272 THE CIRCULATION OP THE BLOOD 



three waves above the line of zero potential and two waves below it. 

 They have been lettered from before backward, P, Q, R, S, and T, 

 and in all such records when correctly obtained, the waves above the 

 line of zero potential indicate that the base of the heart is negative to 

 the apex. The exact cause of each wave has been ascertained by taking 

 simultaneously with the electrocardiogram a record of the mechanical 

 changes occurring in the heart during each cardiac cycle. Such records 

 have been secured by taking intracardiac pressure curves with the results 

 as shown in Fig. 83. The top curve represents auricular and the second 



m m 



ff i T 



^ 



m 



<*- 



T *T T 



Fig. 82. Normal electrocardiogram. Leads 1, 2, 3. Note that the height of the R deflection in 

 lead 3 equals the difference between the height of RI and R 2 . 



one ventricular pressure, whereas the lowest is an electrocardiogram. 

 It will be observed: (1) that the P-wave occurs just antecedent to con- 

 traction of the auricles; (2) that the small positive wave, Q, which is ab- 

 sent in these tracings, must occur just before the beginning of the con- 

 traction of the ventricles; (3) that the negative wave, R, occurs just be- 

 fore and during the early part of ventricular systole that is, during 

 the presphygmic period; and (4) that the long upward wave, T, culmi- 

 nates at the moment the ventricle begins relaxing. 



Although such comparisons give us considerable insight into the cause 

 of several of the waves, there yet remain certain peculiarities of the 

 electrocardiogram to be considered. These are: (1) the cause of the 



