An electrocardiogram (ECG) is a recording of the electrical changes that occur in the myocardium during a cardiac cycle. These changes result from depolarization and repolarization as action potentials occur in contracting cardiac muscle fibers. Because body fluids can conduct electrical currents, such changes can be detected on the surface of the body.

To record an ECG, electrodes are placed on the skin and connected by wires to an instrument that responds to very weak electrical changes by moving a pen or stylus on a moving strip of paper. Up-and-down movements of the pen correspond to electrical changes within the body that reflect myocardial activity. Because the paper moves past the pen at a known rate, the distance between pen deflections indicates time elapsing between phases of the cardiac cycle.

A normal ECG pattern includes several deflections, or waves, during each cardiac cycle. Between cycles, the muscle fibers remain polarized, with no detectable electrical changes. Consequently, the pen does not move and simply marks along the baseline as the paper passes through the instrument. However, when the S-A node triggers a cardiac impulse, the atrial fibers depolarize, producing an electrical change. The pen moves, and when the electrical change completes, the pen returns to the base position. This first pen movement produces a P wave, corresponding to depolarization of the atrial fibers just before they contract.

When the cardiac impulse reaches the ventricular fibers, they rapidly depolarize. Because the ventricular walls are of much greater mass than those of the atria, the amount of electrical change is greater, and the pen is deflected to a greater degree than before. When the electrical change finishes, the pen returns to the baseline, leaving a mark called the QRS complex, which usually consists of a Q wave, an R wave, and an S wave. This complex appears due to depolarization of the ventricular fibers just prior to the contraction of ventricular walls.

The electrical changes occurring as the ventricular muscle fibers repolarize slowly produces a T wave as the pen defects again, ending the ECG pattern. The record of the atrial repolarization is missing from the pattern because the atrial fibers repolarize at the same time that the ventricular fibers depolarize. Thus, the QRS complex obscures the recording of the atrial repolarization.

ECG patterns allow a physician to assess the heart's ability to conduct impulses. For example, the time period between the beginning of a P wave and the beginning of a QRS complex is the P-Q interval (P-R interval if the initial portion of the QRS wave is upright). This indicates how long it takes for the cardiac implies to travel from the S-A node through the A-V node. Ischemia or other problems affecting the fiber of the A-V conduction pathways can increase this P-Q interval. Similarly, injury to the A-V bundle can increase durations of the QRS complex, because it may take longer for an impulse to spread throughout the ventricular walls.

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