As noted in Chap. 1, propagation of the electrical stimulus in the heart can be represented as dipoles. For simplicity’s sake, each dipole is represented as a vector with a negatively charged tail and a positively charged head (Fig. 2.1). ECG electrodes are attached to the surfaces of specific body areas and connected to one another to form leads. The electrical potentials in the heart are recorded by the leads and transmitted them to the electrocardiograph, which is a galvanometric recording device (Fig. 2.2). Vectors travelling away from the recording point are represented by negative deflections; those moving toward the recording point appear as positive waves (Fig. 2.3).

The standard leads include three bipolar limb leads and nine “unipolar” leads, three limb leads and six precordial leads. Each bipolar lead records the difference between the electrical potentials detected by two electrodes, one positive, the other negative, placed on two different points of the body surface (Fig. 2.4). If the potential difference detected by the positive electrode is greater than that detected by the negative electrode, the electrocardiograph records an upward (or positive) deflection. If the positive electrode detects a smaller potential difference is smaller than that detected by the negative electrode, a downward or negative deflection will be recorded. As for the so-called unipolar leads, the exploring (or positive) electrode records voltage at one site relative to an electrode with zero potential, which is ideally located over the center of the heart. The zero potential is achieved by joining the nonexploring electrodes to a central terminal known as the Wilson central terminal (Fig. 2.5).