ELECTROCARDIOGRAPHY 



401 



ative electrocardiography is of considerable interest. 

 A third reason could be that, out of comparative 

 study, a further elucidation of electrocardiographic 

 theory could possibly be gained, insofar as the close 

 connections between anatomical and electrocardio- 

 graphic data could easily be demonstrated. The litera- 

 ture, however, supports us only with some (and 

 mostly poor) figures concerning the duration of the 

 various intervals and the patterns and magnitudes of 

 the respective deflections. An exhaustive description 

 of the ECG of various animals lies beyond the scope of 

 this review. For veterinary use, some monographs are 

 available (5, 43, 46), and the literature up to 1940 

 has been quoted by Schaefer (57). A collection of 

 ECG tracings of numerous animals, from the arthro- 

 pod to the elephant, is given by Zuckermann (74). The 

 ECG of small laboratory animals (272, 288), and 

 especially of the dog, (2, 266, 268) has been carefully 

 described. 



There are some details, remarkable beyond their 

 descriptive value, which shall be mentioned here 

 briefly. In the first place, the vector directions, es- 

 pecially of QRS, are in most animals very different 

 from those of man. In arthropods (74), as well as in 

 birds (287) and cattle, the main deflections of QRS 

 in what may be called the analogue of the Einthoven 

 leads is negative. This has been interpreted as a 

 preferably caudocranial conduction of the excitation. 

 According to our description of the dog's heart (fig. 

 34), the negativity of QRS means only that the point 

 of distribution (''Quellpunkt") of the excitation is 

 shifted slightly towards the apex of these hearts. The 

 anatomy, as well as the relatively low potential, of 

 QRS points to the fact that the principle of mutual 

 cancellation is valid in these hearts. This shift of tlie 

 distribution point therefore is not necessarily very 

 large. Only a slight shift of this center of distribution 

 toward the apex will cause the average direction to 

 reverse and run toward the base (fig. 34). The reason 

 is, of course, a similar slight difference in the ana- 

 tomical distribution of the specific conducting system, 

 which may even be hard to detect. No detailed inves- 

 tigation, however, has been made on the subject. 



The duration of all phases of the ECG is, of course, 

 strongly dependent on the absolute size of the heart. 

 It is, nevertheless, surprising how small the variations 

 are. The QT duration varies preferably with the heart 

 rate, for obvious reasons, but, in the elephant, QT is 

 clearly longer (0.65 sec) than it would be in man with 

 similar heart rates, where it would be 0.5 sec. The 

 relative bradycardia of big animals allows such pro- 

 longations of QT, but apparently there is an adapta- 



tion of QT to the highest heart rate which may occur 

 in that particular species. The duration of QRS is de- 

 termined mainly by the amount of synchronization in 

 the various compartments of the ventricular wall. 

 This synchronization is the more perfect the bigger 

 the heart; but the general laws governing distribution 

 of excitation are completely identical in small and 

 excessively big hearts. If we extrapolate in table 2 the 

 duration of QRS, assuming that it depends on the 

 sixth root of the heart weight, we get for the elephant 

 (the heart of which weighs approximately 20 kg) a 

 QR.S duration of about 0.15 sec, which is nearly the 

 observed value of 0.16 sec. 



Time relations vary, of course, with body tempera- 

 ture. In poikilothermic animals, figures that do not 

 account for blood temperature are therefore useless. 

 In homoiothermic animals, the pigeon has a QRS 

 duration of 0.04 sec, the seagull 0.02 to 0.026 sec, 

 a colibri, in spite of its light weight, 0.03 sec (74). 

 Some data on domestic animals are listed in table 10. 

 The range of QRS durations and voltages is astonish- 

 ingly small. 



There are some evaluations of the vectorial data, 

 especially in the most common laboratory animals, 

 which cannot be referred to here (see 5, 266, 327). 

 The much bigger deviations in the field contour from 

 ideal surfaces, compared with man, condemn all vec- 

 torial data in animals to be rather unreliable. The 

 question of the ventricular gradient is likewise fairly 

 unknown in animals. As far as the general pattern of 

 QRS and T is concerned, nearly all species seem to 

 have at least a gradient different from zero, but in 

 many cases (as in the dog) considerably less than in 

 man. Some animals, preferably birds, seem to have 

 predominantly discordant T waves (74, 287) whereas 

 in larger animals like horse and cattle, T is at least 

 not strictly discordant. A relation between the size of 

 the heart and its ventricular gradient has never been 

 established and most probably does not exist: the 

 small heart of a frog has a very big ventricular gradi- 

 ent, the big heart of a dog has a small one. In some 

 animals (e.g., the boa constrictor (74)), ECG's with 

 local leads show a very large discordant T, the ven- 

 tricular gradient of which must be comparatively 

 large in spite of a strict discordance of the T wave. 



Many species show a very sensitive pacemaker, 

 leading to marked disturbances of rhythmicity under 

 the influence of external stimuli. In Ijirds, the heart 

 rate may change instantaneously l^etween very high 

 values and near standstill (287). The dog shows an 



