Decreased Voltage of the Electrocardiogram

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Decreased Voltage Caused by Cardiac Myopathies. One of the most common causes of decreased voltage of the QRS complex is a series of old myocardial artery infarctions with resultant diminished muscle mass. This also causes the depolarization wave to move through the ventricles slowly and prevents major portions of the heart from becoming massively depolarized all at once. Consequently, this condition causes some prolongation of the QRS complex along with the decreased voltage. Figure 12-16 shows a typical low-voltage electrocardiogram with prolongation of the QRS complex, which is common after multiple small infarctions of the heart have caused local delays of impulse conduction and reduced voltages due to loss of muscle mass throughout the ventricles.

Decreased Voltage Caused by Conditions Surrounding the Heart. One of the most important causes of decreased voltage in electrocardiographic leads is fluid in the pericardium. Because extracellular fluid conducts electrical currents with great ease, a large portion of the electricity flowing out of the heart is conducted from one part of the heart to another through the pericar-dial fluid.Thus, this effusion effectively "short-circuits" the electrical potentials generated by the heart, decreasing the electrocardiographic voltages that reach the outside surfaces of the body. Pleural effusion, to a lesser extent, also can "short-circuit" the electricity around the heart, so that the voltages at the

Right axis deviation caused by right bundle branch block. Note also the greatly prolonged QRS complex.

Low-voltage electrocardiogram following local damage throughout the ventricles caused by previous myocardial infarction.

Right axis deviation caused by right bundle branch block. Note also the greatly prolonged QRS complex.

Low-voltage electrocardiogram following local damage throughout the ventricles caused by previous myocardial infarction.

surface of the body and in the electrocardiograms are decreased.

Pulmonary emphysema can decrease the electrocar-diographic potentials, but by a different method from that of pericardial effusion. In pulmonary emphysema, conduction of electrical current through the lungs is depressed considerably because of excessive quantity of air in the lungs. Also, the chest cavity enlarges, and the lungs tend to envelop the heart to a greater extent than normally. Therefore, the lungs act as an insulator to prevent spread of electrical voltage from the heart to the surface of the body, and this results in decreased electrocardiographic potentials in the various leads.

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