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This is done with the use of the height or depth, independent of the wave (QRS, P of T). At the beginning of every lead is a vertical block that shows with what amplitude a 1 mV signal is drawn. So the height and depth of these signals are a measurement for the voltage. If this is not set at 10 mm, there is something wrong with the machine setting. The inside of the cell is negatively charged relative to the outside (resting potential). If the cardiac muscle cells are electrically stimulated, they depolarize (the resting potential changes from negative to positive) and contract. The electrical activity of a single cell can be registered as the action potential. As the electrical impulse spreads through the heart, the electrical field changes continually in size and direction. Through the depolarization by surrounding cells they become positively charged and they contract. The glass electrode measures the electrical current in the heart cell (with the patch-clamp method ). The electrical signal is written in blue and shows the action potential. Courtesy of Arie Verkerk and Antoni van Ginneken, AMC, Amsterdam, The Netherlands. The final result, which is shown on the ECG, is actually the average of billions of microscopic electrical signals. During repolarization the ion concentration returns to its precontraction state. On the ECG, an action potential wave coming toward the electrode is shown as a positive (upwards) signal. The electrical signal between the atria and the ventricles goes from the sinus node via the atria to the AV-node (atrioventricular transition) to the His bundle and subsequently to the right and left bundle branches, which end in a dense network of Purkinje fibers. The depolarization of the heart results in an electrical force which has a direction and magnitude; an electrical vector. In the animation vectors for atrial depolarization, ventricular depolarization and ventricular repolarization are shown. Normal atrial repolarization is not visible on the ECG (but can be visible during atrial infarction and pericarditis ). As the endocardial cardiomyocytes depolarize slightly earlier than the outer layers, a typical QRS pattern occurs (figure). This wave possibly results from afterdepolarizations of the ventricles. The electrocardiogram (ECG) registers these activities from electrodes which have been attached onto different places on the body. However, it is best to be uniform in this. Official recommendations are to place V4 under the breast in women.
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