In the normal ECG (Figure 5), (Table 9), the P wave resulting from depolarization of the atria, precedes the QRS complex, resulting from depolarization of the ventricles. The last major wave seen in a cardiac cycle is the T wave which results from repolarization of the ventricles. U waves are rarely seen. Repolarization of the atria occurs, but its wave is normally small and is obscured by the QRS complex. The Q wave is the first downward deflection of the QRS complex; there is never an upward deflecting Q wave! The R wave is the first upward deflection of the QRS complex; thus, a downward deflecting R wave is impossible. Finally, the S wave is the second downward deflection of the QRS complex.
An ECG in any given lead may or may not contain all three, Q, R, and S waves. T waves may be either upward or downward deflections. The height or depth of waves is a measure of voltage, while the spacing of waves is a measure of temporal sequencing (i.e. time).
The P-R interval extends from the beginning of the P wave to the beginning of the QRS complex (whether or not there is a Q wave), normally 120-200 msec in length (Table 10). The P-R interval is heart rate dependent. The P-R segment includes no waves, is normally an isoelectric line, and represents conduction of the wave of depolarization from the atria to the ventricles. Electrical activity during this period can only be detected by using intra-cardiac electrodes. The S-T segment and S-T intervals are usually longer and are also heart rate dependent. The Q-T interval defines electrical systole.
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