ECG Abnormalities Part 1 PDF

Cardiovascular physiology Assist prof Dr-Zainab Falah Hassan LEC 5 ECG abnormalities part I Objectives : Some condition that cause abnormal ECG -Bundle branch block -decrease voltage of ECG -Current injury - Some T wave abnormality Some Conditions That C...

Cardiovascular physiology Assist prof Dr-Zainab Falah Hassan LEC 5 ECG abnormalities part I Objectives : Some condition that cause abnormal ECG -Bundle branch block -decrease voltage of ECG -Current injury - Some T wave abnormality Some Conditions That Cause abnormal ECG 1- Bundle branch block Usually Purkinje system transmit the cardiac impulse to the two ventricular walls at almost the same instant. As a result, the potentials generated by the two ventricles (on the two opposite sides of the heart) almost neutralize each other. But if only one of the major bundle branches is blocked, the cardiac impulse spreads through the normal ventricle long before it spreads through the other. A-Left Axis Deviation in Left Bundle Branch Block. When the left bundle branch is blocked, cardiac depolarization spreads through the right ventricle two to three times as rapidly as through the left ventricle. Consequently, much of the left ventricle remains polarized for as long as 0.1 second after the right ventricle has become totally depolarized. this is cause left axis deviation and the duration of the QRS complex is greatly prolonged because of extreme slowness of depolarization in the affected side of the heart. Figure1 1 Cardiovascular physiology Assist prof Dr-Zainab Falah Hassan Figure 1: Left axis deviation caused by left bundle branch block. Note also the greatly prolonged QRS complex. b-Right Axis Deviation in RightBundle BranchBlock. When the right bundle branch is blocked, the left ventricle depolarizes far more rapidly than the right ventricle, so that the left side of the ventricles becomes electronegative as long as 0.1 second before the right. This lead to right axis deviation and a prolonged QRS complex because of slow conduction.Figure2 Figure 2.Right axis deviation caused by right bundle branch block. Note 2 Cardiovascular physiology Assist prof Dr-Zainab Falah Hassan also the greatly prolonged QRS complex. 2- Decreased Voltage of the Electrocardiogram a- 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. This condition causes some prolongation of the QRS complex along with the decreased voltage. As show in figure 3 Figure 3 Low-voltage electrocardiogram following local damage throughout the ventricles caused by previous myocardial infarction. b- 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 -Pleural effusion. -Pulmonary emphysema 3- Current injury (Myocardial ischemia ).The J Point—The Zero Reference Potential for Analyzing Current of Injury It is the exact point at which the wave of depolarization just completes its passage through the heart, which occurs at the end of the QRS complex. For analysis of the electrical axis of the injury potential caused by a current of injury, a horizontal line is drawn in the electrocardiogram for 3 Cardiovascular physiology Assist prof Dr-Zainab Falah Hassan each lead at the level of the J point. This horizontal line is then the zero potential level in the electrocardiogram from which all potentials caused by currents of injury must be measured.Figure4 Figure 4. Point as the zero reference potential of the electrocardiograms for leads I and II 4- Some T wave abnormalities a- Effect of Slow Conduction of the Depolarization Wave on the Characteristics of the T Wave when conduction of the depolarization impulse through the ventricles is greatly delayed , the T wave is almost always of opposite polarity to that of the QRS complex figure 1 b- T wave inversion in myocardial ischemia figure 5 Figure 5. T wave inversion in myocardial ischemia 4 Cardiovascular physiology Assist prof Dr-Zainab Falah Hassan c- Effect of Digitalis on the T Wave. Digitalis is a drug that can be used during coronary insufficiency to increase the strength of cardiac muscle contraction. But when over dosages of digitalis are given, depolarization duration in one part of the ventricles may be increased out of proportion to that of other parts. As a result, nonspecific changes, such as T wave inversion or biphasic T waves, may occur in one or more of the electrocardiographic leads.Figure6 Figure 6. Biphasic T wave caused by digitalis toxicity 5