Myocardial repolarization on the ECG

Essence of the process and its reflection on the curve

Myocardial repolarization is a crucial physiological process of restoring the initial electrical charge of the cardiomyocyte cell membrane after their active contraction. On the surface electrocardiogram, this prolonged stage begins immediately after the completion of the QRS complex, which is responsible for the depolarization of the bulk of the myocardium.

The transition from active excitation to gradual recovery is marked by the J point. It is followed by the ST segment, an isoelectric line section showing that the ventricles are fully depolarized and in a refractory state. The process concludes with the formation of the T wave, which corresponds to the final stage of potential recovery. In some cases, a late repolarization phase is recorded, manifesting on the tracing as the U wave. The entire period from the onset of depolarization (Q wave) to the end of repolarization (end of the T wave) is grouped into the QT interval.

Correlation with the cardiomyocyte action potential

The total electrical activity of the heart, which we see on the ECG paper strip, directly consists of the transmembrane action potentials (APs) of millions of individual cells. Each graphic element of the cardiogram strictly corresponds to a specific phase of the ventricular cardiomyocyte AP:

• Phase 0 (depolarization): forms the QRS complex.
• Phase 1 (rapid initial repolarization): coincides with the J point.
• Phase 2 (slow repolarization, or plateau phase): generates the ST segment. At this moment, the electrical charge is stable, and the approximate duration of the phase is about 120 ms.
• Phase 3 (rapid terminal repolarization): reflected as the T wave. The process takes approximately 130 ms.
• Phase 4 (resting potential): corresponds to the isoelectric line on the tracing (the interval from the end of the T wave to the beginning of the next Q wave).

Differences between atria and ventricles

The processes of electrical excitation and subsequent recovery occur in all heart chambers without exception, but on a standard recording, they look completely different. This is due to the colossal difference in myocardial mass between the various sections.

• Atria: their depolarization forms the P wave (duration about 80 ms). However, atrial repolarization occurs strictly simultaneously with the formation of the QRS complex. Since the electrical vector of the ventricles is significantly more powerful, it completely masks the atrial recovery process, so we do not see a separate wave on the baseline.
• Ventricles: the spread of excitation through the interventricular septum and free walls forms the QRS complex (about 80 ms). Subsequent recovery takes much longer and forms distinct elements: the ST segment and the T wave.

Relationship between electrical and acoustic phenomena

Electrical processes in the heart muscle are always the primary triggering mechanism for subsequent mechanical contractions. This inextricable link can be easily traced by synchronous comparison of the surface ECG and phonocardiogram (PCG).

• First heart sound coincides in time with the QRS complex, i.e., with the moment of ventricular depolarization.
• Second heart sound occurs after the end of the T wave, which corresponds to the completion of ventricular repolarization.

It is important to note that a physiological delay exists between the end of electrical systole (end of the T wave) and the mechanical closure of the semilunar valves (second sound). This time interval is approximately 40 ms.

Rules for evaluating intervals and segments

For competent and error-free cardiogram interpretation, it is necessary to clearly distinguish the basic concepts of an interval and a segment, as they are often confused.

• Segment is exclusively the section of the isoelectric line between two waves. For example, the PQ(R) segment reflects the time of impulse propagation through the AV node and bundle of His, while the ST segment reflects the plateau phase.
• Interval is a broader concept that includes at least one wave and the adjacent section of the isoelectric line. For example, the PQ(R) interval is measured from the beginning of the P wave to the beginning of the QRS complex (lasting about 40–60 ms), while the RR interval reflects the distance between two adjacent cardiac cycles.

Class and exam questions

What else to study on this topic

• Atrial depolarization and formation of the P wave
• Propagation of excitation through the AV node and bundle of His
• Formation of the QRS complex (ventricular depolarization)
• Relationship of the first heart sound with the QRS complex
• Measurement and clinical significance of the QT interval

Other topics in the "Cardiac Physiology" section

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