Fundamentals of Electrocardiography

Cardiac Depolarization Waves Versus Repolarization Waves

Figure 11-2 shows a single cardiac muscle fiber in four stages of depolarization and repolarization, with the color red designating depolarization. During depolarization, the normal negative potential inside the fiber reverses and becomes slightly positive inside and negative outside.

In Figure 11-2 A , depolarization, demonstrated by red positive charges inside and red negative charges outside, is traveling from left to right. The first half of the fiber has already depolarized while the remaining half is still polarized. Therefore, the left electrode on the outside of the fiber is in an area of negativity, and the right electrode is in an area of positivity, which causes the meter to record positively. To the right of the muscle fiber is shown a record of changes in potential between the two electrodes, as recorded by a high-speed recording meter. Note that when depolarization has reached the halfway mark in Figure 11-2 A , the recording on the right has risen to a maximum positive value.

In Figure 11-2 B , depolarization has extended over the entire muscle fiber, and the recording to the right has returned to the zero baseline because both electrodes are now in areas of equal negativity. The completed wave is a depolarization wave because it results from the spread of depolarization along the muscle fiber membrane.

Figure 11-2 C shows halfway repolarization of the same muscle fiber, with positivity returning to the outside of the fiber. At this point, the left electrode is in an area of positivity, and the right electrode is in an area of negativity. This polarity is opposite to the polarity in Figure 11-2 A . Consequently, the recording, as shown to the right, becomes negative.

In Figure 11-2 D , the muscle fiber has completely repolarized, and both electrodes are now in areas of positivity so that no potential difference is recorded between them. Thus, in the recording on the right, the potential returns once more to zero. This completed negative wave is a repolarization wave because it results from the spread of repolarization along the muscle fiber membrane.

Relation of the Monophasic Action Potential of Ventricular Muscle to the QRS and T Waves in the Standard Electrocardiogram

The monophasic action potential of ventricular muscle, discussed in Chapter 10 , normally lasts between 0.25 and 0.35 second. The top part of Figure 11-3 shows a monophasic action potential recorded from a microelectrode inserted into the inside of a single ventricular muscle fiber. The upsweep of this action potential is caused by depolarization, and the return of the potential to the baseline is caused by repolarization.

The lower half of Figure 11-3 shows a simultaneous recording of the ECG from this same ventricle. Note that the QRS waves appear at the beginning of the monophasic action potential, and the T wave appears at the end. Note especially that no potential is recorded in the ECG when the ventricular muscle is either completely polarized or completely depolarized . Only when the muscle is partly polarized and partly depolarized does current flow from one part of the ventricles to another part, and therefore current also flows to the surface of the body to produce the ECG.

Relationship of Atrial and Ventricular Contraction to the Waves of the Electrocardiogram

Before contraction of muscle can occur, depolarization must spread through the muscle to initiate the chemical processes of contraction. Refer again to Figure 11-1 ; the P wave occurs at the beginning of contraction of the atria, and the QRS complex of waves occurs at the beginning of contraction of the ventricles. The ventricles remain contracted until after repolarization has occurred—that is, until after the end of the T wave.

The atria repolarize about 0.15 to 0.20 second after termination of the P wave, which is also approximately when the QRS complex is being recorded in the ECG. Therefore, the atrial repolarization wave, known as the atrial T wave , is usually obscured by the much larger QRS complex. For this reason, an atrial T wave is seldom observed on the ECG.

The ventricular repolarization wave is the T wave of the normal ECG. Ordinarily, ventricular muscle begins to repolarize in some fibers about 0.20 second after the beginning of the depolarization wave (the QRS complex), but in many other fibers, it takes as long as 0.35 second. Thus, the process of ventricular repolarization extends over a long period, about 0.15 second. For this reason, the T wave in the normal ECG is a prolonged wave, but the voltage of the T wave is considerably less than the voltage of the QRS complex, partly because of its prolonged length.

Electrocardiographic Calibration and Display

All recordings of ECGs are made with appropriate calibration lines on the display grid. Historically, ECGs were recorded electronically and printed onto paper; ECGs are now usually displayed digitally. As shown in Figure 11-1 , the horizontal calibration lines are arranged so that 10 of the small line divisions upward or downward in the standard ECG represent 1 millivolt, with positivity in the upward direction and negativity in the downward direction.

The vertical lines on the ECG are time calibration lines. A typical ECG is run at a speed of 25 millimeters per second, although faster speeds are sometimes used. Therefore, each 25 millimeters in the horizontal direction is 1 second, and each 5-millimeter segment, indicated by the dark vertical lines, represents 0.20 second. The 0.20-second intervals are then broken into five smaller intervals by thin lines, each of which represents 0.04 second.