Electrocardiography (ECG) relies on accurate detection of cardiac electrical activity through surface electrodes. However, every electrode-skin interface generates a spontaneous voltage known as the polarization voltage or half-cell potential. Understanding this electrochemical phenomenon is crucial for minimizing artifacts and ensuring reliable signal acquisition.

When a metal electrode contacts an electrolyte (such as the conductive gel or skin moisture), an oxidation-reduction reaction occurs at the interface. This reaction creates a potential difference between the metal and the electrolyte, determined by the Nernst equation. For instance, a silver-silver chloride (Ag/AgCl) electrode, the most common type in clinical ECG, develops a stable half-cell potential of approximately +0.222 V relative to the standard hydrogen electrode. This stability arises because the Ag/AgCl system is a reversible electrode: the reaction Ag + Cl⁻ ⇌ AgCl + e⁻ is well-defined and reproducible.

The polarization voltage is not constant; it can drift due to changes in temperature, electrolyte concentration, or mechanical stress on the electrode. When a pair of electrodes with slightly different half-cell potentials is used, a DC offset voltage appears in the ECG signal. Modern ECG amplifiers are designed to reject this DC offset, but large or rapidly changing polarization voltages can saturate the input stage, leading to baseline wander or signal clipping.

Two types of polarization—electrolytic and activation polarization—further influence measurement stability. Electrolytic polarization occurs when current flows through the electrode, causing accumulation of reaction products near the surface. This changes local ion concentration and shifts the half-cell potential. Activation polarization relates to the energy barrier required for the charge transfer reaction; slower reaction kinetics increase this voltage, especially at high current densities. In practice, ECG signals draw minimal current (microamps), but motion artifacts or poor electrode contact can temporarily elevate current demand, inducing transient polarization shifts.

To combat these issues, manufacturers pre-coat Ag/AgCl electrodes with a thin layer of silver chloride and use a hydrogel containing a fixed chloride concentration. This design maintains a stable chloride activity at the interface, keeping the polarization voltage within ±50 mV. Additionally, “offset-free” electrodes are conditioned with a bypass resistor to equalize potentials before attachment. Clinicians also use skin preparation—rubbing to remove dead skin cells—to reduce the skin’s own polarization (a different phenomenon involving the stratum corneum’s capacitance) and lower the source impedance.

In summary, ECG electrode polarization voltages are an inescapable consequence of metal-electrolyte thermodynamics. Their scientific basis lies in electrochemistry, specifically reversible half-cell reactions. By choosing stable electrode materials, optimizing gel composition, and preparing the skin properly, we can minimize polarization-induced artifacts. This understanding enables engineers to design better amplifiers and clinicians to obtain cleaner ECG traces, ultimately improving diagnostic accuracy in cardiac care.