Page 3, Electrocardiography, Dr. D. Penney

The Electrocardiogram (ECG)

Due to changes in electric field, the electrical activity of the heart (an other organs) can be sensed at the body surface by using appropriate apparatus. At the turn of the century, Einthoven made measurements of the ECG pattern produced by the contracting heart by using specific lead placement. This was called the Einthoven Triangle (Figure 3). This was based upon three assumptions, all of which were later shown to be wrong or to be gross over-simplifications (Table 7).

Movement of charge can be characterized in terms of a number of dipoles, each dipole having a positive head and a negative tail (Figure 4). When a dipole is approaching an exploratory electrode, (+) charge is encountered and a positive deflection results(Table 8). When a dipole is moving away from an electrode, (-) charge is encountered and a negative deflection results. An electrode placed perpendicular to a dipole records no deflection. Knowing this, electrode placement in the various ECG leads, and the way in which the heart depolarizes will allow you to understand the patterns produced.



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