ELECTROCARDIOGRAPHY 



363 



f^ 



O.t 



0.2 



0.3 



O.i sec 



0.1 



0.2 



0.3 



O.i sec 



FIG. 46. The differences in the 

 superposition of R and T of two 

 single fibers. The desynchroniza- 

 tion of the fibers leads in both 

 cases to a complete separation of 

 R, whereas the T deflections are 

 summed to a higher potential in 

 case of short intervals (B), to a 

 smooth cumulative curve in case 

 of long intervals (A). 



An augmentation of the QRS area may be caused 

 by the following events: /) a diminution of cancella- 

 tion l^y a reversal of excitation waves in certain parts 

 of the heart (extrasystoles, blocks); 2) slowing of 

 conduction velocity; 3) augmentation of all active 

 cross sections of the muscular mass, i.e., a real hyper- 

 trophy. 



An increase in the duration of QRS may be caused 

 by /) an increase in heart weight and muscle mass; 

 2) detours in the spread of excitation, with an in- 

 version of direction in some parts of the heart; j) 

 retardation of conduction velocity. 



The direction of the heart vector and the type of 

 QRS are changed by /) a local (not general) hyper- 

 trophy and a changed preponderance pattern of the 

 myocardial fil)ers; 2) changes in the temporal and or 

 spatial distribution of the excitatory processes of the 

 single fibers; j) changes in the anatomical position of 

 the whole heart. 



General Principles Underlying Interpretation 

 of the T Wave 



The rules which govern the interpretation ot T are 

 completely different from those valid for QRS. The 

 main reason is that the repolarization process is of 

 such a long duration that shifts in the synchronization 

 of various fibers have no influence on the form of T, 

 whereas they have great influence on QRS. This 

 may be exemplified as follows. In figure 46, the 

 bipolarly recorded action potential of a very small 

 bundle of muscle fibers is given, which shows the 

 big differences in the time course Ri and Ti of a single 

 fiber. If two fibers' were to depolarize a short time 



' The term "fiber" needs a brief definition: it is, in general, 

 a short part of the semisyncytial structure of the heart, of a 

 length of about 1-5 mm, through which the excitation wave 

 usually runs unbranched, with a thoroughly homogeneous 

 velocity of about i m/sec. At the cross-sectional circumference 

 the fiber is limited by a "membrane" which gives rise to the 

 usual resting and action potential. 



apart, the two Ri waves would be completely sepa- 

 rated, resulting in a double-spiked potential; whereas 

 the two T waves from these fibers would simply sum 

 up to double their individual potentials. The T 

 wave, therefore, does not reflect the degree of de- 

 synchronization of the \'arious fibers. It is the result 

 of a more or less complete superposition of all indi- 

 vidual Ti potentials. Whereas the desynchronization 

 of fibers increases mainly the duration of QRS, it 

 increases mainly the (percentile) amplitude of T. 



It may be of interest to have some more precise 

 data in this respect. Let us a.ssume that the total 

 duration of QRST may be about 0.4 sec (the normal 

 \alue for a heart rate of about 56 min, fig. 65). Then 

 the QRS complex will ha\e a normal duration of 

 about O.I sec. This means that the earliest fiber starts 

 its excitation with its R 0.099 sec earlier than the 

 fiber of highest latency. The mean duration of the 

 individual monophasic action potential therefore is 

 0.4 — 0.1 = 0.3 sec. TheT wave with its main, steep 

 part has a duration of about 0.15, so that the peak of 

 T in the earliest fiijer is 0.05 sec later than the start 

 of T in the latest fiber. Whereas the R spikes are 

 separated h\ a long inter\al, the T waves overlap 

 and form a smooth superposition (fig. 46). In reality, 

 the inter\al between the R's in figure 46 are filled 

 with the innumerable R,"s of the mass of the myo- 

 cardium, but QRS in its form is an image of the 

 desynchronization pattern, and T is not. 



The cancellation of all individual fiber voltages 

 remains true for T as well as for R. This means that 

 the area of T would be reduced in the same manner 

 as the area of QRS, if all fibers would behave homo- 

 geneously. The shape of the T wave thus reveals the 

 pattern of repolarization of the uncancelled fibers. 



If all monophasic action potentials were shorter, 

 as their fillers approached the epicardial surface, a 

 gradient of the repolarization velocity would exist 

 of a quite uniform direction across the heart wall. 

 As we found (234), the potential lasts longer, the 

 nearer to the base it is recorded. Monophasic action 



