We have. used the general term “electrical stimulation” to refer to the spread of electrical stimuli through the atria and ventricles. The technical term for this cardiac electrical stimulation is depolarization. The return of heart muscle cells to their resting state following stimulation (depolarization) is called repolarization. These terms are derived from the fact that the normal myocardial cells (atrial and ventricular) are polarized; that is, they carry electrical charges on their surface. The resting polarized state of a normal heart muscle cell. Notice that the outside of the resting cell is positive and the inside of the resting cell is positive and the inside of the cell is negative (about –90mV).
When a heart muscle cell is stimulated, it depolarizes. As a result, the outside of the cell, in the area where the stimulation has occurred, becomes negative, while the inside of the cell becomes positive. This produces a difference in electrical voltage on the outside of the cell between the stimulated depolarized area and the unstimulated polarized area. As a result, a small electrical current is formed. This electrical current spreads along the length of the cell as stimulation and depolarized. The path of depolarization can be represented by an arrow. Ffor individual myocardial cells (fibers) depolarization and repolarization proceed in the same direction. However, for the entire myocardium depolarization proceeds from innermost layer (endocardium) to outermost layer (epicardium) while repolarization proceeds in the opposite direction. The mechanism of this difference is mot well understood.
This depolarizing electrical current is recorded by the ECG as a P wave (when the atria are stimulated and depolarize) and as a QRS complex (when the ventricles are stimulated and depolarize).
After a period of time, the fully stimulated and depolarized cell begins to return to the resting state. This is known as repolarization. A small area on the outside of the cell becomes positive again. The repolarization spreads along the length of the cell until the entire cell is once again fully repolarized. Ventricular repolarizarion is recorded by the ECG as the ST segment, T wave, and U wave. (Atrial repolarization is usually obscured by ventricular potentials)
The ECG records the electrical activity of a large mass of atrial and ventricular cells, not just the electrical activity of a single cell. Since cardiac depolarization and repolarization normally occur in a synchronized fashion, the ECG is able to record these electrical currents as specific waves (P wave, QRS complex, ST segment, T wave, and U wave).
To summarize, regardless of whether the ECG is normal or abnormal, it merely records two basic events: (1) depolarization, the spread of a stimulus through the heart muscle, and (2) repolarization, the return of the stimulated heart muscle to the resting state.
This spread of a stimulus through the atria and ventricles and the return of the stimulated atrial and ventricular muscle to the resting state produce, as noted previously, the electrical currents recorded on the ECG. Furthermore each phase of cardiac electrical activity produces a specific wave or complex. These basic ECG waves are labeled alphabetically and begin with the P wave.
The P wave represents the spread of a stimulus through the atria (atrial depolarization). The QRS complex represents the spread of a stimulus through the ventricles (ventricular depolarization). The ST segment and T wave represent return of the stimulated ventricular muscle to the resting state (ventricular repolarization). The U wave is a small deflection sometimes seen just after the T wave. It represents the final phase of ventricular repolarization, although its exact significance is not known.
You are probably wondering why there is no wave or complex representing the return of the stimulated atria to the resting state. The atrial ST segment (STa) and atrial T wave (Ts) are generally not observed on the normal ECG because of their low amplitudes. Similarly the routine ECG is not sensitive enough to record any electrical activity during the spread of the stimulus through the AV junction (AV node and bundle of His). The spread of the electrical stimulus through the AV junction occurs between the beginning of the P wave and the beginning of the QRS complex. This interval, which is known as the PR interval, is a measure of the time it takes for the stimulus to spread through the atria and pass through the AV junction.
To summarize, the P-QRS-ST-T-U sequence represents the repetitive cycle of the electrical activity in the heart, beginning with the spread of a stimulus through the atria (P wave) and ending with the return of the stimulated ventricular muscle to the resting state (ST-T-U sequence). This cardiac cycle repeats itself again and again.
The P-QRS-T sequence is recorded on special ECG paper. This paper is divided into gridlike boxes. Each of the small boxes is 1 millimeter square (1 mm2). The paper usually moves out of the electrocardiograph at a speed of 25 mm/sec. Therefore, horizontally, each millimeter of the ECG paper is equal to 0.04 second (25 mm/sec x 0.04 sec = 1 mm). Notice also that between every five boxes there are heavier lines, so each of the 5 mm units horizontally corresponds to 0.2 second (5 x 0.04 = 0.2).
The ECG can therefore be regarded as a moving graph, which horizontally corresponds to time, with 0.04 and 0.2 second divisions. Vertically, the ECG graph measures the voltage, or amplitudes, of the ECG waves or deflections. The exact voltage can be measured because the electrocardiograph is so standardized that 1 millivolt (1mV) produces a deflection of 10 mm amplitude (1 mV = 10 mm). (in most electrocardiographs, the standardization can also be set at one-half or two times normal sensitivity.)