178 Mechanical and Electrical Character of the Heartbeat /9 : 8 



vertebrate circulatory- system. Its physical activity- may be described in 

 terms of the velocities and pressures acquired by the blood at various 

 points of the circulatory system and also in terms of the power expended. 

 The heart not only does work but also contains tissues which produce 

 periodic beats in a fashion similar to that of a series of electronic multi- 

 vibrators. The firing rate of the normal control element, the s-a node, 

 can be increased or decreased both by the nervous system and by 

 certain hormones. 



Like the fibers of all striated muscle, the heart fibers are traversed by 

 a spike potential before contraction. These spike potentials appear as 

 current sources immersed in the surrounding fluid. The resulting body 

 surface potentials are called electrocardiographic potentials. These are of 

 such a form that the heart may be well approximated by a single dipole. 

 Systems to find the orientation and magnitude of best equivalent dipole 

 are called vectorcardiography. Although clinically useful and challenging 

 to the imagination of the physicist, the equivalent heart dipole seems to 

 lack any basic relationship to the heart itself. 



REFERENCES 



1. The following monograph is very complete and well worth reading by any- 

 one wishing to pursue the subject in detail. A large part of the material 

 in this chapter is based on it. 



Whitelock, O. v. S., ed., "The Electrophysiology of the Heart," Ann. New 

 York Acad. Sc. 65: 653-1145 (Aug. 1957). 



2. Best, C. H., and N. B. Taylor, The Physiological Basis of Medical Practice 

 (Baltimore, Maryland: Williams & Wilkins Company), 7th ed., 1961. 



Read the chapters on the heart and circulatory system. 



3. Glasser, Otto, ed., Medical Physics (Chicago, Illinois: Year Book Publishers, 

 Inc., 1950) Vol. 2. 



a. Hamilton, W. F., "Circulatory System: Arterial Pulse," pp. 186-188. 



b. King, A. L., "Circulatorv System: Arterial Pulse; Wave Velocity," 

 pp. 188-191. 



c. Hamilton, W. F., "Circulatory System: Heart Output," pp. 191-194. 



d. Landowne. M., and L. N. Katz, "Circulatory System: Heart; Work 

 and Failure," pp. 194-206. 



e. Green, H. D., "Circulatory System: Methods," pp. 208-222. 



f. Nickerson, J. L., "Circulatory System: Methods; Ballistocardio- 

 graph," pp. 222-225. 



g. Jochim, K. E. } "Circulatory System: Methods; Electromagnetic 

 Flowmeter," pp. 225-228. 



h. Green, H. D., "Circulatory System: Physical Principles," pp. 228- 

 251. 



