A patient was admitted to the hospital, and ECG lead III was recorded. It was found to contain no S wave. The P, R, and T waves appeared normal. What conclusions can you draw?
Correct Answer: There are no indications of cardiac abnormalities
Description: The standard limb leads I, II, and III--each record the differences in potential between two limbs. Because current flows only in the body fluids, the records obtained are those that would be obtained if the electrodes were at the points of attachment of the limbs, no matter where on the limbs the electrodes are placed. In lead I, the electrodes are connected so that an upward deflection is inscribed when the left arm becomes positive relative to the right (left arm positive). In lead II, the electrodes are on the right arm and left leg, with the leg positive; and in lead III, the electrodes are on the left arm and left leg, with the leg positive. The P wave is produced by atrial depolarization, the QRS complex by ventricular depolarization, and the T wave by ventricular repolarization. The U wave is an inconstant finding, believed to be due to slow repolarization of the papillary muscles. The atria are located posteriorly in the chest. The ventricles from the base and anterior surface of the hea, and the right ventricle is anterolateral to the left. Thus, aVR "looks at" the cavities of the ventricles. Atrial depolarization, ventricular depolarization, and ventricular repolarization move away from the exploring electrode, and the P wave, QRS complex, and T wave are therefore all negative (downward) deflections; aVL and aVF look at the ventricles, and the deflections are therefore predominantly positive or biphasic. There is no Q wave in V1 and V2 and the initial poion of the QRS complex is a small upward deflection because ventricular depolarization first moves across the midpoion of the septum from left to right toward the exploring electrode. The wave of excitation then moves down the septum and into the left ventricle away from the electrode, producing a large S wave. Finally, it moves back along the ventricular wall toward the electrode, producing the return to the isoelectric line. Conversely, in the left ventricular leads (V4-V6)there may be an initial small Q wave (left to right septal depolarization), and there is a large R wave (septal and left ventricular depolarization) followed in V4 and V5 by a moderate S wave (late depolarization of the ventricular walls moving back toward the AV junction). There is considerable variation in the position of the normal hea, and the position affects the configuration of the electrocardiographic complexes in the various leads.REF: GANONG&;S REVIEW OF MEDICAL PHYSIOLOGY, KIM BARRETT, HEDDWEN BROOKS, SCOTT BOITANO, SUSAN BARMANTWENTY THIRD EDITIONPAGE NO: 492,494
Category:
Physiology
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