Pre-Lab: the Electrocardiogram

An ECG (electrocardiogram) is a tool used to evaluate the heart’s electrical activity which measures the current produced in the extracellular fluid. Action potentials that occur in the myocardial cells can be detected by electrodes or leads placed at key points on the skin’s surface (reference text book pg. 371) **add figure of a typical EKG trace** The ECG records 5 different waves that correspond with myocardial activity. The first wave is the P wave which occurs in response to atrial depolarization. Next the QRS complex occurs as a result of ventricular depolarization and may or may not include Q and S portions of the wave. Finally, the T wave is in response to ventricular repolarization. Atrial repolarization is not typically seen on the ECG because it happens around the same time as the QRS complex(reference text book). 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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