ELECTROCARDIOGRAPHY 



355 



KIG. 38. A plot of total ventricular activation. Seventeen 

 electrode tracks are shown in four cross sections of the heart. 

 The time of activity (in milliseconds) before or after the time 

 reference was noted at each terminal of the same electrodes 

 which were used in obtaining fig. 36. All times are referred to 

 the beginning of QR.S in lead II, and points of equal latency 

 are connected with each other. Tissue activated within each 

 5-msec interval is shaded in the same manner in all sections. 

 Above and to the right is the lead II QRS and the shading of 

 time intervals. [From Scher & Young (418).] 



myocardial fiber of a cross section Q and a lengtli 

 sufficiently short is given by the formula : (see equa- 

 tion (2.4)) 



M, 



mi) 



(9O 



where m is the membrane potential at the beginning 

 (mi) and the end (m,) of the respective fiber. The 

 dimension of Mi is mv • cm-. Mi is the momentary 

 value, which of course changes with time. No special 

 assumptions are made in this connection about the 

 form (time course) of the action potential. A good 

 illustration of the magnitude of the development of 

 Mi during time is the difference between the mono- 



FiG. 39. Schematic drawing of the pathways along which 

 the excitation most probably spreads, shown in a cross section 

 perpendicular to the heart axis. IP is an inversion point, at 

 which on the surface the excitation seems to diverge to the left 

 and to the right. 



phasic action potentials at the beginning and the 

 end of the fiber in question (fig. 40). These momentary 

 values are drawn in figure 40 for two distinct in- 

 stances. If one considers the time course of this 

 moment Mi for the whole duration of the e.xcitation 

 process, the time-voltage area of M, equals 



/ M,dt = Q- / (m.;-m,)dt = Q- j 



m.dt - 



m,dt 



(9.2) 



if Q is considered as constant. The value of this 

 expression is equal to the shaded area in figure 40. 

 If /mo dt equals exactly /mj dt, taken over the whole 

 of the excitatory cycle, the resulting integral of M; 

 becomes zero. This is the case when the forms of the 

 monophasic action potentials are completely identical. 

 We define excitations with such an identity of their 

 voltage curves as '"homogeneous" and call a region 

 in which all fibers behave in such an identical manner 

 an area of "homogeneous excitation." In such an 

 area, under the assumption of constancy in time of 

 the cross sections of all fibers, the total shaded area 

 for every fiber (fig. 40) would be alike and of opposite 

 sign. Under the special assumption that the mono- 

 phasic action potential has the form of figure 40 

 (i.e., reveals a plateau of sufficient duration), the 

 time course of Mi shows two separate peaks, of 

 opposite direction, the first of which may be called 

 R, the second one T. Both belong to the single fiber, 

 and may therefore he called local (or individual) 

 Ri and Ti. In a homogeneous region of the heart 

 the time-voltage areas of Ri and Ti are equal and of 

 opposite polarity: Ri = — Ti. (The sign ^-- is generally 

 adopted to characterize the time-voltage areas.) 

 The innumerable fibers of the heart act together 



