The electrocardiogram (ECG) is primarily a tool for evaluating the electrical events within the heart. The action potentials of cardiac muscle cells can be viewed as batteries that cause charge to move throughout the body fluids. These moving charges—currents, in other words—are caused by all the action potentials occurring simultaneously in many individual myocardial cells and can be detected by recording electrodes at the surface of the skin. A typical normal ECG recorded as the potential difference between the right and left wrists. The first deflection, the P wave, corresponds to current flows during atrial depolarization. The second deflection, the QRS complex, occurring approximately 0.15 s later, is the result of ventricular depolarization. It is a complex deflection because the paths taken by the wave of depolarization through the thick ventricular walls differ from instant to instant, and the currents generated in the body fluids change direction accordingly. Regardless of its form (for example, the Q and/or S portions may be absent), the deflection is still called a QRS complex. The final deflection, the T wave, is the result of ventricular repolarization. Atrial repolarization is usually not evident on the ECG because it occurs at the same time as the QRS complex.
A typical clinical electrocardiogram makes use of multiple combinations of recording locations on the limbs and chest (called ECG leads) so as to obtain as much information as possible concerning different areas of the heart. The shapes and sizes of the P wave, QRS complex, and T wave vary with the electrode locations. Table 12–2 has been included for your reference; it describes the placement of electrodes for the different ECG leads. To reiterate, the ECG is not a direct record of the changes in membrane potential across individual cardiac muscle cells. Instead, it is a measure of the currents generated in the extracellular fluid by the changes occurring simultaneously in many cardiac cells. To emphasize this point, the bottom of Figure 12–15 shows the simultaneously occurring changes in membrane potential in a single ventricular cell. Because many myocardial defects alter normal impulse propagation, and thereby the shapes and timing of the waves, the ECG is a powerful tool for diagnosing certain types of heart disease.