The thorax can be considered to be an equilateral triangle, with the two shoulders and pubis representing the apices and the heart in its center. When electrodes are attached to the wrists and ankles, the limbs act as extensions of the wires such that the voltage at the wrist (or ankle) is identical to that at the shoulder (or pubis). The right leg is always attached to an electrical "ground" to reduce electrical interference. The electrocardiograph is normally calibrated to deflect the pen 1 cm vertically for each millivolt difference between the electrodes. The paper moves at 25 mm/sec and a 1-mm grid is printed on the paper. Thus, each small box is 0.1 mV high and 0.04 sec wide. The electrocardiograph can record the voltage differences between any two electrodes with the aid of an electrical switch. When lead I is selected, voltages are being measured between the left and right arms, as shown in Fig. 3. Lead II is between the left foot and the right arm and lead III is between the left foot and the left arm. Notice that each of these leads is oriented at 60° to the other two leads. The other three leads in the figure, aVL, aVR, and aVF are explained later in the chapter as are the chest leads, V1-V6.
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