Electrocardiogram: Normal vector analysis

2021-06-01 01:57 PM

Since the lateral aspect of the ventricular apex depolarizes in front of the medial surface, during repolarization, all ventricular vectors are positive and directed toward the apex.

Diagnosisbook supporting video - Electrocardiogram (ECG_EKG)

QRS complex: vector occurs consecutively during ventricular polarization

When cardiac impulses reach the ventricles through the atrioventricular bundle, the first part of the ventricles depolarizes in the left endocardial portion of the interventricular septum. The depolarization then spreads rapidly to both endocardial sides of the interventricular septum and is shown in figure A as a darkened area of ​​the ventricles. Next, depolarization spreads along the endocardial surface to the rest of the ventricles (Figures B, C). It eventually spreads through the ventricular muscle to the outside of the heart (remaining figures).

At each stage in the figure, sections A and E, the instantaneous mean voltage of the ventricles is the red arrow above the ventricle in each figure. Each of these vectors was analyzed to determine the potential at each time point in one of the three leads on the electrocardiogram. To the right of each figure is the variation of the QRS complex on the electrocardiogram. Remember that the positive vector in one lead, in the electrocardiogram will be recorded above the isoelectric line, while the negative vector recorded below the isoelectric line.

The dark area of ​​the ventricle

Figure: The dark area of ​​the ventricle is the depolarized region; the rest is the still polarized area. 

Vector of ventricle and QRS complex 0.01s after depolarization (A), after 0.02s (B), after 0.035s (C), after 0.05s (D), after 0.06s- ventricular depolarization completely (E).

In Figure A, the ventricular muscle begins to depolarize, as shown at approximately 0.01 s after depolarization begins. At that point, the vector is short because only a small portion of the ventricles is depolarized. Therefore, all the potentials on the electrocardiogram are low. The potential in leads II is greater than in leads I and III because the vector of the heart expands to coincide with the direction of the axis of lead II.

In Figure B, at 0.02s after depolarization, the heart vector is long because of the large volume of depolarized myocardium. As a result, the potential at all leads on the electrocardiogram increases.

In Figure C, about 0.37 s after depolarization, the heart vector is shorter and the potentials recorded on the electrocardiogram are smaller because, at this point, the outer apex of the heart is negatively charged, neutralizing many of the surrounding positive charges the outer surface of the heart. Likewise, the axis of the vector begins to shift to the left thorax because the left ventricle depolarizes more slowly than the right ventricle. Therefore, the ratio of potentials between leads I and II increases.

In Figure D, 0.05 s after depolarization, the heart vector points to the left ventricular base, which is short because part of the ventricular muscle is still being depolarized. Because of the direction of the vector at that time, the potentials recorded in leads II and III are both negative, while the potential in leads I remains positive.

 In Figure E, 0.06 s after depolarization, the entire ventricular muscle has been depolarized, so there is no cardiac current. The vector returns to zero and the potential in all leads is zero. Occasionally the QRS complex is negative at its origin in one or more leads, the descending part being the Q wave.

T wave: repolarization process

After the ventricular muscle is depolarized at about 0.15 s, repolarization begins and occurs in 0.35 s. Repolarization induces T waves on the electrocardiogram.

 T wave formation during repolarization

Figure: T wave formation during repolarization. The total time is 0.15s.

Since the interventricular septum and endocardium of the ventricles are depolarized first, it sounds quite logical. However, this is not normal because the interventricular septum and endocardial regions have a longer contractile duration than most of the outer surface of the heart. Thus, most of the first repolarizing ventricular muscle is the entire outer surface of the ventricles, especially near the apex of the heart. In the medial region, the opposite is true, repolarizing eventually. This sequence of repolarization is necessarily caused by high blood pressure in the ventricles during contraction, significantly reducing coronary flow to the endocardium. , that way, depolarization occurs slowly in the endocardium.

Since the lateral aspect of the ventricular apex depolarizes in front of the medial surface, during repolarization, all ventricular vectors are positive and directed toward the apex. Consequently, the normal T waves in all three bipolar limb leads are positive.

In the figure, the five phases of ventricular repolarization are represented by the extension of the to- that is the repolarized region. Firstly, the vector is small because the repolarization region is small, then gradually the vector becomes larger. Finally, the vector weakens again because the depolarization region becomes too small compared to the total current being reduced. These changes are represented by a larger vector when half of the heart is in polarization and the other half is depolarized.

 The electrocardiographic changes of the three-limb lead during repolarization are shown below each ventricular image, describing each stage of the repolarization process. So, 0.15s is the time it takes for the whole process to take place, and a T wave is generated.

P wave: atrial depolarization

Depolarization of the atria and generation of P waves

Figure. Depolarization of the atria and generation of P waves, showing peak vector across the atria and vector in the standard three leads. 

On the right are the P and T waves of the atria. SA, sinoatrial node.

Atrial depolarization is initiated at the sinus node and spreads in all directions of the atria. Therefore, the first negative atrium point is the point above the vena cava junction, where the sinus node is located, and the direction of initial depolarization is represented by the black vector in the figure. In addition, the vector is mostly located in that direction during atrial depolarization. Since it is axially positive in the 3 bipolar limb leads I, II, and III, the electrocardiogram records positive atrial depolarization in all 3 leads (figure). Atrial depolarization produces P waves on the electrocardiogram.

T wave: atrial repolarization

Depolarization spreads through the atrial muscle more slowly than it does into the ventricles because the atria do not have the Purkinje network, which transmits impulses more quickly. As a result, the muscles surrounding the sinus node are depolarized for a long time before other muscle parts of the atria. Consequently, the first repolarized atrial region is the sinus node region, which is the first depolarized region. Thus, when repolarization begins, the region around the sinus node becomes positive relative to the rest of the atria. Therefore, the atrial repolarization vector is in the opposite direction to the depolarization vector. Atrial repolarization produces a T wave on the electrocardiogram, 0.15 s after the P wave, but the T wave is opposite the P wave across the isoelectric line, and it is usually negative on the three bipolar limb leads.

On a normal electrocardiogram, T waves appear at the same time as the QRS complex. Therefore, it is often obscured by the QRS complex and appears in some abnormal electrocardiograms.

Electrocardiogram axis: normal electrocardiogram

Vector QRS and BILLION

Figure. Vector QRS and BILLION

As mentioned, the vector of electric current through the heart changes very rapidly as the impulse propagates through the heart muscle. It changes through two shapes: First, the vector increases or decreases in length as the vector's potential increases or decreases. Second, the vector changes direction as the mean direction of the cardiac potential changes. The electrocardiographic axis describes these changes during the different phases of the cardiac cycle.

When the interventricular septum first depolarizes, the vector extends to the apex of the ventricle, but it is relatively weak, thus forming the first part of the ventricular electrical axis (vector 1). As more parts of the ventricles are then depolarized, the vector becomes even stronger. Thus, vector 2 in the figure shows depolarization of the ventricles, about 0.02s after vector 1. Then 0.02 and 0.01s are followed by vectors 3 and 4 respectively. Finally, the ventricles are depolarized. completely polar, the vector becomes 0, represented by point 5.

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