9: 5/ Mechanical and Electrical Character of the Heartbeat 



169 



d-c potentials is not well understood. The d-c potential between the 

 two arms of many women shows a sharp maximum on one day during 

 the middle of the menstrual cycle. At one time, it was believed that 

 these were associated with ovulation, but the correlation is very poor.) 



The ekg potentials can be observed between almost any pair of points 

 on the surface of the human body. If the two points are reasonably 

 separated, the maximum potential difference observed is of the order of 

 1.0 mv. The ekg has the same period as the heart. Traditionally, 

 three wires were attached to the subject, one to each arm, and the third 

 to the left leg. The ekg was then recorded between the members of 

 each of the three resulting pairs of leads. 



Whether the electrocardiogram is recorded between two points on the 

 surface of the body or between one point and a neutral electrode, it 



Figure 6. A typical ekg. P wave precedes auricular contrac- 

 tion and QRS complex is associated with ventricular contrac- 

 tion. Exact height of wave depends on lead used. 



always has the shape shown in Figure 6. The neutral electrode can be 

 formed by immersing the subject in a tub of water and placing the 

 electrode far from the body. Provided low resistance electrodes are 

 used, the curve will always have the general shape shown. 



The various bumps on the ekg are called waves. The P-wave occurs 

 just before auricular contraction. The QRS-complex is associated with 

 the start of ventricular contraction, and the T-wave occurs at the end 

 of ventricular contraction. The amplitude of the ekg waves is shown in 

 the table on page 170. In addition, a smaller U-wave follows the 

 T-wave after ventricular relaxation. 



Most frequently, electrodes are placed on both arms and on the left 

 foot, and quite commonly are also placed on the back and on the chest. 

 The ekg's are usually described in terms of leads, which means the poten- 

 tial difference between two points. This is confusing terminology 

 because two wires, each ordinarily called a lead, are necessary for one 

 ekg lead. 



