ELECTROCARDIOGRAM 



273 



potential difference capable of being recorded at any time, and will be recorded 

 in full only when the electrical axis lies parallel to the line of the lead. The actual 

 potential difference set up within the heart and to which the manifest value bears a 

 constant relationship, is unable at any time to be recorded. 



Description of the Various Waves in the Tracing 



It will be observed (Fig. 82) that there are 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. The time relations of each 

 wave have been ascertained by taking simultaneously with the electro- 

 cardiogram a record of the mechanical changes occurring in the heart 



Fig. 82. Normal electrocardiogram. Leads I, II, III. Note that the height of any upward 

 deflection or the depth of any downward deflection in lead II equals the sum of the correspond- 

 ing deflections in leads I and III. 



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 one ventricular pressure, 

 whereas the lowest is an electrocardiogram. It will be observed: (1) 

 that the P-wave occurs just antecedent to contraction of the auricles; 



(2) that the small positive wave, Q, which is absent in these tracings, 

 must occur just before the beginning of the contraction of the ventricles; 



(3) that the upstroke of wave R is inscribed before the commencement 

 I of ventricular systole; and (4) that the long upward wave T, culminates 



at the moment the ventricle begins relaxing. 



