# Pre-Lab: the Electrocardiogram

**Topics:**QRS complex, Electrocardiography, Electrophysiology

**Pages:**2 (474 words)

**Published:**September 20, 2011

In order to record an ECG, electrodes(leads) must be placed in appropriate places. (include diagram of proper lead placement). The twelve total leads each use a reference pole (negative) and a recording pole(positive) to give different views of the heart activity. There are three standard limb leads: Lead I with right arm(negative) and left arm(positive), Lead II with right arm(negative) and left leg(positive), and Lead III with left arm(negative) and left leg(positive). (**link from Einthoven Triangle for further elaboration)

The mean electrical axis gives us the direction of the repolarization or depolarization. To determine the mean electrical axis, first two leads or needed from an ECG. The S wave height is subtracted from the Q wave height, which subtracted from the R wave height. This is to be performed for both, or every lead. Resulting values are plotted as horizontal lines on the graph. The zero point is determined by the intersection of all the leads. A line is then drawn through the axis of these points, perpendicular to the axis. Finally, a line is drawn from the zero point to the intersection at which the two...

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