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HANDBOOK OF PHYSIOLOGY 



CIRCULATION 1 



in autonomic tonic innervation. They concern 

 primarily the T wave. The type of change is subject 

 to disagreement. Perhaps it is maximal in the after- 

 noon to midnight (i86). These variations are much 

 higher than the changes of T during respiration, but 

 a strict correlation with daytime records is not always 

 present. A shift in the subjective daytime (changing 

 the simulated diurnal cycle) had no influence on the 

 variations of T (117). Food intake leads to some 

 effects on the T wave, which decreases (439) and 

 causes an increase in the R wave and a shift to the 

 right in its axis (451). All these influences are maximal 

 in patients with neurocirculatory disturbances (186). 



Changes in the Course of Age {j2, 52, ij8, 

 165, 270, 326, 346, 432, 320) 



The influence of age on the vectorial data has been 

 mentioned already in section 10. There are some 

 general trends in these variations in the course of 

 age, the electrical axis of QRS shifting to the left, 

 but by no means gradually or constantly. After the 

 first year of life, the axis has reached a somewhat 

 stable position (a in the frontal plane averages 60°). 

 In the newborn child, a has a value between 120° 

 and 150°, which corresponds to the relative hyper- 

 trophy of the right heart. In the horizontal plane, 

 the axis shifts from an anterior direction toward the 

 back, but also changes from posteriorly directed 

 positions into less markedly posterior positions are 

 reported (456). The vector having passed its acute 

 changes of position in the first year, the elevation 

 (angle a of Einthoven) becomes smaller year by 

 year. This shift corresponds fairly well with pro- 

 gressive anatomical preponderance of the left ven- 

 tricle, the weight of which increases continuously. 

 Perhaps the rising blood pressure plays a role in this 

 connection. The maximal vector of T does not 

 change its position as mucli with increasing age as 

 QR.S does, though the average shift is in the same 

 direction (76, 456). 



There is almost no separate feature of the ECG 

 which does not change some during a lifetime. The 

 QT duration increases gradually, T decreases, the 

 PQ interval and the duration of P increase. Some of 

 these changes are due to the marked decrease in heart 

 rate during the first two decades of life, but there are 

 real influences of age remaining, even after correction 

 for the frequency shift has been made (346). The QRS 

 duration is remarkably long even in very small hearts: 

 in the newborn child about 0.04 sec. The amplitudes 

 are nearly those of adult hearts, if one disregards the 



difference in vector position. The reason is obvious: 

 the relation of heart diameter to thora.x diameter re- 

 mains more or less constant during life. The duration, 

 however, depends upon the velocity of the excitation 

 wave, which increases with the fiber diameters and 

 therefore accelerates with the growing heart [(58, p. 

 83), and see table 2]. 



15. VARIOUS INFLUENCES ON THE ECG 



The ECG is influenced directly and indirectly by 

 all agents which vary the frequency of the heart, the 

 autonomic innervation, the time pattern of the action 

 potentials, or the spread of excitation, as to its path- 

 way and/or its propagation velocity. Very few agents 

 are known to influence the factors which determine 

 QRS, whereas changes in T are most common. The 

 reason obviously is that the plateau of the monophasic 

 action potential is extremely sensitive to all kinds of 

 influences, i:>ecause drugs, ions, hormones, tempera- 

 ture changes, etc. vary the action potential pattern 

 considerably, perhaps by changing the cellular 

 metabolism. In all such cases it is decisive to know 

 if and to what an extent these influences are acting 

 inhomogeneously on the heart. Every influence which 

 does not act on all parts of the myocardium in a 

 similar manner will alter the degree of inhomogeneity 

 of the repolarization and thereby alter the ventricular 

 gradient. 



Some general remarks may be made in advance 

 which might contribute to a better understanding of 

 the various influences on the ECG. It will be shown 

 in the following pages that most of the effects may be 

 due to a very simple basic phenomenon : the change 

 of ion permeability. A simplified picture of the de- 

 polarization and repolarization mechanisms may be 

 given as follows. The magnitude of the action potential 

 depends on the ion battery, which, in turn, depends 

 on intact carrier mechanisms or ion pumps, which 

 restore the ion balance as soon as it is disturbed. 

 Every increase in extracellular potassium leads to a 

 better and earlier repolarization; every improvement 

 of a potassiimt shift into the cell lengthens the plateau. 

 Every decrease in the intracellular potassium acts in 

 the same direction as an increase of intracellular 

 sodium. Acetylcholine seems to augment the potas- 

 sium permeability during systole, thus reducing the 

 plateau via an increase in the external potassium. 

 Adrenaline seems to augment primarily the anaerobic 

 metabolism, thus perhaps decreasing the external 

 potassium by an improvement of potassium intake 



