ELECTROCARDIOGRAMS 277 



The second phase of the ventricular complex the T wave is ascribed 

 to the disturbance of electrical balance which again ensues upon the 

 retreat of the excitation wave. The retreat of the wave would be 

 expected to occur in the same order as the invasion, that is, the region 

 first excited during the advance of the wave would be the first to pass 

 into the unexcited state during the retreat and the region at the base of 

 each ventricle would remain excited longer than the apex. Such an order 

 would cause a rotation of the electrical axes in the reverse direction to 

 that described as occurring during the invasion of the wave and would 

 be expected to produce a second phase in the ventricular complex iden- 

 tical with the first but with its deflections inverted. The T wave, how- 

 ever, is a single prolonged upward deflection which indicates that the 

 retreat of the excitation wave is much slower than the invasion. The 

 slower retreat would tend to cause the deflections to fuse with one an- 

 other, and to flatten out the curve. Again the excited state probably does 

 not retire from all portions of the ventricular muscle at the same rate 

 but dies away more rapidly from apical regions than from regions nearer 

 the base. In the latter region, consequently, negativity would persist 

 for a relatively longer period and the base to apex direction of the elec- 

 trical axis would exert a greater influence upon the electrocardiogram. 

 A slower and less uniform dying away of the excitation process during 

 its retreat would in this way account for the differences in contour of 

 the records inscribed, during these two periods. That the T wave in the 

 human electrocardiogram is probably dependent upon the longer con- 

 tinuance of the excited state at the base than at the apex and that dif- 

 ferent rates of retirement from the two regions affect this wave has 

 been demonstrated by Mines. When the electrical changes at the apex of 

 a frog's heart, which previously had shown no T wave, were hastened 

 by the local application of heat, a typical T wave appeared. The local 

 application of cold to the region of the base had the same effect. It has 

 also been shown by others that vagal stimulation or the exhibition of 

 drugs (e.g., digitalis) may affect the form and direction of the T wave. 

 These experiments indicate that the condition of particular portions of 

 the cardiac musculature may hasten or retard its return to the unexcited 

 state and may be a factor in the production of abnormal T waves ap- 

 pearing in clinical curves. 



