waves, best seen in leads can be seen in acute myocardial ischemia (where they are insensitive but relatively specific markers of left anterior descending coronary artery disease) and left ventricular hypertrophy from any cause. [...]

[...] Newer ECG techniques such as body surface mapping (in which instantaneous depolarization and repolarization are plotted) and signal-averaged electrocardiography (in which the P wave and QRS complex are filtered to assess the presence of abnormal low-amplitude terminal potentials to predict the risk of an arrhythmic event) are now available. The availability of computerized ECG interpretation allows for rapid initial screening, which may be useful in some clinical circumstances; however, physician overread is mandatory for accurate interpretation. Conclusions Steps on Understanding the ECG Identify the atrial rhythm and measure its rate. [...]

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