Clinical significance and physics of the process
ECG allows for the detection of a wide range of pathologies: heart rhythm and conduction disturbances, atrial or ventricular hypertrophy, and coronary artery disease (including myocardial infarction).
The physical essence of the method lies in recording the electrical field. Excitation encompasses the myocardium sequentially, forming a wave front. This front divides the tissue into two zones: on one side, the cell surface is charged negatively, and on the other, positively. How this potential difference is reflected on the body surface depends on two factors: the direction of the excitation vector relative to the recording point and the mass of the heart muscle involved in the process.
Transmembrane action potential
The electrical activity of the heart is a constant shift in action potential phases, which is provided by the movement of ions across the cell membrane. Cardiomyocytes cyclically undergo polarization (rest), depolarization (excitation), and repolarization (recovery).
- Phase 4 (Rest or diastole). Permeability to sodium ions ($Na^+$) is minimal. The sodium-potassium pump exchanges intracellular sodium for extracellular potassium ($K^+$), maintaining a negative charge on the cell surface. In the cells of the conduction system during this phase, calcium ($Ca^{2+}$) enters through slow channels, causing spontaneous diastolic depolarization—the basis of cardiac automatism.
- Phase 0 (Depolarization). There is a sharp increase in $Na^+$ permeability. A massive influx of sodium into the cell changes the charge of the outer membrane to positive.
- Phase 1 (Partial rapid repolarization). Begins immediately after the peak of excitation. Chloride ions ($Cl^-$) enter the cell, and $K^+$ begins to leave the cell.
- Phase 2 (Slow repolarization or "Plateau"). The potential is maintained at a stable level due to the balance of currents: $K^+$ leaves the cell, while $Ca^{2+}$ and $Na^+$ enter it.
- Phase 3 (Rapid repolarization). Calcium channels close. The prevailing efflux of $K^+$ returns the cell to its original resting potential.
Anatomy of the conduction system
The conduction system is capable of independently generating impulses without the participation of the nervous system (automatism). The impulse originates in the sinoatrial node, located in the wall of the right atrium near the mouth of the superior vena cava. Here, the rate of spontaneous depolarization is maximal.
Then the wave envelops the atria, causing their contraction, and reaches the atrioventricular (AV) node. After a short delay, the excitation passes to the bundle of His trunk and its branches (right and left). The left branch additionally divides into anterior and posterior fascicles. The conduction pathways lie in the subendocardial layer of the interventricular septum. Conduction ends in the Purkinje fibers, which transmit the impulse directly to the working cardiomyocytes in the direction from the endocardium to the epicardium.
Formation of ECG elements
Cellular-level processes and impulse propagation through anatomical structures are strictly synchronized with the electrocardiogram trace:
- P wave: reflects atrial excitation (depolarization).
- Isoelectric segment from P to QRS: corresponds to the passage of the impulse through the AV node and the bundle of His trunk.
- QRS complex: demonstrates the spread of excitation over the ventricles. The beginning of the complex is the activation of the interventricular septum, the R wave is formed during the depolarization of the free wall of the left ventricle.
- ST segment: the period of complete ventricular excitation.
- T wave: reflects the process of ventricular repolarization. The late phase of repolarization sometimes forms a U wave.
Briefly: answers to topical questions
Which leads are used in standard ECG recording?
Generally accepted clinical ECG practice involves recording 12 leads. They are divided into the following groups:
- Standard bipolar leads — leads I, II, III (from the limbs, recording the potential difference between two points).
- Unipolar augmented leads — aVR, aVL, aVF (from the limbs, according to Goldberger).
- Unipolar chest leads — V1, V2, V3, V4, V5, V6 (according to Wilson, reflecting activity in the horizontal plane).
What is the normal width (duration) of the QRS complex?
The normal duration (width) of the QRS complex is 0.06–0.1 s. When assessing the morphology of the ventricular complex, it is taken into account that its waves should normally be clean, without splitting or notches (with the exception of permissible small notches in standard lead III and aVF). The voltage of the complex normally ranges between 5–20 mm.
What is the impulse conduction velocity in the atrioventricular (AV) node?
The impulse conduction velocity in the atrioventricular node is 0.05 m/s. This is the minimum velocity in the heart's conduction system, causing the physiological atrioventricular delay (necessary for pumping blood from the atria into the ventricles before the systole of the latter). The low velocity is explained by the following factors:
- Absence of nexuses — deficiency of gap junctions between cells.
- Reduced excitability — initially low excitability of the nodal cells.
- Anatomical arrangement — atypical cardiomyocytes are located transversely to the direction of conduction.
The relationship between AP phases and ECG: systole (cellular activity, phases 0–3) forms the large QRST complex, while diastole (cellular rest, phase 4) falls into the quiet T-Q interval.
Class and exam questions
What is cardiac automatism?
This is the ability of the conduction system structures to independently generate electrical impulses without the control of the external nervous system. The physiological basis of automatism is the process of slow diastolic depolarization.
Where is the primary pacemaker located?
Normally, the primary pacemaker is the sinoatrial node. It is located in the wall of the right atrium near the mouth of the superior vena cava.
What is the direction of impulse conduction in the ventricles?
The impulse propagates along the conduction pathways in the subendocardial layer. At the final stage, through the Purkinje fibers, excitation is transmitted to the contractile cells in the direction from the endocardium to the epicardium.
What else to study on this topic
- Pathological changes of the P wave and atrial rhythm disturbances.
- Ischemic changes of the ST segment and T wave in myocardial infarction.
- ECG signs of left and right ventricular hypertrophy.
- Bundle branch blocks: effect on the shape of the QRS complex.
- Mechanisms of formation of automatism and conduction disorders.
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