348 



HANDBOOK OF PHVSIOLOGV 



CIRCULATION I 



FIG. 30. Superposition of extrinsic and intrinsic potential 

 in a unipolar record from the ventricular surface of a dog. The 

 intrinsic deflection starts far behind the top of the R wave 

 with a slur at the beginning of the very rapid downstroke. 

 [From Schaefer (58).] 



the heart muscle mass appears has reached its ma.ximal 

 value of 2ir. 



The solid angle li, under which the active cross 

 section of a fiber bundle appears from the surface 

 electrode, can never e.xceed the value of 27r, according 

 to a hemisphere, and in most cases is much less. The 

 unipolar potential therefore becomes V,, = m-Q = 

 ('V'/4ir)-Sl, and amounts maximally to one-half of 

 the intracellular action potential \', i.e., 100/2 = 

 50 mv. In most cases it is much less, but may go as 

 high as 25 mv. This means that the weighted active 

 cross section of fibers sending their excitation waves 

 towards the electrode has a solid angle of about tt. 

 The nearest fibers influence the total potential 

 pattern with a very characteristic event, the "in- 

 trinsic potential" (322, 531). In the record, a sudden 

 and very steep downstroke occurs, indicating that the 

 bulk of nearby fiijers is about to be activated. The 

 downstroke (fig. 30) may be finished in such a short 

 time as 3 to 5 msec. This shows that the shell of active 

 cross sections around the electrode suddenly dis- 

 appears, as the depolarization wave front reaches the 

 electrode. The heart mass around the electrode is 

 then totally active and without any membrane 

 potential differences. The intrinsic downstroke is 

 usually followed immediately by a local potential of 

 opposite sign, indicating that a certain number of 

 fibers are now activated bv excitation waves running 



away from the electrode. Thus the zero point of the 

 downstroke indicates that moment in time at which 

 the weighted active cross section of the approaching 

 fibers equals exactly the corresponding cross section 

 of the receding fibers. 



In the older literature it is often claimed that 

 summit of the spike of the unipolar action potential 

 is recorded at the "time of arrival" of the excitation 

 at the electrode point. This, however, may be quite 

 wrong, as simultaneous recordings with unipolar 

 and close bipolar electrodes have shown (413, 501). 

 In most cases, the beginning of the downstroke is 

 induced by depolarization of relatively remote but 

 large muscle masses, the excitation waves of which 

 had been approaching the electrode. Only the starting 

 point of a "very sudden" downstroke may be more 

 or less correct as a measure of the local activation 

 time. For that reason, we regret to say that many 

 classical determinations of the time course of ven- 

 tricular activation cannot be accepted. 



As unipolar electrodes are often used nowadays in 

 close contact with the heart, even in man (30), it 

 seems necessary to discuss briefly how curves so 

 obtained are to be interpreted. The first argument to 

 contest is that only certain parts of the heart con- 

 tribute to a unipolar potential. This is true neither 

 for extremity leads nor for a direct electrode on the 

 heart's surface. Even from the epicardial surface of 

 the heart, a resultant electromotive force is recorded 

 which is more or less identical with the force rep- 

 resented by the heart vector. The amount of prox- 

 imity potential, in other words, is smaller than 

 expected by many authors. Therefore, although 

 Groedel claims that the precordial potential contains 

 a high local effect because of its similarity to the 

 epicardial record taken from a point exactly below 

 the precordial point, the correct conclusion should 

 have been drawn in the opposite direction : even the 

 epicardial record is largely determined by the re- 

 sultant heart vector with comparati\ely small prox- 

 imity influences (94). 



The interpretation of unipolar surface derivations 

 is difficult. If a record from the cava! surface is 

 purely negative, it may be that the ijulk of fibers 

 having direct contact with the electrode conduct 

 their excitation waves strictly away from the elec- 

 trode, as at the sinus node region (239, 278) or at the 

 site of origin of an extrasystolic beat (416). The same 

 record, however, would be obtained where the 

 inner surface layer of fibers runs parallel to the surface 

 and therefore perpendicularly to the lead vector: 

 their potentials will not be recorded, and the record 



