168 Mechanical and Electrical Character of the Heartbeat /9 : 5 



to the blood is negligible, whereas during vigorous exercise it is the major 

 type of hydrodynamic energy. 



5. Electrocardiograph/ 



Every time the heart beats, electrical potential changes occur within it. 

 These potentials spread to the surface of the body. Electrodes at almost 

 any pair of points on the surface of the body will show potential differ- 

 ences related in time to the heartbeat. A record of these potential 

 differences is called an electrocardiogram; the recording equipment is an 

 electrocardiograph. The recording equipment and the records are often 

 indicated by the abbreviations ekg or ecg. 



Electrical changes at the surface of the heart were first demonstrated 

 in 1856. Electrocardiography, the science of measuring the associated 

 potentials, did not really develop until physical instrumentation made 

 possible the detection of these small potentials. The first big step was 

 the application of the string galvanometer to electrocardiography in 

 1903. This was the work of Einthoven, whose ideas dominated the 

 field for many years. Today, all electrocardiographs depend on the 

 action of electronic amplifiers. In this field, as is the case in so many 

 others, the rapid advances have resulted from the widespread application 

 of electronic techniques. The electrocardiogram is used in many clinical 

 diagnoses of heart ailments. It is widely used because of its convenience 

 and also because of the large amount of information which can be 

 obtained without any surgical procedures or any discomfort to the 

 patient. 



The electrocardiogram is a record of electrical potential differences 

 at the surface of the body. The heart, however, is not the only source of 

 potentials at the body surface ; it is necessary to distinguish between those 

 potentials due to the heart and those originating from other organs. 

 Every muscle within the body undergoes potential changes as its fibers 

 contract. The magnitude of the action potentials for all nerves and all 

 muscle fibers is about 120 mv. The motion of any skeletal muscle can 

 give rise to body-surface potential differences comparable to the ekg 

 potentials. To limit this source of distortion, the ekg is often recorded 

 with the patient lying down. 



In addition to potentials of muscular origin, there are also d-c body 

 surface potentials. These exist between the two hands, the hands and 

 the feet, and so forth, and may be as large as 0. 1 mv. These potentials 

 can be eliminated by suitable electronic design of the recording appara- 

 tus. (It is interesting to note parenthetically that the origin of these 



