What is the electrical axis of the heart - its positions and deviations

What is the electrical axis of the heart?

EOS - the total (prevailing) vector of all electrical impulses that are observed in the cardiac conduction system in one cycle of contraction. Often this indicator is identical to the electrical position of the heart (EPS) - the orientation of the resulting vector of impulses from the ventricles relative to the lead I axis on the ECG.

In the myocardium, like other muscles of the body, bioelectric currents (action potentials) arise during contraction. It is their electrocardiograph that registers and records on a specialized film in the form of an ECG.

The impulse is generated by the pacemaker (sinus node), from where excitation reaches the atrium along the nerve pathways of the heart, and then the atrioventricular node (AV). This compound inhibits transmission so that the contraction follows after the relaxation of the atria, which provides a unilateral and continuous flow of blood through the heart chambers.

On the ECG, electrical impulses are displayed in the form of multidirectional waves:

• positive - P, R, T - directed upwards in relation to the isoline;
• negative - Q, S.

Electrocardiographic recording is a recording of changes in potential differences during the process of excitation and relaxation of the atria and ventricles, due to the electromotive force of the heart (EMF) from the surface of the human body.

EMF is an unstable value, its direction changes throughout the entire cardiac cycle. When all momentary orientations of the impulses are summed up (according to the addition rules), a vector is obtained that corresponds to the average EMF during the full depolarization period - EOS (direction of the electric motor force during QRS registration on the ECG).

When recording an ECG, the electrodes are located in three leads, recording the potential difference:

• I - left-right hand;
• II - left leg - right hand;
• III - left leg - left hand.

This placement forms a three-dimensional arrangement of the EMF vectors on the body, which forms the "Einthoven triangle". If we put the EDS in such a shape, then the angle α (alpha) between the electromotive force and the horizontal line of the 1st lead will express the EOS deviation.

Also, the angle α is roughly determined by the Bailey six-axis coordinate system or using special tables. In the absence of the above devices at hand, the orientation of the EOS is set by measuring the height of the R and S teeth in I and III standard leads:

• RII = RI + RIII - normal axis position;
• RI> RII> RIII, SIII> RIII - left-sided deviation of EOS;
• RIII> RI, SI> SIII - EOS deviates to the right.

What EOS positions exist in the norm and what is the difference between them?

The muscle mass of the left ventricle (LV) is commensurately larger than the right one. Therefore, the electrical processes that occur in the LV are stronger, and the EOS vector will be directed in this direction. If you project the heart to a coordinate system, then the left ventricle will be located in the range of +40⁰+70⁰ (which is considered normal axis orientation).

However, the individual features of the structure of the heart and the physique of each patient vary the position of the EOS in the range from 0⁰ up to 90⁰.

EOS Normal Position Options

EOS normal position - angle α from 30⁰ up to 69⁰, the height RII≥RI> RIII, and in III and VL the R and S waves are approximately the same. The cardiac axis is clearly perpendicular to lead III.

EOS horizontal position - axis orientation coincides with the placement of I standard lead (RIII> SIII), angle α from 0 to + 30⁰... It occurs in hypersthenics or short people with a wide chest, as well as at the peak of expiration, with abdominal obesity, in the II and III trimester of pregnancy. The heart "lies" on the dome of the diaphragm.

Semi-horizontal position of EOS - the cardiac axis is at an angle of 90⁰ to standard lead III (RIII = SIII), angle α = + 30⁰.

Vertical electrical position of the heart - the direction of the EDS is perpendicular to the I assignment (RI = SI), angle α = + 90⁰... This type is typical for tall asthenic people with a narrow chest, at the end of a deep breath. The heart "hangs" between the roots of the lungs on the vascular bundle.

Semi-vertical electrical position of the heart - axis direction parallel to II and indistinctly perpendicular to lead I (RII> RIII> RI), angle α from +70⁰ up to +90⁰.

The presence of transitional types of EOS position is explained by the fact that pure asthenics or hypersthenics are rare, and "intermediate" types of constitution are widespread.

Rotation around its horizontal or vertical axis is also sometimes determined (rotation of the apex anteriorly or posteriorly relative to its normal position).

The horizontal axis of the heart is the symbolic bisector through the apex and base.

The longitudinal axis is characterized by the location of the gastric QRS complex in the thoracic leads, the axes of which are located frontally. It is necessary to designate the turning zone and assess the QRS structure in V6.

Types of orientation of the heart in the frontal plane:

1. Normal position - the pivotal zone is located in lead V3, R and S waves of identical height are noted. In V6, the QRS complex acquires a qR or qRs configuration.
2. Clockwise rotation - the turning zone in the area of ​​leads V4-V5, and in V6 the complex looks like RS. It is often combined with the vertical position of the EOS and its deviation to the right.
3. Counterclockwise rotation - the pivot zone is shifted by V2. Deepening Q is observed in leads V5-V6 (not to be confused with coronary), and the QRS complex acquires the qR form. It is combined with the horizontal position of the EOS and its deviation to the left.

Rotation of the heart along the vertical axis:

1. Apex anteriorly - the QRS complex in leads I-III takes the form of qRI, qRII, qRIII.
2. Apex posteriorly - the QRS complex takes the form of RSI, RSII, RSIII.

Pathological axis deviations: what do they talk about and what are the consequences?

The situation itself cannot serve as a basis for making a specific diagnosis, only indicating the presence of electrical disorders. Not a single cardiologist will convince you of the presence of pathology only by EOS. To establish the fact of the disease, it is necessary to support the conclusion of the examination with the correct clinical questioning and additional diagnostic measures.

The position of the EOS is influenced by a number of factors:

• congenital heart defects;
• secondary changes in the anatomical relationships between the right and left heart;
• abnormal arrangement of organs in the chest cavity (dextrocardia, vicar emphysema after lobectomy);
• deformation of the chest (kyphosis, scoliosis, keeled or funnel-shaped curvature);
• failures in the conducting system of the organ (especially in the bundles of Giss), which cause disturbances in the heartbeat;
• cardiomyopathy of various origins;
• long history of hypertensive and coronary heart disease (CHD);
• chronic heart failure;
• respiratory diseases with an obstructive component (COPD, bronchial asthma, emphysema);
• decompensated liver failure (ascites, flatulence).

What diseases are there for?

Deviation of the electrical axis of the heart to the left (levogram) (angle α from 0 to -30⁰) has several reasons:

1. Hypertrophy of the left half of the heart.Angle α is directly proportional to the growth rate of LV mass. Pathology develops with idiopathic cardiomyopathy, arterial hypertension, excessive exercise ("sports heart"), coronary artery disease, cardiosclerosis.
2. Myocardial infarction (with posterior necrosis).
3. Pathology of intracardiac conduction. Most often it is a blockade of the left leg or antero-superior branch of the His bundle.
4. Ventricular tachycardia.
5. Valvular heart disease.
6. Myocarditis.

A sharp deviation of the EOS to the left is also distinguished when the angle α> -30⁰.

Deviation of the electrical axis of the heart to the right (pravogram) (angle α> +90⁰) is observed when:

1. Failures in the conduction of a nerve impulse along the fibers of the Giss bundle.
2. Pulmonary stenosis (when the pressure in the right ventricle rises).
3. Ischemic heart disease.
4. Right myocardial infarction.
5. Cardiorespiratory diseases, which formed the "cor pulmonale" (in this case, the LV is malfunctioning and there is an overload of the right ventricle).
6. Thromboembolism of the branches of the pulmonary artery (due to blockage, gas exchange in the lungs is disturbed, the vessels of the pulmonary circulation are narrowed and the pancreas is overloaded).
7. Mitral valve stenosis (after rheumatic fever). The fusion of the leaflets prevents the full expulsion of blood from the left atrium, which causes pulmonary hypertension and overloads the pancreas.

A sharp deviation of the EOS to the right is observed at an angle value α = +120⁰.

It is worth remembering that none of the above diseases can be diagnosed based solely on the position of the EOS. This parameter is only an auxiliary criterion in identifying any pathological process.

Conclusions

Axis deviation is often not a sign of an acute condition. But if a sharp violation of the EOS is registered with a value of more than +90⁰, then this may indicate a sudden disturbance of conduction in the myocardium and threaten with cardiac arrest. Such patients require immediate specialized medical attention in order to find the reason for such a sharp change in the direction of the current.