Abstract:
Because the body fluid is good conductors, algebraic sum of the action potentials of myocardial fibers can be recorded externally. The record of this potential fluctuation during cardiac cycle is called electrocardiogram (ECG). Most electrocardiograph machines record these fluctuations on moving strip of paper. The names of these waves and segments are:
1- P wave .
2- PR segment.
3- QRS complex
4- ST segment .
5- T wave .
6- U wave .
ECG leads: The electrocardiograph is a modified galvanometer that measures the difference in action potential between two points.
Bipolar limb leads and cardiac vector:
Because the standard limb leads are records the potential difference between two points, the deflection in each lead indicates magnitude of direction in the axis of the lead of electromotive force generated in the heart (cardiac vector or axis). Cardiac vector can be calculated from standard limb leads, if it is assumed that the three electrodes location from the points of an equilateral triangle (Einthoven s triangle), and that the heart lies in the center of triangle. Mean QRS vector (electrical axis of the heart) is plotted by using the average QRS deflection in each lead. The average QRS deflection should be measured by integrating the QRS complex, measuring the net differences between the positive and negative peak of QRS. Calculate the mean QRS of lead I, II, and III. Draw perpendicular lines from the potential L I, II, III.