ELECTROCARDIOGRAPHY 



359 



area of a normal heart, with minimal cancellation 

 effect in tracings of extrasystolic beats with the 

 largest QRS areas. It has been shown (416) that in 

 ventricular extrasystoles the intramural propagation 

 runs more or less in one single direction, if the stimulus 

 lines near the apex or the base and unipolar observa- 

 tions point in the same direction (468) : the polarity 

 in the septal region is reversed if the stimulus activates 

 the base, and the septal wa\"e is conducted from the 

 base to the apex. Under optimal conditions, the 

 time-voltage area in the frontal projection may 

 increase to 300 ^ivsec (microvolt-seconds) and is up 

 to 25 times higher in extrasystolic than in normal 

 beats, with healthy hearts (410). This means that 

 the total time-\oltage production of the heart can 

 be reduced to an amount of i :25 or 4 per cent, or 

 even less, of the inaximal possible amount. "Or even 

 less" means that even in an optimally situated 

 ectopic focus the excitation wave does not travel in 

 completely parallel fiber bundles, so that a certain 

 amount of cancellation still remains. The \alue of 

 4 per cent is borderline; the average ratio of ectopic 

 to normal beats is 10:1, if only large ventricular 

 extrasystoles are selected. The mechanisms which 

 govern the extrasystolic ECG are drawn schematically 

 in figure 42. 



Influence of Conduclion Velocity 

 Upon the Q_RS Complex 



It is ob\ious that in the case of a slowing of the 

 conduction velocity, the depolarization dipole would 

 remain longer on the myocardial fiber, so that the 

 time-voltage area would be enlarged inversely 

 proportional to the \elocity. If therefore a QR.S 

 complex shows an augmented area, it is unknown 

 whether this is due to diminution of cancellation or 

 to slowing of conduction. Only from indirect signs 

 is discrimination possible. Every change in cancel- 

 lation necessarily means that parts of the heart 

 muscle conduct their excitation in a direction opposite 

 to the normal. This is the case, for example, in the 

 true bundle branch block. It has often been shown 

 that after an experimental block the excitation wave 

 is inverted in certain areas (486). A block, however, 

 inevitably changes the position of the resultant 



vector QRSs, because it is impossible that in a 

 certain set of dipole components a part of these 

 components would change its direction withotit a 

 change in tlie direction of the resultant dipole. The 

 slowing of the excitation wave, however, if homo- 



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tSOmi^ 

 Chinidin 



FIG. 43. Influence of quinidine on the conduction velocity 

 and the bipolar action potential of single fibers. Action po- 

 tentials at four diflferent instances are given in the left part of 

 the figure. The broadening of the potentials (duration t) and 

 the increase in latency (measured according fig. 33) indicate a 

 slowed conduction velocity v. The length 1 of the excitation 

 wave (fig. 33) remains nearly constant. Strophanthine reduces 

 these effects instantaneously to normal. [From Trautwein 

 (485)-] 



geneous all over the heart, would act in the same 

 manner as if the speed of the recording instrument 



had been increased. QRS would be enlarged, but 

 the \'ector direction unchanged. A simple test of 

 whether the vector is changed or not may be made 

 by a calculation of the type of QRS (see section 9). 

 The whole consideration, of course, must assume 

 that the area of the action potential, recorded with 

 microelectrodes at a single point of the fiber, remains 

 unchanged, perhaps not a \alid assumption in all 

 cases. 



The propagation \elocity has always been dis- 

 regarded in textbooks on the ECG because we know 

 little about processes causing the slowing of con- 

 duction. Nevertheless, it can be found experimentally 

 that drugs like quinine act in this way (166, 485). 

 As figure 43 indicates, the propagation velocity 

 decreases from 0.6 to about o.i m per sec after 

 injection of 50 mg quinidine. At the same time, the 

 action potentials recorded with close bipolar elec- 



