ECG waves and segments

Atrial complex: P wave and PQ segment

The process begins when an impulse originates in the sinoatrial node and spreads first to the right and then to the left atrium. On the ECG, this is recorded as the P wave (duration about 40 ms). It reflects depolarization and the earliest rapid repolarization of atrial cardiomyocyte membranes.

When both atria are completely excited, local currents disappear, and an isoline is traced on the tracing—the PQ segment (60 ms). Physiologically, this is the phase of slow atrial repolarization (plateau phase). At this moment, the impulse slowly passes through a small group of cells of the atrioventricular (AV) node. Due to the small mass of these cells, no wave arises, and the low conduction velocity forms the atrioventricular delay. A potential difference between the excited atria and unexcited ventricles exists, but the electromagnetic field itself does not change.

Ventricular complex: Depolarization (QRS)

The QRS complex lasts 80 ms and corresponds to the spread of excitation through the ventricles, their initial rapid repolarization, and the final repolarization of the atria.

• Q wave: The first negative wave. The impulse travels via the bundle of His to the interventricular septum, papillary muscles, and trabeculae. The vector is directed opposite to the main direction, which is why the wave is negative. Its small amplitude is explained by the small number of involved cells. Function: excitation of the papillary muscles at the beginning of systole prevents the atrioventricular valves from prolapsing into the atria.
• R wave: The tallest positive wave, reflecting the excitation of the bulk of the ventricular myocardium. The ascending limb is due to conduction along the septum toward the apex and from the endocardium to the epicardium. The descending limb is due to the transition of excitation to the lateral walls.
• S wave: A negative wave marking the completion of depolarization. Excitation travels from the apex to the base, covering the basal regions of the left ventricle and the conus arteriosus. The peak of the S wave corresponds to the excitation of virtually all ventricular cardiomyocytes. Mechanically, at this moment, asynchronous contraction gives way to isometric contraction.

Repolarization and electrical diastole

After the ventricles are fully excited, the plateau phase of the action potential sets in. The potential difference disappears, forming the ST segment—an isoelectric line lasting 120 ms. During this period, all ventricles are excited, and local currents do not arise.

The process of rapid restoration of the membrane potential to its baseline level is called repolarization and forms the T wave (130 ms). First, excitation disappears in the apex of the heart, then in the interventricular septum.

The interval between the end of ventricular repolarization and the beginning of the next cycle (until the new P wave) is the TP segment. The heart is in a state of rest (polarization), with no electrical activity present. This is the time of electrical diastole.

Time intervals

Intervals combine waves and isoelectric segments, showing the total duration of processes in the chambers of the heart:

1. PQ interval (P wave + PQ segment): Lasts 100 ms and precisely corresponds to the time of atrial systole.
2. QT interval (QRS complex + ST segment + T wave): Lasts 330 ms and reflects the duration of ventricular electrical systole.

Briefly: answers to topical questions

Class and exam questions

What else to study on this topic

• Mechanism of atrioventricular delay formation at the cellular level
• Correlation of cardiomyocyte action potential phases with ECG elements
• Significance of papillary muscle excitation in preventing valve prolapse
• Vector analysis of excitation spread during QRS complex formation

Other topics in the "Cardiac Physiology" section

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