Main components of a normal ECG
The electrocardiogram records the total electrical activity of the heart. Graphically, its structure consists of three main types of elements: waves, segments, and intervals.
- Waves are any deflections of the curve from the baseline (isoline). Normally, in most leads, the P, T and U waves are positive (directed upward), while the Q and S waves are negative. The R wave represents the main positive peak of the ventricular complex.
- Segments and intervals are time periods. A segment is an exclusively flat section of the baseline between two waves. In turn, an interval is a broader concept that necessarily includes a wave (or several waves) along with the adjacent section of the baseline.
Standard educational time values for various ECG elements when measured using a calibration grid are about 0.12, 0.2, 0.3, and 0.35 seconds.
Graphical representation of repolarization
The process of the ventricular myocardium returning to the resting state forms a strict sequence of graphic elements on the scalar cardiogram.
- J point. Serves as the precise boundary of the transition from myocardial depolarization to repolarization.
- ST segment. This is the section of the isoline from the J point to the onset of the T wave. During this period, the myocardium is completely depolarized, so there is no potential difference, and the curve does not deviate from the zero level. The duration of the segment is approximately 120 ms.
- T wave. Coincides with the final stage of cardiomyocyte charge recovery. Its duration is about 130 ms.
The combination of excitation processes (the QRS complex) and recovery processes (the ST segment and T wave) forms the QT interval—the span from the beginning of the Q wave to the end of the T wave. Physiologically, this is the electrical systole of the ventricles.
Interesting fact: the atria also undergo their repolarization phase. However, this process is not visible on a standard ECG because it occurs simultaneously with ventricular depolarization and is completely masked by the powerful signal of the QRS complex.
Synchronization with the action potential
The processes visible on the surface cardiogram can be precisely correlated with the phases of the transmembrane action potential (AP) of individual cardiomyocytes.
| AP Phase | Cellular process | ECG element |
|---|---|---|
| Phase 0 | Depolarization | QRS complex |
| Phase 1 | Rapid initial repolarization (RIR) | J point |
| Phase 2 | Slow repolarization (SR, "plateau") | ST segment |
| Phase 3 | Rapid final repolarization (RFR) | T wave |
| Phase 4 | Resting potential (RP) | Isoline |
Before repolarization begins, excitation must fully encompass the ventricles (Phase 0). The signal spreads from the interventricular septum and apex (the Q wave) through the inner surface (the R wave) to the outer layer of the heart (the S wave). Electrical impulse transmission between myocardial cells occurs through specialized intercellular junctions—gap junctions (nexus). After this, the cells enter a refractory state (Phase 2) and then finally restore their initial charge (Phase 3).
REMEMBER: An interval always includes a wave (or a complex of waves) along with a section of the isoline, whereas a segment is exclusively a "clean" section of the isoline between waves.
Class and exam questions
What does the J point mean on an ECG?
The J point is the boundary on the cardiogram that marks the moment of complete completion of ventricular depolarization and the very beginning of their repolarization. At the cellular level, it corresponds to phase 1 of the action potential.
How does the QT interval differ from the ST segment?
The ST segment reflects only the "plateau" phase, when the myocardium is completely depolarized and generates no potential difference. The QT interval is the entire electrical systole as a whole: it includes both the excitation itself (the QRS complex) and the subsequent recovery (ST and T).
Which action potential phase corresponds to the T wave?
The T wave is formed at the moment of rapid final repolarization of cardiomyocytes, which strictly corresponds to phase 3 of the transmembrane action potential.
Why is atrial repolarization not visible on an ECG?
The atrial charge recovery process coincides in time with the depolarization of the bulk of the ventricular myocardium. The powerful electrical signal of the QRS complex completely masks atrial repolarization.
What else to study on this topic
- Ventricular depolarization (formation of the QRS complex)
- Atrial depolarization (P wave and PQ segment)
- Significance and measurement methods of the RR interval
- Propagation of excitation through the cardiac conduction system
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