Depolarization and repolarization waves: normal ECG

2021-06-01 01:02 PM

A normal ECG consists of a P wave, a QRS complex, and a T wave. The QRS complex usually has, but not always, three distinct waves: the Q wave, the R wave, and the S wave.

As the cardiac impulse passes through the heart, the electrical current also propagates from the heart into the adjacent tissues surrounding the heart.

A small fraction of the current propagates all the way to the surface of the body. If electrodes are placed on the skin on opposite sides of the heart, the potential generated by the current can be recorded; This recording is known as an electrocardiogram (ECG). A normal ECG for two heartbeats is shown in the figure.

 

Figure. Electrocardiogram normal.

A normal ECG consists of a P wave, a QRS complex, and a T wave. The QRS complex usually has, but not always, three distinct waves: the Q wave, the R wave, and the S wave.

P waves are caused by electrical potentials generated when the atria depolarize before the atria begin to contract. The QRS complex is caused by the potential generated when the ventricles depolarize before contraction, i.e., as well as by the depolarization wave propagating through the ventricles. Therefore, both the P wave and the components of the QRS complex are depolarized waves.

The T wave is caused by the potential generated as the ventricles recover from the depolarized state. This process usually occurs in the ventricular myocardium 0.25 to 0.35 s after depolarization.

The T wave is known as a repolarization wave.

Thus, the electrocardiogram includes both depolarized and repolarized waves. Distinguishing between depolarized and repolarized waves is so important in the electrocardiogram that further elucidation is needed.

The figure shows a single myocardium in the four depolarisation and repolarisation stages, with red indicating depolarization. During depolarization, the normally negative potential inside the fibre reverses and becomes slightly positive inside and negative outside.

Figure. Record of depolarization (A and B) and repolarization (C and D) waves from a myocardium.

In Figure A, the depolarization process, represented by the red positive charges inside and the red negative charges on the outside, moves from left to right. The first half of the fibre is depolarized, while the other half is still polarized. Thus, the left electrode on the outer surface of the fibre is in a region of a negative charge, and the right electrode is in a region of positive charge, causing the meter to record a positive charge. To the right of the body, fibre shows a record of the changes in potential between the two electrodes, as recorded by a high-speed clock. Note that the maximum positive charge gain is recorded when the depolarization has reached the half-target in figure A.

In Figure B, the depolarization has extended over the entire muscle fibre, and the recording on the right has returned to the isoelectric line because both electrodes are now in an equally electronegative region. The complete wave is a depolarizing wave because it results from the propagation of depolarization along the muscle fibre cell membrane.

Figure C shows still halfway repolarization of the upper muscle fibre, with a positive electrical return of the fibre. At this point, the left electrode is in a region of positive charge and the right electrode is in a region of negative charge. This polarization direction is opposite to that shown in figure A. Thus, the recording, as shown on the right, becomes negative.

In Figure D, the muscle fibre has completed repolarization, and both electrodes are now in the region of positive charge so that no other potential is recorded between them. Thus, in the record on the right, the potential once again goes back to 0. This complete sound wave is a repolarization wave because it results from the propagation of repolarization along with the fibrous membrane muscle.

Relationship of the single-phase action potential of the ventricular muscle to the T wave and QRS complex in a standard electrocardiogram:

The single-phase action potential of the ventricular muscle normally exists between 0.25 and 0.35s. The upper part of

The figure shows a single-phase action potential recorded from a microelectrode inserted into the interior of a single ventricular muscle fibre. The increase in action potential is caused by depolarization, and the return of the potential to baseline is due to repolarization.

Figure. Above, a single-phase action potential from the ventricular myocardium when cardiac function is normal shows rapid depolarization and then repolarization that occurs slowly during the plateau phase but rapidly towards the end. Below, the electrocardiogram records simultaneously.

The lower half of the figure shows a simultaneous ECG recording from this same ventricle. Note that the QRS waves appear at the beginning of the single-phase action potential and the T waves appear at the end. Of note is that no potential is recorded in the ECG when the ventricular muscle either completes polarization or completes depolarization.

It is only when the muscle is partially polarized and partly depolarized that current flows from one part of the ventricle to another, and therefore also to the surface of the body, to produce an ECG.