170 Mechanical and Electrical Character of the Heartbeat /9 : 5 



TABLE I 

 Normal Human Electrocardiogram Patterns 



In ekg terminology, the potential differences in the three leads are 

 numbered as 



Lead I : V 1 = V L - V R 



(7) 



where L, R, and F refer to the left arm, right arm, and foot, respectively, 

 and the potentials with the three subscripts refer to the values between 

 these points and a neutral electrode. Elementary algebra reduces 

 these three equations to 



V n = V m + V 1 (8) 



that is, if any two of the three "standard" leads are measured the third 

 is thereby determined. This seems trivial, and probably did also to 

 Einthoven, who first pointed it out, but physiologists have dignified 

 Equation 8 by the name "Einthoven's law." 



In the following sections of this chapter, the heart is approximated by 

 an equivalent dipole. This equivalent dipole is constant for the QRS- 

 complex and is similar for the P- and T-waves. On the cellular level, 

 the heart cannot be regarded as a mere dipole. It was noted in the last 

 section that at the start of every heartbeat, an electrical spike potential 

 originates at the s-a node and spreads out in all directions over the 

 auricle. Thereafter, the a-v node emits a pulse which travels as a spike 

 potential down the Purkinje fibers of the auriculoventricular bundle of 

 His to initiate a contraction of the muscle fibers of the septum between 

 the two ventricles. The spike potential travels down around the septum 

 and then up the outer sides of the ventricles. In every region, the 

 appearance of the spike potential is followed by a contraction. The 



