Myocardial contraction is triggered whenever a wave of electrical impulses passes through the heart. The pattern of electrical excitation spreads coordinately over the structure of the heart. This results in a measurable change in potential across the surface of the body of a subject. The recording of the resultant signal from specific body points is known as an electrocardiogram (ECG). Students can directly record an ECG signal from the headset input of a smartphone with an electric lead and a suitable sound app. This allows interdisciplinary teaching by applying physics concepts to physiology.

In a first approximation, the total electric charge of the heart can be modeled as an electric dipole. This heart dipole changes both direction and magnitude during the cardiac cycle and generates a variable potential on any point of the body (see Fig.).

Differences in the concentration of ions between the interior and the exterior of a cell lead to a voltage called the membrane potential. Myocardial cells at rest have a negative membrane potential (greater positive charge just outside the cell boundary than just inside). Stimulation above a threshold value induces the opening of voltage-gated ion channels and a flood of cations into the cell. The positively charged ions entering the cell cause the depolarization characteristic of an action potential. After a delay, potassium channels reopen, and the resulting flow of cations out of the cell causes repolarization to the resting state.

Each cardiac cycle begins with the spontaneous generation of an action potential in the sinoatrial node, which is located in the superior lateral wall of the right atrium. The action potential travels from there at 3 to 5 m/s through the walls of the atria and then through the AV node into the ventricles (Fig.). The impulse then moves through the His bundle and further along to the Purkinje fibers, where the contraction is triggered.The propagation of electric impulses establishes a complicated charge and potential distribution that changes in time as different parts of the heart are stimulated.

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When the cardiac impulse passes through the heart, electrical current also spreads from the heart to the adjacent tissue surrounding the heart and extends to the surface of the body. If two electrodes are placed on the skin on opposite sides of the heart, electrical potentials generated by the field of the dipole can be recorded.

The detected waveform features depend not only on the magnitude variation of the dipole in the heart, but also on the orientation of the electrodes with respect to the dipole. At any instant, the potential difference recorded between two electrodes is proportional to the component of the dipole moment vector of the heart projected on the connection line of the two electrodes. In other words, the ECG waveform will look slightly different when measured from different electrode positions.

3 Willem Einthoven and the birth of clinical electrocardiography a hundred years ago ,” Cardiac Electrophysiol. Rev. 7, 99– 104 (2003). 3. S. S. Barold, “,” Cardiac Electrophysiol. Rev., 99–(2003). https://doi.org/10.1023/A:1023667812925 3 As far back as 100 years ago, Einthoven showed that the projections on the three sides of a triangle of the cardiac dipole are sufficient to perform a useful electrocardiogram.Figureshows the electrical connections between the patient’s limbs and the electrocardiograph from the standard bipolar so-called “limb lead I.” In recording limb lead I, the negative terminal of the electrocardiograph is connected to the right arm and the positive terminal to the left arm. Limb lead I gives a very good indication of the electrical phenomena at play when moving from left to right in the heart, but a poor view of events moving perpendicular to the limb lead I axis.

Different views of the heart dipole are accomplished by the other two Einthoven leads: limb lead II (right arm to left leg) and III (left arm to left leg). The electrodes can be placed anywhere on the arms as long as the left and the right electrode have the same distance from the heart. The signals in the leads may be regarded as projections of the electric heart vector on the respective lead vectors. Although the power of the measured signal decreases with increasing distance between the electrodes and the heart, the shape of the signal remains almost unchanged. The intensity of the signal also depends on the amplification of the measuring system and the electrode-skin contact resistance.