As cardiomyocytes depolarize and propagate action potentials throughout the heart, a charge separation (or dipole) is created. By utilizing the resultant electrical field present in the body, electrodes can be placed around the heart to measure potential differences as the heart depolarizes and repolarizes. This measurement gives rise to the ECG, normally consisting of the P wave for atrial depolarization, the QRS complex for ventricular depolarization, and the T wave for ventricular repolar-ization.
There are up to 12 different standard positions, or leads, available to detect the ECG. Most commonly, a 5-wire system is used clinically and can be used for the 3 bipolar limb leads (which make up Einthoven's triangle), the 3 unipolar limb leads, and 1 precordial (chest) lead at a time. At least two lead traces are needed to calculate the heart's electrical axis, which gives the general direction of the heart's dipole at any given instant. The heart's mean electrical axis is then the average dipole direction during the cardiac cycle (or more commonly, during ventricular depolarization).
The recorded ECG remains one of the most vital monitors of a patient's cardiovascular status and is used today in nearly every clinical setting. Electrocardiography has come a long way since it was first used in the early 1900s. New instruments and analysis techniques are continually being developed. The ECG has also been used in combination with other implantable devices, such as pacemakers and defibrillators. The trend has been toward developing smaller, easier-to-use devices that can gather a wealth of information for use in patient diagnosis and treatment.
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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.