When the normal ECG tracing is at baseline, the heart is in a state in which no current is flowing toward the positive or the negative terminals of the ECG leads. This is known as the isoelectric line (net voltage measurement is 0 millivolts). If the electric current is traveling toward a positive electrode during the cardiac cycle, an upward deflection is observed. If the electric current is traveling away from a positive electrode, a downward deflection is observed (see Figure 2.2).

Figure 2.3 The Rule of Electrical Flow

ECG Vector