392 



HANDBOOK OF PHYSIOLOGY 



CIRCULATION I 



explained as the result of an increased vagal tone, 

 because they disappear partially or wholly after 

 atropinization (30-2). With strophanthine, similar 

 though less marked effects occur. The effects are 

 probably of peripheral origin, for the action potential 

 is shortened and the plateau flattened. This is the 

 consequence of a reduced potassium and sodium 

 permeability of the muscle fiber (174). The conduc- 

 tion velocity is decreased for the same reason, so that 

 the QRS duration and the P-Q interval are increased 

 until a complete atrioventricular block exists. Al- 

 though the ECG looks rather abnormal, these events 

 cannot be interpreted as an impairment of the cardiac 

 function as long as no blocks occur. 



Quinine and quinidine have a peculiar action, in 

 that higher concentrations strongly reduce the con- 

 duction velocity. The result is an ECG looking like a 

 "bundle branch block," which in reality shows only 

 an over-all slowing in the spread of excitation. The 

 T wave may be greatly distorted, showing a character- 

 istic widening of its summit, or even notches. The 

 relative Q-T interval increases as a consequence of 

 disturbed conduction. Strophanthine abolishes these 

 effects very quickly (485). 



Hypothermia 



The importance of hypothermia in modern surgery 

 justifies a brief discussion of its influence on the ECG. 

 There is much literature on this topic, which is nearly 

 completely referred to by modern authors (185, 222). 

 As had been stated long ago (329), hypothermia in- 

 duces a picture quite similar to that of ventricular 

 blocks with QT prolonged much more than QRS 

 which might be due to the prolongation of the mono- 

 phasic action potential (426), but the results depend 

 on individual factors. ST may be elevated, preferably 

 in lead II and III, and pronounced only within 

 narrow limits of body temperature (32 "-34° C). T is 

 augmented in such cases (222), its vector rotated 

 clockwise in the frontal projection (220). A very un- 

 usual pattern of QRS is seen in a series of cases : QRS, 

 which, by the way, remains surprisingly short and 

 seldom exceeds 0.05 sec from Q to the peak of S ( 1 85), 

 is followed by a very high deflection, mostly in the 

 same direction as R, and increasing with falling tem- 

 perature. This deflection, well known for many years, 

 but nevertheless called "Osborn-wave" by some 

 authors, has no connection with atrial repolarization, 

 as has been argued, and is not an indicator of immi- 

 nent ventricular fibrillation. It becomes very high at 

 the verv moment when T becomes inverted. The 



mechanism of this deflection is unknown. It is not con- 

 ditioned by an acidosis (185). The degree of abnor- 

 mality depends completely, of course, on the tempera- 

 tures reached in cooling, the QRS duration increasing 

 gradually up to 0.4 sec and QT to more than 2.0 sec 

 in dogs in extremely low temperatures, down to 3° C. 

 E\'en after cooling to 1.5° C, all dogs sur\ived (496). 



Acceleration 



If a subject is exposed to acceleration forces of 3 to 

 4 g, surprisingly few effects are observed. An increase 

 in pulse rate apparently is induced by baroceptor re- 

 flexes, but only minimal effects on the ECG can be 

 observed. The electrical axes of QRS are shifted in 

 some cases up to 25°, but in most cases the deviations 

 are much less, and do not surpass deviations of the 

 axes seen in deep respiration (133, here the older 

 literature). 



Further data on how the ECG is influenced by 

 various items may be found in Lepeschkin's textbook. 



The Fetal ECG, and the ECG in Pregnancy 



During pregnancy, the maternal ECG (39, 47, 

 542) is mainly determined by the position of the 

 heart, which shows a left axis deviation according to 

 the shift of the anatomical axis. The axis deviation is, 

 however, very small in most cases (15°). Qm there- 

 fore is deepened. Major changes of the ECG in preg- 

 nanc\-, which cannot be related to the shift of the ana- 

 tomical axis, are always to he regarded with suspicion 



(39. P- 475)- 



The fetal ECG can easily be recorded during the 



fifth month of pregnancy but never before the fourth 



month (47). The electrodes have to be put on the belly 



of the mother, and bipolar electrodes are the best. A 



bipolar derivation in the craniocaudal direction just 



above the symphysis seems to be most successful, init 



vaginal electrodes are also useful. The position of the 



fetus may be determined quite correctly by vectorial 



analvsis (309), and twins can be identified as they 



were in one case after only 16 weeks of pregnancy. 



16. THE THEORY OF NORMAL .AND .^BNORM.'^L 

 RH\THMS (60, 66, 71, 395) 



The Pacemaker 



The excitation of the heart starts in a circumscribed 

 region called the pacemaker, which is normally the 



