Atrial Complex: P Wave and PR Segment
The process begins when an impulse originates in the sinoatrial node and spreads first to the right atrium and then to the left atrium. On the ECG, this is recorded as the P wave (duration approximately 40 ms). It reflects atrial cardiomyocyte depolarization and the earliest rapid repolarization of their membranes.
When both atria are fully excited, local currents disappear, forming an isoelectric line on the tracing—the PR segment (60 ms). Physiologically, this corresponds to the phase of slow repolarization (plateau phase) of the atria. At this moment, the impulse slowly passes through a small group of cells in the atrioventriculare (AV) node. Due to the small mass of these cells, no distinct wave is generated, and the slow conduction velocity creates the atrioventricular delay. A potential difference between the excited atria and unexcited ventricles exists, but the overall electromagnetic field remains unchanged.
Ventricular Complex: Depolarization (QRS)
The QRS complex lasts 80 ms and corresponds to ventricular activation, their initial rapid repolarization, and the final stages of atrial repolarization.
- Q wave: The first negative deflection. The impulse travels via the bundle of His to the interventricular septum, papillary muscles, and trabeculae. The vector is directed oppositely to the main electrical axis, making the wave negative. Its small amplitude is explained by the limited number of initially involved cells. Function: excitation of the papillary muscles at the onset of systole prevents the atrioventricular valves from prolapsing into the atria.
- R wave: The tallest positive wave, reflecting excitation of the bulk of the ventricular myocardium. The ascending limb is due to conduction through the septum toward the apex and from endocardium to epicardium. The descending limb corresponds to the spread of excitation to the lateral walls.
- S wave: The negative wave marking the completion of depolarization. Excitation travels from the apex toward the base, encompassing the basal regions of the left ventricle and the arterial conus. The peak of the S wave corresponds to the activation of virtually all ventricular cardiomyocytes. Mechanically, asynchronous contraction transitions into isometric contraction at this point.
Repolarization and Electrical Diastole
Following the complete excitation of the ventricles, the plateau phase of the action potential occurs. Potential differences vanish, forming the ST segment—an isoelectric line lasting 120 ms. During this period, all ventricular cells are depolarized, and local currents do not arise.
The process of rapidly restoring the membrane potential back to its resting level is known as repolarization and forms the T wave (130 ms). Excitation first subsides at the cardiac apex, followed by the interventricular septum.
The interval between the end of ventricular repolarization and the beginning of the next cycle (until the next P wave) is the TP segment. The heart is in a state of rest (polarization), and electrical activity is absent. This period represents electrical diastole.
Time Intervals
Intervals combine waves and isoelectric segments, showing the total duration of electrical events in various heart chambers:
- PR interval (P wave + PR segment): Lasts 100 ms and precisely matches the duration of atrial systole.
- QT interval (QRS complex + ST segment + T wave): Lasts 330 ms and reflects the total duration of electrical ventricular systole.