ELECTROCARDIOGRAPHY 



387 



into the cell. As the metabolism acts in the same way 

 (perhaps by producing more intracellular hydrogen 

 ions which in turn exchange with external potassium), 

 a sequence of events is possible which may explain, 

 at least to a certain extent, the multifold actions on 

 the ECG. The picture given here is neither complete 

 nor fully understood as far as the mechanisms acting 

 are concerned (for detail see 162, 488, and the pre- 

 ceding chapter). In particular, the action of potassium 

 brought into the heart from outside is much less 

 marked than would be expected if the changes in re- 

 polarization were brought about by an accumulation 

 of potassium which left the cell during the systole 

 (5'2). 



Bodily J fork 



One of the ijest known influences on the ECG is 

 that of bodily work (40, 59, 311, 381, 421, 455)- Its 

 most characteristic consequences in normal hearts are 

 a shortening of the QRS complex and of the A-V- 

 conduction time, beyond the effect of the rising fre- 

 quency. QRS always shows a decrease of at least 2 

 msec after heavy work. The relative QT duration 

 (corrected for heart rate) is lengthened at first and 

 shortened in the second phase of exercise, an effect to 

 be observed to a maximal degree in athletes. The 

 amount of such changes during and after exercise of 

 course depends mainly on the amount of work per- 

 formed. A widely accepted test is Master's two-step 

 method. With such a standardized exercise, in a 

 group of healthy persons at the age of 50 to 60, the 

 maximal momentary spatial QRS and T vector 

 changes show a posterior rotation of an azimuthal 

 angle of —6° for QRS, —2.4° for T (average values). 

 In the frontal projection, the maximal QRS vector is 

 shifted to the right, the Einthoven angle showing an 

 increase of 2.4° in QRS, 8.8° in T (455). In normal 

 young persons, ST is never depressed (333), though 

 in older persons a slight ST depression seems to be 

 the rule. T is depressed nearly always in moderate 

 exercise. During heavy (anaerobic) work (running on 

 a treadmill), the maximal T vector increases re- 

 markably (to 165% of its initial value) and rotates 

 forward (20° immediately after work) and to the 

 right (280). The difference in the behavior of T after 

 moderate and heavy bodily work is obvious. In the 

 latter, apparently strong inhomogeneities occur, 

 shifting the vector direction and indicating that the 

 ventricular gradient must rotate anteriorly and in- 

 feriorly, i.e., in a direction which is mainly identical 

 with its normal position. The conclusion may be 



drawn, therefore, that the normal inhomogeneities 

 are merely exaggerated by the exercise. 



These effects most probably can be interpreted on 

 the basis of changes in the autonomic tonic innerva- 

 tion on the one hand, or the oxygen supplied on the 

 other. The strong increase in the sympathetic tone 

 during work, indicated by the augmentation of fre- 

 quency, leads to a decrease in all time factors (dura- 

 tion of QRS, QT, A-V-conduction time). Especially 

 in well-trained persons, the diminution of the very 

 strong vagal tone of the heart leads to exaggerated 

 effects. The oxygen lack adds a second effect, increas- 

 ing, for example, the antagonism between inner and 

 outer layers of the ventricular wall insofar as the 

 augmented oxygen requirement increases the normal 

 differences in oxygen supply between these layers. If 

 certain parts of the myocardium are suffering from a 

 relative coronary insufficiency, the increased oxygen 

 requirement magnifies this local oxygen lack, leading 

 to local damage of the well-known coronary type, 

 and revealing vectorial shifts of the ventricular gra- 

 dient which point, in such cases, to the location of the 

 oxygen deficiency. This can be clearly distinguished 

 by the abnormal position of the vectors as compared 

 with the usual effects of exercise. 



Atioxia, Hypoxia, Carbon Dioxide 



The effects of blood gases on the ECG are intricate 

 (50, 148, 470, 474), for the same reason as is the 

 influence of bodily work. Several mechanisms are af- 

 fected, cellular metabolism as well as the CNS cen- 

 ters, the latter inducing marked changes in the 

 autonomic stimulation of the heart. The breathing of 

 low oxygen concentrations has been used as a suit- 

 able test for coronary insufficiencies, although the 

 effects of breathing CO2 and even high o.xygen pres- 

 sures are nearly identical (522). This indicates that 

 the influence of blood gas concentration mav act 

 unspecificaliy or even indirectly. Particularly CO 2 

 seems to act more on the circulatory centers than on 

 the heart itself, because vagotomy greatly reduces all 

 effects of CO 2. However, a small effect persists which 

 is therefore apparently of peripheral or sympathetic 

 origin. The use of hypoxia as a test for coronary in- 

 sufficiency may be understood on an historical basis. 

 From the viewpoint of the physiologist, hypoxia alone 

 does not sufficiently imitate the events in coronary 

 diseases, where ischemia (i.e., simultaneous hypoxia 

 and hypercapnia) is obviously the traumatic agent. 

 If a dog is artificially ventilated with low oxygen ten- 

 sion, the mechanical performance of the heart does 



