ELECTROCARDIOGRAPHY 



391 



their plasma concentrations at the same time, d) 

 Even the action of different ionic plasma concentra- 

 tions on single fibers x'aries according to the type of 

 fibers investigated (auricle, Purkinje fibers, myo- 

 cardial fibers). 



In most cases, in the human heart, only moderate 

 changes of the ECG have been described. Among these, 

 the effects of calcium and potassium prevail. The 

 QRS vector is comparatively stable, remaining un- 

 changed by calcium lack (100), but QRS is prolonged 

 by an increase in plasma potassium (254). Here, as in 

 other influences of potassium, the absolute plasma 

 concentrations do not show a close correlation with 

 the shift in ECG, which depends more or less on the 

 ratio between intracellular and extracellular potas- 

 sium, as altered in dogs (301). In human hypopotas- 

 semia QRS is nearly unchanged, as is also true for 

 changes of Ca, Na, CI, and inorganic P. In the rabbit 

 heart, however, the results are somewhat different 

 (477). Only a severe sodium deficiency prolongs the 

 human QRS, because the rapidity of depolarization 

 in single fibers is determined by the sodium influx, 

 and is diminished by decreasing external sodium. 

 This leads to a reduced conduction velocity and pro- 

 longation of all conduction times, QRS included. 



The QT duration is unchanged by alterations in 

 the plasma content of Na, K, CI, and bicarbonate; 

 calcium deficiency prolongs QT (47, 135, 391), al- 

 though in man in a not very impressive manner (254). 

 An increase of calcium above the normal level in- 

 creases QT even more. Potassium prolongs QT if its 

 concentration rises, but here, too, it is not the abso- 

 lute concentration that determines the effect, but the 

 ratio between intracellular and extracellular potas- 

 sium (301). An antagonism between calcium and 

 potassium exists only in a very narrow band of con- 

 centrations (135). 



The T wave is also rather sensitive to ionic balance. 

 T is augmented by increased potassium concentra- 

 tions, with tent-shaped T waves in severe potassium 

 intoxication. Such variations of T occur as soon as the 

 plasma content exceeds 6.5 meq per liter (254). A 

 diminution of plasma potassium, especially if the quo- 

 tient between cellular and extracellular potassium is 

 raised decreases T in most cases (254, 301). Changes 

 of Na, CI, and P, within physiological limits, produce 

 no distinct change in T (254); however, a rise in 

 calcium may depress it somewhat, or even invert it 

 and round its contour. Decrease of calcium may act 

 in a similar manner, sometimes decreasing T but in- 

 creasing it in other cases. This strengthens our suspi- 

 cion that so many of the clinical findings are indirect. 



complicated by other events, and scarcely predictable 

 from the effects on the single fiber. 



All these effects are so inconstant and depend in 

 such a complicated manner on the concentration 

 gradients across the membrane of the myocardial 

 fibers, that only in hypopotassemia is a close correla- 

 tion found between the plasma concentrations and 

 the form of the ECG (172, 476). But even the effect 

 of potassium is complicated concerning its action on 

 the U wave: U is strongly augmented by hypopotas- 

 semia, as it is by adrenaline (see above), so that a 

 prolongation of QT may be simulated. In the whole, 

 we may conclude that the ECG reveals only the pres- 

 ence rather than the special form of an ionic dis- 

 turbance (136). 



Some special remarks may be made on the mecha- 

 nism by which calcium acts. The lower the calcium 

 concentration, the higher is the sodium influx into the 

 fiber during depolarization in nerves (208, 511), and 

 most probably also in heart muscle fiber. It is as- 

 sumed that Ca interferes with the carrier mechanism 

 which supports the Na influx. An increa.se of calcium, 

 therefore, leads to a slowing of the rising phase of the 

 action potential and thereby slows down the conduc- 

 tion velocity, lengthening PQ and QRS in a manner 

 similar to that of sodium deficiency. The role calcium 

 plays in the repolarization process is very uncertain. 

 Results of experimental changes in Ca concentrations 

 on the action potential are contradictory. There is 

 some evidence that, within physiological limits, Ca 

 does not influence the action potential at all, but 

 greatly changes the electromechanical coupling (328, 

 511, 513) and most probably the conduction in the 

 specific system. 



Drugs 



The action of numerous drugs on the ECG has been 

 described in so many papers that we can refer only to 

 the review by Lepeschkin concerning the details (47). 

 Drugs influencing the autonomic innervation, or 

 transmission in general, exert the same influence as 

 that part of the autonomic system which they do not 

 block, or which they do activate. (There are excep- 

 tions, however.) The action of cardiac glycosides re- 

 quires special discussion (53). Digitalis depresses T, 

 may even invert it, and leads to a depression of the 

 S-T segment. In this action, four stages may be dif- 

 ferentiated : the stage of T depression, of S-T depres- 

 sion, of aggrevated S-T depression with a steep up- 

 stroke of T, and of diphasic T (47). These changes, 

 which may resemble a pathological ECG, have been 



