The atria are large enough to produce a low-amplitude signal called the P wave. The P wave is the first deflection of the ECG and represents depolarization of the atria. The termination of the P wave does not represent relaxation of the atria (a common misconception), but rather the time when all the muscle cells in the atria are depolarized and contracting (analogous to panel A3 in Fig. 1). Thus, the duration of the P wave indicates the speed of propagation of action potentials through the atria.
Soon after action potentials begin to spread across the atria, they reach the atrioventricular (AV) node. Action potentials propagate through the AV node and the bundle of His very slowly; this continues even after the entire atrium is depolarized. There are no potential changes in the ECG during this period because of the small size of the AV node/common bundle apparatus; therefore, the deflection briefly returns to baseline after the P wave.
The QRS complex represents the propagation of the wave of depolarization across the ventricles. If the trace begins with a negative deflection it is called a Q wave. The Q wave is absent if the first deflection is positive rather than negative. The first positive deflection, if one is present, is named the R wave. If a negative deflection follows an R wave, then it is called an S wave, even if it is the first negative deflection. As explained later, the shape of the QRS complex depends on where the electrodes are attached. As a result, a trace may have only an R wave or only a Q wave comprising the entire QRS complex. The QRS complex normally has a greater voltage than the P wave because the mass of the ventricles is much larger than that of the atria. The QRS complex is also shorter than the P wave (<0.1 sec) because of the presence of the Purkinje network, which rapidly propagates the wave of excitation to all the cells in the ventricles. The QRS complex terminates when all cells in the ventricles are depolarized.
After the QRS complex the deflection again returns to baseline, indicating no potential differences. This, of course, is because the entire ventricle is depolarized and all action potentials are in their phase 2 plateau phase. As the ventricle begins to repolarize, the T wave is created. Figure 1 would lead us to predict that the T wave should be a mirror image of the QRS complex, but instead it usually has the same polarity as the QRS complex. That is because the repolarization, unlike depolarization, is not conducted from cell to cell. Rather, each cell repolarizes independently whenever it is ready to do so. Depolarization begins at the endocardium, where the Purkinje fibers are located, and spreads to the epicar-dium. For an unknown reason, action potentials in the subepicardium are shorter in duration than those in the subendocardium. Thus, repolarization appears to progress in a reverse fashion from the epicardium back toward the endocardium; hence, the upright T wave. The termination of the T wave represents the point in time when all the cells in the ventricles have returned to their resting potential and the heart is fully relaxed. There is no component in the ECG that corresponds to the repolar-ization phase of the atria. The repolarization of the atrial cells is disorganized and sets up negligible net voltage. The voltage between beats is referred to as the baseline or sometimes the TP segment and should be a flat line.
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