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A typical ECG tracing has 4 recognizable waves, called deflection waves. These waves are labeled alphabetically as the P wave, the QRS complex, the T wave, and the U wave.
The first wave is called the P wave. It results from atrial muscle cell depolarization. Beginning with the depolarization of autorhythmic cells of the SA node, and resulting in the spread of depolarization wave through the atria. The P wave lasts less than 120 milliseconds (0.12 seconds). Shortly after the P wave begins, the atria contract, resulting in atrial systole.
The QRS complex is the result of the depolarization of the conduction system in the ventricles, and its spread through the walls of the ventricles. It represents electric activity that precedes the contraction of the ventricles (ventricular systole). Ventricular systole begins shortly after the QRS complex appears. The QRS has a complex shape that may consist of up to 3 waves. This is due to the passage of depolarization wave down the interventricular septum, and through the septum from left to right, upward from the apex, and through the lateral walls of the ventricles.
In a typical ECG, the Q wave is the initial downward deflection. The R wave is an upward deflection, while the S wave is the second downward deflection. The shape of QRS complex varies from one ECG lead to another. For example, it may consist of only an R wave, QS waves with no R wave, or RS waves with no Q wave. The normal duration of a QRS complex is between 70 milliseconds (0.07 seconds) and 100 milliseconds (0.1 seconds). During the formation of the QRS complex, atrial repolarization (electrical recovery) takes place, which is not usually evident on an ECG due to the large QRS complex that masks it.
The T wave reflects ventricular repolarization. Because ventricle repolarization is slower than depolarization, the T wave is more spread out than the QRS complex. A normal T wave is usually smooth and rounded, and has the same deflection orientation (same direction), as the QRS complex. A typical T wave lasts approximately 160 milliseconds (0.16 seconds). Shortly after the T wave begins, the ventricles start to relax.
The U wave is a low-amplitude, rounded deflection that follows the T wave in certain ECGs. It normally has the same deflection orientation as the T wave. The physiological basis of the U wave is not clear. However, an abnormal increase in its amplitude is an indication of hypokalemia, or could be caused by certain drugs, such as quinidine and procainamide.
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