The electrical activity that accompanies the cardiac cycle can easily be measured and used diagnostically. The flow of positive electrical charges can be measured and tracked with strategically placed electrodes attached to the surface of the skin. There are at least 12 different lead pairs or positions for measurement on the body's surface: six limb leads; I, II, III, aVR, aVF and aVL, and six chest leads; V1 - V6.
A wave of depolarization advancing toward a positive skin electrode will cause an upward deflection on an ECG strip. The degree of deflection is related to the thickness or length of muscle through which the positive charge travels. If the depolarization wave moves in the opposite direction away from the positive electrode, the ECG deflection will be below the baseline.
ECG's are used to assess a number of items including: rate, rhythm, heart axis, heart muscle hypertrophy and evidence of ischemia or infarction.
Lead I consists of a positive electrode attached to the left arm or shoulder and a negative one on the right arm or shoulder. A wave of depolarization on the heart that advances toward the positive lead causes a positive deflection on the ECG strip. If we recall the position of the heart in the chest, we would expect to see a moderate upward deflected P-wave for atrial depolarization and also for the R-wave for ventricular depolarization.
Lead II has its positive electrode at the left leg or lower left chest and its negative electrode at the right arm or shoulder. This pair is more in line with the long axis of the heart, thus the upward deflections are greater than in Lead I.
Lead III has its positive electrode at the lower left leg or lower left chest and the negative electrode at the upper left arm or shoulder. The positive deflections of the P- and R-waves are minimal. The leads form a triangle with 60ø at each internal angle. If the sides of the triangle were moved inwardly to bisect the location of the AV node the lines would have external angles of 60ø.