366 HANDBOOK OF PHYSIOLOGY ^^ CIRCULATION I 



SEGMENT 



^RS- T SEGMENT 

 ,r -QRS GROUP 

 Lp R SEGMENT 

 P-R(P-Q)INTERVAL 



FIG. 49. Two normal ECG in standard leads. Upper row: 

 heart sounds. Time marks: o i sec. 



and as it is reflected predominantly (though not 

 exclusively) in the time course of the ECG, a de- 

 scription of voltages as a function of time is still one 

 of the most important things electrocardiography 

 can supply. This is the main reason why some authors 

 are so reluctant to ascribe great clinical value to the 

 vectorcardiogram (7). Also, the ventricular gradient 

 is not directly represented in the vector loop. The 

 same is true for all proximity potentials, because the 

 vector loop has a physical meaning only in the 

 absence of such potentials. We therefore need a 

 double analysis of the ECG: by the conventional 

 curves and by their vectorial counterpart. 



Nomenclature (^sg) 



A normal ECG curve taken from a conventional 

 (standard) extremity or Einthoven lead is given in 

 figure 49. The sequence of deflections was named by 

 Einthoven with the symbols P, Q, R, S, T, U. Some 

 of these deflections are combined in groups or com- 

 plexes: the QRST group, the Q.RS complex. Parts 

 of the curve are called "segments," e.g., PQ segment, 

 ST segment, counting from the end of P to the 

 beginning of QRS or from the end of S to the be- 

 ginning of T. Special difficulty lies in the identifi- 

 cation of the QRS deflections. The current use is 

 merely a descriptive nomenclature, in which the 

 first sharp, downward deflection after the P wave 

 (if followed by an upward deflection) is labeled Q, 



the first upward deflection is R, the second down- 

 ward, S. If there is only one downward deflection, 

 it is labeled QS. If more than the Q, R, and S de- 

 flections are present, the upward deflection following 

 S is labeled R', a downward deflection following 

 R' is S' (if necessary, R" and S" may follow). If 

 one of the deflections is comparatively small, it is 

 sometimes written with a small letter. For example, 

 rS means that a small positive deflection is followed 

 by a large negative one. The normal pattern in 

 extremity leads, then, is qRs. The P wave of atrial 

 depolarization is occasionally followed by an opposite 

 polarity, low-voltage, and long-lasting wave; the 

 repolarization wave of the atria, clearly distinguish- 

 able from QRS only in cases of complete A-V block. 

 It is labeled Tp. If necessary, a polyphasic P wave 

 may be subdivided into Qp, Rp, Sp, R'p, S'p. 

 This, however, will be practically restricted to atrial 

 esopliageal leads. 



A peculiar difficulty is sometimes met in the 

 determination of the zero line or level of reference. 

 Since, in the inajority of ECG tracings, the QRS 

 complex is superimposed on the Tp or Ta deflection, 

 the best level of reference for QRS is the level at the 

 beginning of the first of its deflections. For the ST 

 segment however, this ''zero" le\'el may be wrong. 

 There is no correct procedure applicable in all cases. 

 The best reference point, recoinmended by the inter- 

 national societies, is the le\'el at the beginning of 

 QRS for the determination of the ST junction (i.e., 

 the point where S ends and the steep deflection is 

 replaced more or less abruptly by the more gradual 

 slope which precedes the first limb of T). For the 

 ST segment, the best reference line is the isoelectric 

 le\el T-P or U-P, when this can be determined. But 

 often a persisting U wave is superimposed on the 

 beginning of P. In this case, the beginning of QRS is 

 the best (though not unquestionably correct) zero 

 level. 



A special nomenclature has been adopted to 

 describe the various \'ectors of the ECG. Whenever 

 the \ectors are measured in space, they carry the 

 index s: QRSg, T^. If their time-\'oltage areas are 

 concerned, the symbol Aqrs or A^ is sometimes used, 

 and if observed in space, the inde.x s is added. In 

 soine papers (as in this chapter) the area is indicated 

 by the symbol QRS or T, with s as index in case of 

 spatial values. If the areas are measured in a plane, 

 the indices f for frontal, h for horizontal, and sa for 

 sagittal are used here: QRSf, e.g., means time- 

 voltage area of QRS, taken as frontal projection of 

 its vector. The \entricular gradient is marked with 



