ELECTROCARDIOGRAPHY 



357 



To understand the physiology of the ventricular 

 gradient, we should assume for a moment a lead 

 system which can be used to derive the total heart 

 vector or one of its components in a correct manner. 

 Such a system could be a lead field with parallel 

 flow lines. From such a system recording in the 

 three axes of space, every derivation forms a com- 

 ponent of the heart vector, and in general we observe 

 that the time-voltage area of the component has a 

 definite value different from zero. This value may be 

 regarded as a component of a spatially oriented 

 vector. This vector is the spatial ventricular gradient 

 (Gs).'^ The gradient projects it.self on each lead 

 vector under the same conditions that apply to the 

 heart vector. The projections are simple, if the lead 

 possesses a parallel lead field. Where the heart 

 vector does not project itself in such a simple manner 

 in local leads, a projection of the ventricular gradient 

 is not possible. 



The ventricular gradient concept was propounded 

 for the first time by Wilson, and elaborated in detail 

 by Ashman et al. {84-89) in several papers. The 

 numerical value of the gradient consists of two parts, 

 one of which corresponds to the events of depolariza- 

 tion and is measured by the area of QRS in the 

 derivations mentioned. The second part is the area 

 of T. Recognition of the parts is possible only if and 

 because depolarization is separated from repolariza- 

 tion in the ECG by a time interval (ST), during 

 which the whole myocardial fiber and approximately 

 the whole heart are nearly completely and con- 

 stantly depolarized, indicated by the "plateau" of 

 the monophasic action potential. We therefore ha\'e 



two vectors, Q.RS and T, formed by the components 

 of the areas of QRS and T in the different projec- 

 tions, the vectorial resultant of which is the ven- 

 tricular gradient; Gs = QRS + T. Figure 41 demon- 

 strates this vectorial relationship. 



Provided that conditions i to 4 are realized, 



G*, = QRS + T = o, and therefore T = - QRS, 



or, as in our figure, the vector T is opposite to and of 



equal length with the vector QRS. If in general, 



G. ± o, we can define a value Te = — QRS, which 

 is the virtual resultant of all repolarization processes 

 and assumes these proces.ses to be homogeneous all 

 over the heart, and the form and the orientation of 



' Mathematically: if we put M = (Mx, M,-, M^), it is M = 

 (J M,dt, ; Mydt, / M,dt) = (M„ Mv, M.) = G,. 



QRS 



FIG. 41. The analysis of the vectorial area T, which is the 

 vectorial sum of all time-voltage areas Q-T of all single fibers, 

 with their inhomogeneous repolarization, G, and of the fictive 

 T-areas produ ced b y the homogeneous repolarization process, 



Te. Te equals QRS, because in case of homogeneous beha\ior; 

 the area of T is equal and of opposite polarity to the area of 



QRS ("elementary T"). The real T is the resultant of Te and 



G. Since Te is equal but opposite to QRS, the same vectorial 

 relations may be expressed thus: the gradient G is the vectorial 



sum of T and QRS. 



the fibers to be constant. We will call this virtual 



vector the "elementary T".' This elementary T may 

 be regarded as that component which, together with 



the ventricular gradient, gives the true T. The true 

 T area in any projection, therefore, is the result of 

 two components, one of which depends totally upon 

 the area of QRS. If QRS changes, e.g., if its area gets 

 larger, T undergoes a corresponding change which 

 has no specific significance regarding any change of 

 the repolarization process if G remains constant. For 

 further detail see also section 1 1 . 



The formal reasons for the existence of a \'entricular 

 gradient have already been listed above. Among these 

 four possible reasons, the lack of linearity most 

 probably does not play any role. The relative im- 

 portance of the other three is, however, very difficult 

 to decide. As we calculated (233), the pure geo- 

 metrical factors of the beating heart, i.e., the changes 

 in the form or relative position of the different myo- 

 cardial fibers certainly contriijutes in some degree 

 to the ventricular gradient; the real value of the 

 gradient however cannot be explained liy these 

 factors alone. The onl\- adequate explanation for 

 the liigh \alue of the gradient is inhomogeneit\- in 



' This term "elementary" means that the corresponding 

 part of T stems from the repolarization of the individual fiber 

 elements of the heart, irrespective of their differences in re- 

 polarization velocity. 



