400 



HANDBOOK OF PHYSIOLOGY 



CIRCULATION I 



anterior 



posterior 



FIG. 70. Direction of the vectors of R in the frontal projection 

 of ventricular extrasystoles (circles: monkey; dots: man). Each 

 arrow indicates one experiment in which an extrasystole has 

 been elicited by an external stimulus on the epicardial surface, 

 at the site of the dots or circles. [Experiments with monkeys 

 calculated from records of Storm, cited from Lepeschkin (46). 

 Experiments with men from Barker, Macleod, and Alexander, 

 cited from Schaefer (58).]. 



The QRST complex of a v-entricular extrasystole is 

 very different from the normal, mainly for two rea- 

 sons : as mentioned before, the mutual cancellation of 

 the fibers is greatly diminished (fig. 42) and the QRS 

 area, therefore, highly augmented. T is altered pas- 

 sively by this change of QRS, and actively, also (484), 

 and is discordant with QRS. The excitation wave 

 runs, at least in the very beginning of its pathway, 

 along the ordinary myocardial fibers (209). The 

 spread is much slower than in normal beats and thus 

 the QRS duration is increased. Only after a while 

 does the excitation enter the Purkinje system, thus 

 increasing in velocity but taking big detours (81). The 

 form of QRS depends entirely on the site of the ec- 

 topic stimulus. To a certain degree, this site can be 

 determined by vectorial analysis of QRS. We should 

 never forget, however, that most details in this respect 

 have been investigated in animals (236, 399, 419). 

 Human extrasystoles have been widely interpreted in 

 the light of certain vectorial or other theories (516). 

 In the frontal plane, and for extrasystoles elicited at 

 the epicardial surface, the vectorial data of the QRS 

 area are given in figure 70, which contains experi- 

 ments with monkeys and observations in man (58, 

 p. 382). The direction of the QRS vectors is contrary, 

 in some respects, to that of the excitation wave travel- 

 ing over the heart's surface (fig. 34) : the vectors point 

 in the direction from whence the excitation starts in 

 normal beats (58). 



The activation of the ventricular wall is completely 

 changed in ventricular extrasystoles, as has been de- 

 scribed in Chapter 12 (416). The theory of the extra- 

 systolic QRS seems to be clear at least in principle, if 



one accepts a vectorial theory based on the events in 

 the single cardiac fiber (see sections 7 and 9). The 

 spread of excitation in extrasystoles resembles that in 

 experimental bundle branch block (136). Here, as 

 well as in extrasystoles, the excitation must take a 

 detour, which increases the conduction latencies for 

 two reasons : the distance is increased and the velocity 

 decreased, because much of the spread takes place in 

 the ordinary, slowly conducting myocardium (loi). 

 There are, by the way, hitherto unsolved questions 

 because in some experiments, after cutting the 

 branches of the specific system, QRS is broadened; 

 whereas in other experiments only a very small in- 

 crease of QRS duration is seen (58). The incomplete- 

 ness of the block may be the main reason, as some 

 experiments (465) seem to indicate. In some cases, 

 the pattern of a so-called bundle branch block may be 

 in reality nothing but an over-all diminished conduc- 

 tion velocity (58). 



The short pre-extrasystolic interval as well as the 

 stress of a longer period of frequent extrasystolic beats 

 involve problems of refractoriness and "fatigue," 

 which give rise to various ECG patterns (47). So the 

 repolarization of a premature beat has, in many cases, 

 a different ventricular gradient (105, 410, 484), due 

 to phenomena of refractoriness augmenting the in- 

 homogeneities of repolarization. After a series of 

 extrasystoles, these T changes are even more pro- 

 nounced and, after a long-lasting paroxysmal tachy- 

 cardia, a coronary pattern of ST and T may occur 

 (217). Obviously, the extracellular ionic concentra- 

 tions here play an important role. One should, how- 

 ever, be careful with the term "fatigue,"' because the 

 reduction of heart rate to normal restores the T wave 

 nearl\- immediately. Every shortening of the R-R 

 interval may bring a slightly abnormal heart to a 

 so-called reversible bundle branch block, which most 

 probably is nothing but evidence that the pathway of 

 excitation is temporarily blocked by refractoriness. It 

 is even uncertain whether such "blocks" are only due 

 to an over-all diminuition of conduction velocity. 

 Small local changes of this velocity or refractoriness 

 of small areas lead to considerable changes of QRS, 

 due to alteration of the cancellation effect. 



17. COMPARATIVE ELECTROCARDIOGRAPHY 



The veterinary use of the ECG of domestic animals 

 and the fact that a thorough experimental investiga- 

 tion of electrocardiography is only possible in experi- 

 ments with animals are the main reasons why compar- 



