The Electrocardiogram

An electrocardiogram (ECG; in German, the electrokardio-gram, EKG) is a measure of how the electrical activity of the heart changes over time as action potentials propagate throughout the heart during each cardiac cycle. However, this is not a direct measure of the cellular depolarization and repolarization with the heart, but rather the relative, cumulative magnitude of populations of cells eliciting changes in their membrane potentials at a given point in time; it shows electrical differences across the heart when depolarization and repolarization of these atrial and ventricular cells occur.

The human body can be considered, for the purposes of an ECG, a large-volume conductor. It is basically filled with tissues surrounded by a conductive ionic fluid. You can imagine that the heart is suspended inside of that conductive medium. During the cardiac cycle, the heart contracts in response to action potentials moving along the chambers of the heart. As it moves, there will be one part of the cardiac tissue that is depolarized and another part that is at rest or polarized. This results in a charge separation, or dipole, which is illustrated in Fig. 1.

From: Handbook of Cardiac Anatomy, Physiology, and Devices Edited by: P. A. Iaizzo © Humana Press Inc., Totowa, NJ

The dipole causes current flow in the surrounding body fluids between the ends of the heart, resulting in a fluctuating electric field throughout the body. This is much like the electric field that would result, for example, if a common battery were suspended in a saltwater solution (an electrically conductive medium). The opposite poles of the battery would cause current flow in the surrounding fluid, creating an electric field that could be detected by electrodes placed in the solution. A similar electrical field around the heart can be detected using electrodes attached to the skin. The intensity of the voltage detected depends on the orientation of the electrodes with respect to that of the dipole ends. The amplitude of the signal is proportional to the mass of tissue involved in creating that dipole at any given time. Using electrodes on the surface of the skin to detect the voltage of this electrical field is what provides the electrocardiogram.

It is important to note, as might be expected, that because the ECG is measured on the skin, any potential differences within the body can have an effect on the electrical field detected. This is why it is considered important for diagnostic purposes that, while recording an ECG from an individual, the individual should remain as still as possible. Movements require the use of skeletal muscles, which then contribute to the changes in volt-

Fig. 1. After conduction begins at the sinoatrial (SA) node, cells in the atria begin to depolarize. This creates an electrical wavefront that moves down toward the ventricles, with polarized cells at the front, followed by depolarized cells behind. The separation of charge results in a dipole across the heart (the large black arrow shows its direction). Modified from D.E. Mohrman and L.J. Heller (eds.), Cardiovascular Physiology, 5th Ed., 2003. Reproduced with permission of the McGraw-Hill Companies.

Fig. 1. After conduction begins at the sinoatrial (SA) node, cells in the atria begin to depolarize. This creates an electrical wavefront that moves down toward the ventricles, with polarized cells at the front, followed by depolarized cells behind. The separation of charge results in a dipole across the heart (the large black arrow shows its direction). Modified from D.E. Mohrman and L.J. Heller (eds.), Cardiovascular Physiology, 5th Ed., 2003. Reproduced with permission of the McGraw-Hill Companies.

Fig. 2. A typical ECG waveform for one cardiac cycle measured from the lead II position. The P wave denotes atrial depolarization, the QRS indicates ventricular depolarization, and the T wave denotes ventricular repolarization. The events on the waveform occur on a scale of hundreds of milliseconds. Modified from D.E. Mohrman and L.J. Heller, (eds.), Cardiovascular Physiology, 5th Ed., 2003. Reproduced with permission of the McGraw-Hill Companies.

Fig. 2. A typical ECG waveform for one cardiac cycle measured from the lead II position. The P wave denotes atrial depolarization, the QRS indicates ventricular depolarization, and the T wave denotes ventricular repolarization. The events on the waveform occur on a scale of hundreds of milliseconds. Modified from D.E. Mohrman and L.J. Heller, (eds.), Cardiovascular Physiology, 5th Ed., 2003. Reproduced with permission of the McGraw-Hill Companies.

ages detected using electrodes on the surface of the body. When the monitored patient is essentially motionless, it is considered a "resting ECG," the type of ECG signals discussed in the majority of this chapter.

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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