The Ecg Waveform

When an ECG is recorded, a reading of voltage vs time is produced, which is normally displayed as millivolts (mV) and seconds. A typical lead II ECG waveform is shown in Fig. 2. For this recording, the negative electrode was placed on the right wrist, and the positive electrode was placed on the left ankle, giving the standard lead II ECG (explained in Section 3.1.). It shows a series of peaks and waves that corresponds to ventricular or atrial depolarization and repolarization, with each segment of the signal representing a different event associated with the cardiac cycle.

The cardiac cycle begins with the firing of the sinoatrial node in the right atrium. This firing is not detected by the surface ECG because the sinoatrial node is not composed of an adequately large quantity of cells to create an electrical potential with a high enough amplitude to be recorded with distal electrodes (signal amplitude is lost as it dissipates through the conductive medium). The atria then depolarize, giving rise to the P wave. This represents the coordinated depolarization of the right and left atria and the onset of atrial contraction. The P wave is normally around 80-100 ms in duration. As the P wave ends, the atria are completely depolarized and are beginning contraction.

The signal then returns to baseline, and action potentials (not large enough to be detected) spread to the atrioventricular node and bundle of His. Then, roughly 160 ms after the beginning of the P wave, the right and left ventricles begin to depolarize, resulting in what is called the QRS complex, representing the beginning of ventricular contraction, which is around 80 (60100) ms in duration. Typically, the first negative deflection is the Q wave, the large positive deflection is the R wave, and if there is a negative deflection after the R wave, it is called the S wave. The exact shape of the QRS complex, as explained in Section 3.2., depends on the placement of electrodes from which the signals are recorded.

Simultaneous with the QRS complex, atrial contraction has ended, and the atria are repolarizing. However, the effect of this global atrial repolarization is sufficiently masked by the much larger amount of tissue involved in ventricular depolarization and is thus not normally detected in the ECG. During ventricular contraction, the ECG signal returns to baseline. The ventricles then repolarize after contraction, giving rise to the T wave. Note that the T wave is normally the last-detected potential in the cardiac cycle; thus, it is followed by the P wave of the next cycle, repeating the process.

Of clinical importance in the ECG waveform are several notable parameters (regions), which include the P-R interval, the S-T segment, and the Q-T interval. The P-R interval is measured from the beginning of the P wave to the beginning of the QRS complex and is normally 120-200 ms long. This is basically a measure of the time it takes for an impulse to travel from atrial excitation and through the atria, atrioventricular node, and remaining fibers of the conduction system. The S-T segment is the period of time when the ventricles are completely depolarized and contracting and is measured from the S wave to the beginning of the T wave. The Q-T interval is measured from the beginning of the QRS complex to the end of the T wave; this is the time segment from when the ventricles begin their depolarization to the time when they have repolarized to their resting potentials and is normally about 400 ms in duration.

An obvious observation made concerning the QRS complex is that it has a much higher peak and shorter duration than either the P or T waves. This is because ventricular depolarization occurs over a greater mass of cardiac tissue (i.e., a greater number of myocytes are depolarizing at the same time); furthermore, the ventricular depolarization is much more synchronized than either atrial depolarization or ventricular repolarization. For additional details relative to the types of action potential that occur in various regions of the heart, refer to Chapter 9.

It is also very important to note that deflections in the ECG waveform represent the change in electrical activity caused by atrial or ventricular depolarization and repolarization and not necessarily generalized cardiac contractions or relaxations, which take place on a slightly longer time-scale (see Fig. 3). Shown in Fig. 3 are the certain points on the ECG waveform and how they relate to other events in the heart during the cardiac cycle.

One last thing that should be noted relative to the ECG waveform is the sometimes-detected potential referred to as the U wave. Its presence is not fully understood, but is considered by some to be caused by late repolarization of the Purkinje system. If detected, the U wave will be toward the end of the T wave and have the same polarity (positive deflection). However, it has a much shorter amplitude and usually ascends more rapidly than it descends (which is the opposite of the T wave).

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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