ECG leads

Registration Principles and Core Method

Electrocardiography is based on the fact that the working heart generates an electrical field around itself, changes in which can be recorded on the body surface. Theoretically, the potential difference can be measured between any two areas of the body. However, for doctors worldwide to compare results and draw objective conclusions, strictly standardized electrode placement sites are used in medicine.

Each lead has its own geometric axis—an imaginary line connecting two recording electrodes. In this system, the concept of polarity exists: the pole where the electrical potential is higher at a given time is conventionally considered positive. The direction of current in the heart is always evaluated along the axis of a specific lead.

Standard Leads and Einthoven's Triangle

Historically, the first to be introduced were standard bipolar leads (I, II, and III), proposed by the pioneer of electrocardiography W. Einthoven. To understand how they work, a classic geometric model is used — Einthoven's Triangle. This is an imaginary equilateral triangle whose vertices are the sites of electrode placement on the limbs (right arm, left arm, and left leg). It is assumed that the electrical center of the heart lies exactly in the middle of this figure.

This group of leads records electrical field changes exclusively in the frontal plane. They measure the voltage between two limbs:

• Lead I (upper side of the triangle): measures the potential difference between the right arm (negative pole) and the left arm (positive pole).
• Lead II (right side relative to the patient): connects the right arm (minus) and the left leg (plus).
• Lead III (left side): connects the left arm (minus) and the left leg (plus).

Please note: in the standard lead system, the left leg always acts as the positive pole relative to the upper limbs.

Augmented Unipolar Limb Leads

The main drawback of classic bipolar recording is the inability to isolate and evaluate the potential of one specific region. To solve this problem, unipolar leads were developed. They use one active (exploring) electrode, while the second must have a zero charge.

Since biological tissues do not have a natural point with absolute zero potential, an artificial reference point was created. For this purpose, the wires from three limbs are combined into a central terminal — theoretically, the sum of their electrical potentials cancels each other out and equals zero.

To improve recording quality and increase the voltage of recorded waves, the scheme was slightly modified in practice. The wire from the limb from which the reading is taken is disconnected from the common zero node. This forms augmented leads (marked with the letter a — from augmented):

• aVR (Right, right arm): the vector is directed almost opposite to the axes of leads I and II, which is why the normal ECG curve here appears inverted.
• aVL (Left, left arm): topographically complements the picture of standard lead I.
• aVF (Foot, left leg): complements leads II and III. Clinically, it is most informative for assessing the state of the posterior diaphragmatic (inferoposterior) wall of the left ventricle.

Precordial Leads

To obtain data on the propagation of electrical impulses in the anteroposterior direction (to assess depth), six precordial leads are used (Precordial leads, V1–V6). Unlike previous groups, they record heart activity in the horizontal plane.

These are unipolar systems. The active positive electrode is fixed on the patient's chest, and the neutral reference point is Wilson's central terminal (the sum of limb potentials through additional resistances, yielding zero).

Electrodes are placed along strict anatomical lines:

1. Parasternal group: $V_1$ is located in the 4th intercostal space at the right sternal border, and $V_2$ is symmetrically placed in the 4th intercostal space at the left border.
2. Apical group: $V_4$ is placed in the 5th intercostal space at the left midclavicular line (projection of the apex of the heart). Important nuance: this electrode is always placed before $V_3$.
3. Intermediate position: $V_3$ is attached exactly in the middle between the already placed $V_2$ and $V_4$ (approximately at the level of the 4th rib).
4. Lateral group: placed at the same horizontal level as $V_4$. Electrode $V_5$ is placed at the anterior axillary line, and $V_6$ at the midaxillary line.

Evaluating Graphic Recording Parameters

The electrocardiogram obtained on paper or screen is a graph of the sequential activation of myocardial structures by electrical excitation.

• Vertical axis is responsible for amplitude (voltage). It allows assessing the strength of electrical activity and the muscle mass of various parts of the heart. Baseline calibration: 10 millimeters equals 1 millivolt.
• Horizontal axis reflects duration (time). It is used by doctors to calculate heart rate and impulse conduction velocity. The time scale directly depends on the paper speed of the machine: at standard 25 mm/s, one millimeter of paper corresponds to 0.04 s, and during detailed recording at 50 mm/s, one millimeter means 0.02 s.

Briefly: answers to topical questions

Class and exam questions

What else to study on this topic

• Determination of the alpha angle ($\alpha$) and calculation of the electrical axis of the heart (EAH)
• Topical diagnosis: which leads reflect the anterior, lateral, and posterior walls of the left ventricle
• Formation and clinical significance of zero potential in unipolar recording
• Correlation of wave amplitude with myocardial hypertrophy of the heart chambers

Other topics in the "Cardiovascular system" section

Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.