3IO 



HANDBOOK OF PHYSIOLOGY 



CIRCULATION I 



activity in the dog is usually directed from left to 

 right in the septum and results from earlier and/or 

 greater initial left-to-right activity. This activity, 

 transposed to the human heart, would produce a 

 wave directed to the right, toward the head (since 

 the left side of the septum lies caudally), and possibly 

 slightly anteriorly (fig. 32). This wave will produce 

 an initial negative deflection in all limb leads, termed 

 the Q wave. For the leads on the precordium, the 

 picture is also clear. The leads on the right side of 

 the chest (Vi and V2) face the positive side of the 

 wave front and record an upward deflection, while 

 those on the far left (V's and Ve) record a negative 

 deflection. 



Immediately after invasion of the .septum begins 

 (fig. 33), rapid conduction through the Purkinje 

 system results in an irregular pattern of inside-out 

 spread in the walls; the transition from the first phase 

 of activity to this second and major phase of ven- 

 tricular activity is smooth and gradual. Within the 

 septum, left-to-right activity predominates slightly. 



Arrows drawn perpendicular to the advancing wave 

 front depict the instantaneous vector of depolariza- 

 tion. At 5 msec after the beginning of QRS in the 

 dog (by extrapolation 12 msec after the beginning of 

 QRS in man), the average direction of these arrows 

 indicates a pattern of activity directed slightly forward 

 to the right and from base to apex. Such a pattern 

 will result in negative deflections in leads II and III, 

 and little or no deflection in lead I, which may be 

 positive or negative at this time. The potentials in 

 the leads on the anterior chest surface will differ 

 slightly from those occurring during the earlier phase, 

 since the leads on the right will now "see" both ap- 

 proaching (left to right) and receding (base to apex) 

 activity. The approaching activity will be in the right 

 wall and on the left side of the septum; the receding 

 activity in the left wall and in the right septum. At 

 this time, there may be little or no potential in these 

 leads and a positive deflection in V3. 



At 15 msec after the onset of QRS in the dog, i.e., 

 almost halfway (35 msec in man) through QRS 



-vf^VL 



\, m 



VF 



FIG. 34. .At about the middle of QR.S interval, breakthrough of activity to anterior right ventricle 

 has left forces moving to left posteriorly relatively unopposed. The result is a negative deflection in 

 lead VR and positive deflections in all other limb leads. The leads on far right of chest (Vi and Vj) 

 now see negative activity; potential at Vj is near zero, and potentials at Vj, V5, and Ve are positive. 

 [From Scher (i 14a).] 



FIG. 35. During terminal portion of QRS complex, activity is directed to left and posteriorly in 

 basal left ventricle and basally in upper septum. This condition results in potentials which are small 

 in all leads. Deflection in VR is positi\e, deflections in all other limb leads are negative. Potentials 

 are now returned to zero from negative peak in Vi and Vj and from positive peak in Vj, Vj, and Ve. 

 This activity results in slight negative potentials in Vj, Vj, Vs, and Ve. [From .Scher (i 14a).] 



