ELECTROCARDIOGRAPHY 



385 



diminishes nearly proportionally to the changes in 

 duration of contraction with increasing heart rates 

 (300, 489). The time-voltage area of the action 

 potential varies directly with the tension developed, 

 in anoxia and pyruvate poisoning (509). Adrenaline, 

 the most powerful activator of contraction, increases 

 the action potential area (508) and lengthens the 

 plateau (Trautwein, personal communication), but 

 both only to a rather small degree and primarily in 

 damaged fibers. Nevertheless, it is surprising to what 

 degree mechanical and electrical changes may vary 

 together. A further e.xample of this may be seen in 

 the effects of acetylcholine, which depresses both the 

 action potential and contraction in nearly pro- 

 portional amounts (433). 



In general, these correlations are overruled by 

 the complete separability of action potential and 

 contraction. Fleckenstein (20) argued that the 

 mechanical state changes in close relationship with 

 the membrane potential of the fiber. Such cor- 

 relations cannot be found under all conditions. Neither 

 is the amplitude of the action potential proportional 

 to the strength of contraction (433), nor does a 

 thorough correlation exist between the duration of 

 electrical and mechanical events. A normal ECG 

 may be found in hearts with almost no mechanical 

 contraction although the reverse is never found ! 

 Under the influence of low temperatures, a complete 

 dissociation between potential and contraction occurs : 

 the plateavi can be fixed for a long time in total 

 depolarization, while the heart relaxes (426). On 

 the other hand, using an externally induced depo- 

 larization, the membrane can be depolarized to- 

 gether with a synchronous contraction lasting for 

 about 2 sec (286). There may be, but there need not 

 be, a strict electromechanical correspondence. This 

 may be seen under various concentrations of Ca; 

 an increase in calcium increases the mechanical 

 contraction stronglv with an unchanged action 

 potential (328). These few representative data 

 indicate that in spite of an unquestionable relation- 

 ship between action (membrane) potential and 

 contraction, secondary events may interfere with an 

 electromechanical coupling. This is the reason why 

 correlation between electrical and mechanical events 

 is so extremely complicated. 



14. VARIABILITY OF THE ECG 



The wide range of "normal" values of all electro- 

 cardiographic data is obvious in the figures of tables 



3 to 9 (see section 10). This variability may be 

 understood easily by reconsidering the generation of 

 electrocardiographic potentials. The strongly di- 

 verging excitation waves cancel their respective 

 fields to such a degree that even small individual 

 differences or small changes in the spread of excitation 

 lead to large differences in the potential pattern. 

 The relative constancy of QRS during life indicates 

 that only slight changes occur in the spread of excita- 

 tion, because QRS is strongly bound to structural 

 peculiarities of the heart. Only the T wave shows a 

 high variability with nearly all variants: during 

 daytime, food intake, bodily work, gas exchange, 

 emotional state, etc. The variability of T can always 

 be related to metabolic or chemical changes, and 

 hormonal influences. The independence of changes 

 in QRS and T, statistically proven (454), is therefore 

 not surprising. The number of observations is so 

 great that a quotation of special papers is impossible 

 in this review. Only some recently published literature 

 can be listed. The textbook by Lepeschkin will be a 

 perfect guide to the originals. 



The Individual Properties of the ECG 



Individual variability may be regarded as to the 

 interindividual and the intraindividual variations of 

 ECG. The same person shows changes in the mag- 

 nitude and direction of the various vectors with time 

 which are at least of the same order of magnitude as 

 those revealed by interindividual comparisons (454). 

 After an interval of i week or i year, the variability 

 is similar and amounts, in the horizontal plane, to 

 1 9° for I week interval, for the spatial QRS in con- 

 ventional leads. The same angle varies interindi- 

 vidually ijy only 22°. It may be argued that the 

 variability of the electrode positions is one of the 

 main causes. In the whole, the ECG remains fairly 

 constant, concerning angles and general pattern, in 

 the same person over many years (170). Naturally, 

 the influence of the heart rate on the ECG should 

 be taken into consideration in all such comparisons. 

 A factor of heredity in the determination of the ECG 

 has often been claimed, on the basis of investigations 

 on unioN'ular twins, but the restilts seem at best un- 

 certain (115, 532). A somewhat curious individual 

 factor of the ECG has been found in some colored 

 people, who show persistent or transient T wave 

 in\ersions in precordial leads, as an apparently 

 "normal" variant. An exaggerated vagotonia may 

 be the cause (507). [For literature, see (376).] 



The diurnal variations are mainly due to the shift 



