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HANDBOOK OF PHYSIOLOGY 



CIRCULATION I 



since they were first published. After Einthoven's 

 initial successes, many workers started a thorough in- 

 vestigation of electrocardiographic data, beginning 

 with a detailed study of the action currents from iso- 

 lated frog hearts. Burdon-Sanderson, Samojlofl, Borut- 

 tau, and Bayliss were some of the names closely con- 

 nected with the early development of electrocardiog- 

 raphy. Nevertheless, this entire field of research 

 stagnated for nearly 30 years, until the introduction, 

 by Wilson et al. (528), of local leads and the zero elec- 

 trode in unipolar recordings. In the last decade, a 

 rapid development of the theoretical basis of electro- 

 cardiography has occurred, mainly under the influ- 

 ence of physicists, who applied the principles of field 

 theory to the problems of recording. It is the result of 

 much ingenious endeavor by collaborating physicians, 

 physiologists, and physicists that we are now able 

 to operate with mathematically and physically correct 

 concepts. 



The electrocardiogram has a twofold peculiarity: on 

 the one hand it is a highly theoretical topic, the 

 physical complexity of which covers the most difficult 

 problems in medicine; on the other hand, the value of 

 this branch of medical science consists exclusively of 

 its application to clinical medicine. The EC'.G poses 

 no problems, the investigation of which might lead 

 to the advancement of our basic scientific knowledge 

 in physiology or physics. The conditions under which 

 the normal and abnormal EGG originates are most 

 complex in nature, and their elucidation leads only, 

 under optimal conditions, to the statement that the 

 well-known laws of electrical fields and the electro- 

 physiological behavior of single cells, as reviewed in 

 the preceding chapter, are sufficient for the interpreta- 

 tion of the intricacies of electrocardiographic curves. 

 Whatever an electrocardiographer may expect as the 

 result of his scientific endeavor can scarcely be more 

 than a contribution to the questions concerning the 

 translation of an EGG into the language of physiologi- 

 cal events, such as the spread of excitation waves and 

 their respective velocities and pathways, duration of 

 local action potentials, their deviations and local dif- 

 ferences, and production of excitation in pacemakers. 

 From things like these, perhaps, an indirect conclu- 

 sion may be possible concerning heart contraction, its 

 mechanical forces, work performance, and imminence 

 of heart failure or recovery. But whatever an ECG 

 may indicate, it certainly is no indicator at all of the 

 function of the heart, which can only be determined 

 by the analysis of stroke volumes ejected at known 

 pressures (408). We definitely know that the EGG can 

 be nearly normal in mechanically ineff"ective hearts. 



and, conversely, that greatly disturbed EGG curves 

 may be recorded from an individual with mechanically 

 intact circulation. Even in hearts with scarcely per- 

 ceptible mechanical movement, the EGG may re- 

 main nearly normal (433, 469), though a minimal 

 mechanical movement can always be seen as long as 

 electrical events can be recorded (184). So the very 

 purpose of an EGG is to give information about the 

 direction and velocity of the excitation wave, the in- 

 homogeneities of the excitation process in the various 

 parts of the heart, and the focus which generates this 

 traveling excitation wave. All other conclusions drawn 

 from an EGG are indirect. Observations, theories, 

 and derivations which do not lead to information of 

 this kind are useless. 



Interpretation of an electrocardiographic curve is 

 based on three fundamental elements: the theory of 

 derivation, the form of an individual action potential, 

 and the spread of myocardial activation. If these three 

 items were completely known, the EGG of a particular 

 heart would be predictable in every detail. It is our 

 aim in the following sections of this chapter to show 

 how difficult the study of derivation of potentials is. 

 The form of the thorax, the magnitude of the heart and 

 its position and movement during the cardiac cycle, as 

 well as the inhomogeneity of the field in which the 

 heart is embedded, are so complicated that a strict 

 physical solution of the simple problem as to how the 

 electromotive forces of the heart lead to the recorded 

 surface potential seems for the present (and, as we 

 believe, for the future) impossible. In the absence of 

 such a solution, we may use simplified models which 

 permit prediction of the electrical behavior of the 

 heart, at least within certain limits. The action poten- 

 tials produced by the various myocardial fibers inter- 

 fere in producing a common electric field. If the 

 potential production in every fiber were the same in 

 pattern and strength, the problem could be handled 

 in a reasonably simple way by making certain assump- 

 tions. But the action potential patterns are quite differ- 

 ent in the various parts of the heart. Only a marked 

 inhomogeneity in the form of single fiber action po- 

 tentials can explain the form of the T wave. The 

 spreading process, moreover, cannot be described 

 completely without knowledge of the anatomical 

 structure of the muscular walls, and this is imperfectly 

 known. Even if this knowledge were exhaustive, our 

 problem would still be far from solved : we need de- 

 tailed knowledge of the way the excitation wave 

 travels across the heart walls, including directions, 

 propagation times, and resulting local latencies. 



All this is impossible to achieve. So, every inter- 



