354 



HANDBOOK OF PHYSIOLOOY 



CIRCULATION I 



FIG. 36. Successive positions of 

 depolarizing wa\e front for coronal 

 section of a dog's heart in a normal 

 beat. A multipole needle electrode is 

 inserted into the ventricular wall, with 

 terminals i mm apart. The latencies of 

 bipolar records have been observed. 

 The lines are isochrones of latencies. 

 The black arrows indicate the line 

 connecting the lead II electrodes. 

 [From Scher & Young (416).] 



9. GENERAL PRINCIPLES UNDERLYING INTERPRET.aiTION 

 OF THE ELECTROCARDIOGRAM 



Areas of QRS and T and the I 'entricular Gradient 



In the preceding sections, our emphasis has been 

 on the "activation"' wave, i.e., the depolarization 

 process of the myocardium. A complete theory of 

 the EGG, however, should take into account as well 

 the repolarization and its electrocardiographic 

 counterpart, the T wave. The repolarization phenom- 

 enon is complicated h\ the different manners in 

 which the various parts of the heart behave, whereas 

 depolarization appears to be more homogeneous. 

 This is one of the reasons why the physical laws of 

 electric fields and leads, and the physiology of con- 

 duction, are difficult to correlate with the behavior 

 of T. 



Depolarization and repolarization in the heart 

 can be considered separately because of the form ol 

 the monophasic action potential. Activation accession 

 (i.e., the upstroke of the action potential) is very 

 rapid compared with repolarization, which starts 

 slowly, forms a plateau, and then returns to the base 

 line with the maximal decline of the curve being 

 about I '1000 as fast as the downstroke of the de- 



FiG. 37. Schematic translation of the isochrones of fig. 36 

 into intraventricular pathways of the excitation. The arrows 

 are only an indicator of the average direction in which the 

 wave front of the excitation proceeds. 



polarization process. If we construct the electric 

 field of one single fiber during the whole cardiac 

 cvcle, the same methods can be adopted as were 

 used previously in discussing the dipole during 

 depolarization. 



The dipole moment Mi of a single, cylindric 



