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Lecture III - ECG, The Electrical Axis of the Heart, The Cardiac Cycle.docx

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Biological Sciences
Stephen Reid

Thiruvarangan The Electrical Axis of the Heart At any one instant the ventricles begin to depolarize and continue to conduct, there is a primary direction in which the electrical activity is flow at that very instant. The mean electrical axis is the average of all the instantaneous mean electrical vectors occurring sequentially during depolarization of the ventricles. 1. Conduction down the branch  interventricular septum depolarizes from left to right (Q wave; negative; away from the positive lead II electrode). 2. 20 msec later: Depolarisation toward the apex (vector 2) 3. 20 msec later: Depolarisation toward the left arm (vector 3) (predominantly right to left movement) 4. S Wave (vector 4) (towards the left) Figure 1: The sum of electrical activity during the QRS complex identifies that the major direction of flow of electrical activity is about Diagnostic Use of the Heart’s Electrical Axis 60 degrees  Deviation to the Right (above 120 degrees) o Increased Right Ventricular Mass (Ventricular hypertrophy) due to something preventing the right ventricle to pump blood through the pulmonary circuit. o Chronic obstructive lung disease – hinders breathing out (expiration) and leads to inefficient gas exchange (leads to heart to compensate by increasing force of contraction). o Pulmonary embolism – blockage of blood flow in the lungs which leads the heart to force more blood through the lungs (forcing heart to grow) o Congenital heart defects o Severe pulmonary hypertension – high blood pressure in pulmonary circuit and so the right ventricle will compensate by growing so that it can attempt to more forcefully contract  Deviation to the Left (less than 0 degrees) o Increased left ventricular mass (left ventricular hypertrophy) o Hypertension - high blood pressure in the systemic circulation o Aortic stenosis – where the aortic valves doesn’t open properly (i.e. smaller opening in which blood can flow through) o Ischemic heart disease Thiruvarangan Calculating the Mean Electrical Axis 1. Look at the lead I ECG. Calculate the isoelectric line to R distance. This equals “a” 2. Look at the lead I ECG. Calculate the isoelectric line to S distance. This equals “b” 3. Add “a” plus “b”. Note that “b” is a negative value. 4. Do the same for the lead II and II ECG traces to find “c”, “d”, “e” and “f”. 5. Calculate “c + d” and “e + f” 6. Draw an equilateral triangle. 7. Starting at the centre of each line (which represent leads I, II and II) measure the distance represented by “a + b”, “c + d” and “e + f” (right is positive). 8. Draw a perpendicular line from the end of these vectors into the middle of the triangle. 9. Determine the centre of the triangle. 10. Draw a line from the centre of the triangle to the point at which the perpendicular lines (from the end of a + b, c + d and e + f meet). 11. The line from the centre of the triangle the meeting point of
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