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LETTERS TO THE EDITOR
Letters to the Editor will be published, if suitable, and as space permits. They
should not exceed 1,000 words, double spaced, in length, and may be subject to
editing or abridgment.
understanding of the
Relationship of Surface
Electrogram Recordings to Activity
in the Underlying Specialized
Conducting Tissue
nature
of ventricular
conduction.
ALLEN M. SCHER, PH.D.
MADISON S. SPACH, M.D.
Duke University
Durham, North Carolina
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To the Editor:
A recent article by Myerburg et al. (CIRCULATION 45: 420, 1972) indicates a discrepancy
between the time of local depolarization, as
judged from extracellular electrograms, and the
instant of intracellular depolarization in action
potentials, both recorded from strands of Purkinje
tissue. The paper concludes that extracellular
records taken by catheter electrodes within the
heart, purportedly showing the time of activity of
the atrioventricular conduction system, may
likewise be in error in indicating the time of
activity in the conduction system.
One may envisage a similar experimental result.
Suppose that in open-heart surgery, when an intracellular electrode is placed into a ventricular
cell, its potential is similarly found to differ in its
timing from the QRS complex. One might interpret such data as indicating that the QRS
complex did not arise from ventricular depolar-
The author replies:
To the Editor:
In replying to the comments of Drs. Scher and
Spach, I would like first to place their comments
in the context of the manuscript. Four major
points are made in the paper: (1) loss of
electrogram voltage during early premature
activity; (2) potentially misleading reversal of
polarity during early premature propagation; (3)
the depth to which surface electrograms might
reflect activity within specialized conducting
tissue; and (4) the time relationships between
propagating wavefronts and surface recordings as
analyzed from surface electrograms and transmembrane action potentials. From the comments
contained in their letter, it appears that Drs.
Scher and Spach are primarily concerned only
about the last of these four points.
In the first paragraph of their letter, Drs. Scher
and Spach accurately paraphrase our results and
conclusions on the time relationships, but later
challenge these points. In response, I cite data
from the manuscript (page 426), as well as
subsequent studies from our laboratory,l which
demonstrate that very premature impulses are
often characterized by propagating wavefronts
which are nonuniform across the transverse plane
of the conducting tissue, that is, the plane
perpendicular to the longitudinal axis of propagation. We have shown in many experiments (e.g.,
our fig. 6) that the surface electrogram could
coincide with any one of several transmembrane
potentials which are temporally dissociated from
one another. Since electrograms recorded with
close bipolar surface electrodes register primarily
local events over short distances,2 the position of
the electrodes across the transverse axis will
influence the time of inscription of the electrogram when nonuniform propagation is occurring.
ization.
A recent
paper by Dr. Myerburg, with other
co-workers, in CIRCULATION RESEARCH is a
valuable contribution to the literature about
ventricular activation. The present paper, however, is characterized by a lack of understanding
of elementary potential theory. Further, conclusions in the paper are based on the assumption
that if a recording from a single cell indicates that
activity occurs at one time, while the average
activity of many cells in the electrogram indicated
that activity occurs at another time, the single cell
must be right.
Certainly there are many problems with the
catheter recording procedure for timing and
understanding the function of the conduction
system. Possibly the greatest of these is the
inability to determine accurately which cells are
being recorded from. Myerburg's paper, however,
raises a false issue and contributes nothing to our
206
Circulation, Volume XLVI, July 1972
207
LETTERS TO THE EDITOR
This could clearly be a pitfall for the interpretation of the timing of events recorded on the
surface of conducting tissue during premature
activity, since the leading edge of the wave of
propagation might be significantly distant from
the position of the electrodes.
The second paragraph of the letter is puzzling.
I do not think that Drs. Scher and Spach really
believe that I, or anyone else, would interpret the
hypothetical data they present in the way
suggested by the last sentence. Relating the
timing of a single-cell recording to a peripheral
QRS complex is an entirely different matter from
relating a single-cell recording to a bipolar
electrogram recorded less than 100 g from the
impaled single cell.
Drs. Scher and Spach state in their third
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paragraph that, when we were confronted by a
discrepancy botween the timing of a surface
electrogram and timing of a transmembrane
potential, we assumed that the single cell
reflected the actual time of passage of the
propagating wavefront. This is not an accurate
report of our findings and conclusions. In
our results and discussion, we were more
interested in differences per se, rather than rightversus-wrong. In some experiments, careful analysis of the data suggested that the transmembrane
potentials provided the more accurate estimate of
the time of passage of the propagating wavefront
(e.g., fig. 9). In others, the surface electrogram
seemed to be more accurate.
Circulation, Volume XLVI, July 1972
The last paragraph of the letter is also
confusing. The first two sentences express feelings
which formed part of the impetus that led us into
these studies. I am not sure what Drs. Scher and
Spach mean by "a false issue," but among the
major issues in our minds at the outset of the
study was that mentioned in the first two
sentences of their last paragraph. This is evident
in the last paragraph of our introduction to the
article.
Finally, I must emphasize that the study was
designed as an empiric investigation into possible
pitfalls in the analysis of information collected by
a new clinical tool. It was not intended as a
definitive study of the theory of surface electrography from specialized conducting tissue, nor
as an investigation into the nature of ventricular
conduction.
ROBERT J. MYERBURG, M.D.
Department of Medicine
University of Miami School of Medicine
Veterans Administration Hospital
Miami, Florida
References
1. MYERBuTRG RJ, NILSSON K: Longitudinal dissociation in the distal A-V conducting system.
Circulation 44 (suppl II): II-73, 1971
2. HOFFMAN BF, CRANEFIELD PF: Electrophysiology of the Heart. New York, McGraw-Hill
Book Co., 1960
Relationship of Surface Electrogram Recordings to Activity in the Underlying
Specialized Conducting Tissue: The author replies:
ROBERT J. MYERBURG
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Circulation. 1972;46:206-207
doi: 10.1161/01.CIR.46.1.206-a
Circulation is published by the American Heart Association, 7272 Greenville Avenue, Dallas, TX
75231
Copyright © 1972 American Heart Association, Inc. All rights reserved.
Print ISSN: 0009-7322. Online ISSN: 1524-4539
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