Physical basis of electrocardiography
The electrical activity of the heart generates a complex electromagnetic field. The chain of physical phenomena is as follows:
- A time-varying local current creates an alternating magnetic field around itself.
- An alternating magnetic field induces a vortex electric field in body tissues (its lines of force are closed, and its magnitude is proportional to the rate of change of the magnetic field).
- A unified electromagnetic field is formed around the heart.
The ECG registers exclusively the electrical component of this field. Eddy currents alter charges on the body surface, creating a potential difference. This difference is recorded by the device in the form of waves.
To describe the field, the concept of the electrical vector (EV) is used — it characterizes the intensity of the electrical component and is directed from minus to plus. Due to the complex architectonics of the conduction pathways, the vector constantly changes its amplitude and direction. The projection of these instantaneous changes onto the lines connecting the electrodes forms the ECG curve.
Formation of waves and the isoelectric line
Waves on the ECG occur only when a charge change process takes place: depolarization (excitation) or repolarization (recovery). For the device to record a potential difference, a significant portion of the muscle fibers must be involved in excitation.
Isoelectric line (a straight line without waves) is recorded at moments when the cell membrane charge is stable and there is no potential difference. This happens in three cases:
- Resting potential (no excitation).
- Action potential plateau (cardiomyocyte charge is maintained at a constant level).
- Complete activation of the myocardium.
It is worth noting that the excitation of the heart's conduction system itself (SA node, AV node, bundle of His) is not displayed on the surface ECG. It contains too few atypical cardiomyocytes to create an electromagnetic field of sufficient power.
Relationship between ECG and cellular action potential
The total electrical activity of the heart is strictly synchronized with the action potential (AP) phases of individual cardiomyocytes (at a cycle of 800 ms and HR of 75 bpm):
- P wave (~80 ms): Corresponds to phase 0 (depolarization) of atrial cardiomyocytes. Atrial repolarization is not visible on the ECG because it is masked by the powerful ventricular complex.
- QRS complex (~80 ms): Phase 0 (depolarization) of the working ventricular myocardium.
- ST segment (~120 ms): Phase 2 (plateau, slow repolarization). The ventricles are fully excited and refractory; the isoelectric line is recorded.
- T wave (~130 ms): Phase 3 (rapid terminal repolarization). Restoration of the ventricular resting potential.
ECG and mechanical contraction
The ECG reflects only electrophysiological processes (excitation and conduction) and is not directly linked to the mechanical contraction of the myocardium. The phenomenon of electromechanical dissociation exists — the presence of electrical activity in the absence of pumping function.
Evidence of this dissociation:
- Calcium experiment: If a solution without $Ca^{2+}$ ions is passed through an isolated heart, mechanical contractions stop, but the ECG and cell APs will be recorded unchanged.
- Dying heart: In terminal states, mechanical cardiac arrest often occurs before the electrical generation of impulses stops (ECG waves are recorded in the absence of a pulse).
Leads and Einthoven's triangle
Clinical ECG uses 12 leads for precise localization of cardiac abnormalities:
- 3 standard (bipolar): Measure the potential difference between two limbs. They form Einthoven's triangle. Lead I: right arm (RA) — left arm (LA). Lead II: RA — left leg (LL). Lead III: LA — LL. The vector is projected onto the sides of this triangle.
- 3 augmented (unipolar): aVR, aVL, aVF. The active electrode is on one limb, while the indifferent electrode is a combination of the other two. The vector is projected onto perpendiculars dropped from the corners of the triangle to the opposite sides.
- 6 chest (unipolar): V1 – V6. The active electrode is placed at 6 points on the chest.
To eliminate interference, grounding is used — a separate electrode is always attached to the right shin (RL).
Briefly: answers to topical questions
At which exact points on the chest are the active electrodes of the chest leads V1-V6 located?
The active electrodes of the unipolar chest leads are located on the anterior chest wall at the following points:
- Lead V1 — 4th intercostal space at the right sternal border.
- Lead V2 — 4th intercostal space at the left sternal border.
- Lead V3 — at the 4th rib on the left parasternal line (or midway between the V2 and V4 electrodes).
- Lead V4 — 5th intercostal space on the left midclavicular line (linea medioclavicularis).
- Lead V5 — on the same horizontal level as V4, at the anterior axillary line (linea axillaris anterior).
- Lead V6 — on the same horizontal level as V4 and V5, at the left midaxillary line (linea axillaris media).
How to calculate the alpha angle for precise determination of the electrical axis of the heart?
The angle α is the angle between the vector of the electrical axis of the heart (EAH) and the horizontal zero line corresponding to standard lead I.
For precise determination, the geometric (calculation) method is used: a vector is constructed in Einthoven's triangle using the amplitude of the QRS complex waves in two standard leads. The direction of the resulting vector in the frontal plane corresponds to the EAH, and the angle between it and the lead I line is the angle α.
An approximate position of the EAH can be determined by the ratio of the QRS complex waves:
- +QRS (R > S) — the axis is directed toward the lead;
- ±QRS (R = S) — the axis is perpendicular to the lead;
- –QRS (R < S) — the axis is directed away from the lead.
What do the PQ and QT intervals on the electrocardiogram reflect?
The PQ and QT intervals on the electrocardiogram reflect the systole duration of various parts of the heart.
- PQ interval — corresponds to the atrial systole time. It has a duration of about 100 ms and includes atrial depolarization (P wave) and their slow repolarization against the background of atrioventricular conduction delay (PQ segment).
- QT interval — represents the electrical systole of the ventricles, equal to the time of their depolarization and repolarization. This interval is measured from the onset of the QRS complex to the end of the T wave, and its normal value in adults is 0.35–0.44 s.
Class and exam questions
Why is there no separate atrial repolarization wave on the ECG?
Atrial repolarization occurs simultaneously with ventricular depolarization. The powerful signal of the QRS complex completely masks the weak signal of atrial recovery.
What is the difference between the electrical vector and the electrical axis of the heart (EAH)?
The vector constantly changes its amplitude and direction every fraction of a second — this draws the waves. The EAH is a constant for the patient, reflecting the main direction of excitation. They coincide only at one moment — at the peak of the R wave.
How to determine the position of the electrical axis of the heart?
It is determined by the amplitude of the R wave in any two standard leads. The values are plotted as vectors on the axes, perpendiculars are drawn, and their intersection indicates the main direction in the frontal plane.
What else to study on this topic
- Detailed analysis of the vectorcardiogram and P, QRS, T loops
- Localization of active V1-V6 electrodes on the chest
- Calculation of the alpha angle for precise determination of the electrical axis of the heart
- Pathological changes of the ST segment
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