358 



HANDBOOK OF PHYSIOLOGY 



CIRCULATION I 



FIG. 42. Model of the differences in the excitation process 

 in normal and ventricular ectopic beats. The extrasystolic 

 beat has only a slight mutual cancellation of potentials and 

 therefore a large time-voltage area of QRS and T. E : the site 

 of the ectopic pacemaker. The arrows indicate the average 

 direction of the excitation waves, analogous to fig. 37. 



the pattern of the action potentials. Tlae predominant 

 role of this factor has long been known. In the very 

 beginning of electrophysiology, scientists were aware 

 that monophasic action potentials varied in different 

 parts of the heart and seemed to last longer at the 

 base than at the apex (433, 434)- Such diflferences, 

 however, could not he analyzed with a reliable 

 technique until small electrodes were developed for 

 recording local monophasic action potentials. The 

 first attempt of this kind was made with a small 

 sucking electrode (411), recording the events in very 

 restricted areas. By means of intracellular electrodes 

 it has often been found that various fibers of the 

 heart show differently shaped action potentials 

 (i57-'59. 240, 261, 262, 506, 534, 535). In dogs the 

 action potential gets shorter from the base to the 

 apex on the heart's surface (234). We therefore may 

 conclude that a regular distribution of inhomo- 

 geneity exists, so that perhaps the inner layers re- 

 polarize more slowly than the outer, or the basal 

 fibers more slowly than those of the apex. If, for 

 example, T is flat or negative, the epicardial surface 

 recovers with a mean delay of 1 1 msec, compared to 

 the endocardial level. In positive T waves, the 

 epicardial surface recovers 8 msec in advance (383). 

 Since the recovery (measincd by timing the re- 



fractory period) is closely related to the action 

 potential, the positive T seems to correlate with a 

 shorter action potential at the surface, which fits 

 perfectly well into the general picture. 



The cause of these diflerences in the monophasic 

 action potential can only be speculated upon at 

 present. Neither mechanical factors (173) nor the 

 well-known differences in temperature between inner 

 and outer layers (75, 187, 314, 359), nor differences 

 in blood supply of the inner and outer layers of the 

 heart can explain the vectorial direction and magni- 

 tude of the gradient (58, 216, 297, 371, 407). How 

 far metabolic differences or differences in the content 

 of calcium could be responsible, is quite uncertain. 



Most probably, the inhomogeneous behavior of 

 the action potential is due to a structural factor which 

 we do not know in detail. This is more than likely, 

 as even small parts of the heart, as papillary muscles 

 (213) or muscle strips from the tortoise heart (215), 

 show an upright, concordant T and a high ventricular 

 gradient. All other factors mentioned above may 

 play their limited role as well, but cannot be claimed 

 to be the main cause of the ventricular gradient. 



Amount of Camrllation of Fiber Dipolfs 



The area of QRS is very important for theoretical 

 interpretation of the EGG, because it is completely 

 independent of the degree of desynchronization of 

 the various fibers, provided that the spatial pattern 

 of the spread of excitation remains unchanged. The 

 instantaneous voltage or the form and amplitude of 

 the QRS complex, however, are highly dependent 

 upon the mode of interaction and synchronization of 

 individual fibers. Area and form therefore rexeal 

 totalK' different peculiarities of the spread of excitation 

 which can be analyzed in detail. The area, irrespective 

 of the total duration of QRS, is an excellent measure 

 of the degree of cancellation resulting from the 

 diverging excitation waves: the higher the area, the 

 lower the cancellation of comparable muscle masses 

 provided. The absolute voltage is a measure of the 

 number of simultaneouslv active fibers and their 

 momentary cancellation. 



Quantitative relationships can be elucidated 

 fairly well by an analysis of extrasystolic beats. In a 

 homogeneous infinite medium tmder ideal con- 

 ditions, the cancellation coefficient could be exactly 

 predicted, if the directions of the individual fibers 

 were known. But neither are the fields ideal nor do 

 we know the structure of the heart exactly. Yet we 

 have recorded maximal amounts of time-potential 



