368 



HANDBOOK OF PHYSIOLOGY ^"^ CIRCULATION I 



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FIG. 50. Demonstration of the atrial T (T„) in a case with complete A-V block. Eag: electro- 

 atriogram. Evg: electroventriculogram. [From Holzmann (34).] 



about 45°; in the frontal plane it has an angle a 

 varying in wide limits between 0° and 80° (366), 

 and lies in the majority of cases at about 60° {541). 

 In the sagittal plane it has an elevation of 70 to 80° 

 measured in a clockwise rotation from the right end 

 of the z axis, looking at the sagittal plane from the 

 right side of the erect subject (193, 497)- The vector 

 has a relatively constant direction during the whole 

 atrial systole, but shows a rounded contour of its 

 frontal vector loop projection in most cases of a left 

 atrial hypertrophy (108). 



The duration of P depends on both the distance of 

 the most remote auricular point from the sinus node 

 and the conduction velocity. The total duration of P 

 is maximal in leads deriving the y axis (0.096 sec), 

 and shorter along the x and z axis (0.079 ^^'^ 0.072, 

 respectively) (374). In an isolated auricle, the con- 

 duction time is increased by stretching the muscle 

 fibers, most probably by lengthening the conduction 

 path (258). In left atrial enlargement, the P wave is 

 also prolonged over the normal value of o. i sec 

 (■'mitral" P). It is always slightly notched especially 

 in cases of dilated left atria. In right atrial enlarge- 

 ment the P wave is often tall and sharply pointed, 

 but there are frequent exceptions of these rules. The 

 mitral P may be due at least in .some ca.ses to a left 

 atrial hypertrophy, whereas the tall pointed P is 

 most frequently found in pulmonary heart disea.se 

 ("P pulmonale"). It is very important in regard to 

 the theory behind the ECG that the explanation for 

 these facts is uncertain. We may assume that strong 

 dilatations or hypertrophy in mitral defects may fa\or 

 an atrial hypertrophy with a prolonged conduction 

 time, so that the area of P is increased. 



In the normal ECG, the atrial complex is largely 

 obscured by ventricular activation. There are two 

 possibilities for separation of atrial and ventricular 

 potentials : by putting electrodes on or near the atrial 

 surface (e.g., by esophageal leads), whereby the 

 atrial component of the ECG is augmented, the 

 ventricular part, however, is by no means reduced; and 

 by oijserving the isolated atrial activity in atrio- 

 ventricular block. We obtained our precise knowledge 

 of the whole atrial complex from observations with 

 block (fig. 50). It became clear that the P wave is 

 followed by Tp (or Ta). This Tp is always opposite 

 to P, so that the "atrial gradient," an analogue of the 

 ventricular gradient, is always small if not zero and 

 averages 2 nv sec (106), whereas the area of P alone 

 is about 4 /jvscc (155). There is, in other words, only 

 a small inhomogeneity, if any, in atrial repolarization, 

 since these small gradients may perhaps he explained 

 by a shift in the atrial position during atrial systole. 



The monophasic action potential of atrial fibers 

 has no, or only a very short, "plateau." Correspond- 

 ingly, Tp develops directly after P, without an 

 isoelectric interval. This means that QRS is always 

 superimposed on Tp, and that the PQ interval 

 cannot be strictly isoelectric (210, 232). The larger 

 P, the deeper the PQ displacement (423). 



The (IRS Complex 



The main part of the spread of excitation is pre- 

 ceded by the activation of the septal and papillary 

 regions, which causes the very first vectors of the 

 ventricular activation to go upward and sometimes 

 to the right, or, more often, to the left, so that standard 



