ELECTROCARDIOGRAPHY 



367 



G. It is always a time-voltage in- 

 tegral, per definitionem . It is indi- 

 cated by f, h, sa, s in the same 

 manner as QRS or T. 



The Normal P Wave (j^, 

 77, 297, 374) 



The P wave corresponds to 

 atrial activation and is very small 

 in the standard surface leads, 

 where the amplitude seldom ex- 

 ceeds o. I mv and never is higher 

 than 0.5 mv in normal hearts 

 (table 3J. In orthogonal leads, 

 P is maximal along the y axis (on 

 the average o. 1 1 mv with a max- 

 imum of 0.24 mv) (399). The 

 reason is obvious : the muscle mass 

 of the auricles is too small to pro- 

 duce a very strong electric field. 

 This is even more the case with 

 the sinus region, from which the 

 directly recorded monophasic ac- 

 tion potentials are well known 

 (see preceding chapter), whereas 

 the EGG most probably never 

 contains any sign of the sinus 

 action. In high amplifications and 

 high fidelity records a very low 

 initial portion of P may be 

 ascribed to the sinus (473), but 

 this assumption seems at least un- 

 certain. The amplitude of P is 

 maximal in the standard ex- 

 tremity lead II; the resultant vec- 

 tor of the atrial fibers coincides 

 therefore more or less with the 

 anatomical axis of the heart as a 

 result of the symmetrical struc- 

 ture of both atria. The oblique 

 position of the heart under these 

 circumstances leads to the fact 

 that the right atrium is activated 

 in a preferably craniocaudal 

 direction, the left in a right-to- 

 left direction, as has been ob- 

 served in man (517). The vector 

 of P (193, 401) runs in the hor- 

 izontal projection from the 

 back to the front, with an azi- 

 muthal angle (after fig. 58) of 



