EXCITATION OF THE HEART 



309 



reversals of polarity were preserved, and these results 

 were thought to reflect direct stimulation of the 

 Purkinje fibers. 



As indicated earlier, a variety of experiments in- 

 dicate that conduction in the atria is more rapid 

 along the long axes of the fibers. Similar data has 

 been supplied for the ventricles by Sano and co- 

 workers (log), who found velocities generally 2 to 10 

 times higher parallel to the fiber direction than per- 

 pendicular to it. 



Ventricular Activation and the QRS Complex 



The shape of the ventricular complex recorded at 

 the Iwdy surface is determined by the pattern of 

 ventricular activation, the particular ECG lead 

 which is recorded, and the position of the heart 

 within the chest. Direction of tlie activation wave 

 will therefore vary with respect to recording points 

 on the body surface according to the position of the 

 heart; also the activation process undoubtedly varies 

 from individual to indi\idual with such anatomic 



features as wall thickness, distribution of Purkinje 

 tissues, etc. A detailed discussion of the electro- 

 cardiogram will follow in the next chapter. However, 

 a brief description of the origin of the human ventricu- 

 lar (QRS) complex seems appropriate here. The 

 consideration is directed toward the potentials 

 recorded at conventional sites on the body surface. 

 As is explained in the preceding chapter, the "V" 

 designation on an electrocardiographic lead means 

 that the potential is recorded between a body surface 

 point and a terminal which averages the potential of 

 the two upper extremities and the left leg. In con- 

 sidering the relation of depolarization to potentials 

 recorded at the i:)ody surface, it must be remembered 

 that the right ventricle lies anteriorly and the left 

 posteriorly. The septum is tilted slightly forward 

 apically, and the base-to-apex axis of the heart is 

 often quite parallel to the diaphragm. In this pre- 

 sentation, data from the dog heart are transposed to 

 the human and the duration of the QR.S complex 

 (40 msec in the dog) is extended to 80 msec. 



As indicated earlier, the initial phase of ventricular 



VL 



•y 'm 



^VF 



FIG. 32. Mean direction of activity during earliest portion of QRS transposed from canine to 

 human heart. First activity goes from left to right in septum. Because of position of septum in human 

 chest, this results in negative deflection in all bipolar limb leads, positive deflection in VR and in 

 leads on right side of precordium (Vi through Vi), and negative deflection in V5, Ve, VL, and VF. 

 [From Scher (i 14a).] 



FIG. 33. When about one-quarter of QR.S interval has passed, acti\ ity is proceeding from left to 

 right in septum, and activity from inside out in wall has begun. Total activity is such that potentials 

 are near zero in all limb leads, both bipolar and unipolar, and in V] and Vj. Other leads on chest 

 are positive because activity proceeds toward the apex and free left wall. [From Scher (i 14a).] 



