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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 Downloaded from http://circ.ahajournals.org/ by guest on June 16, 2017 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 Downloaded from http://circ.ahajournals.org/ by guest on June 16, 2017 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 Downloaded from http://circ.ahajournals.org/ by guest on June 16, 2017 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 The online version of this article, along with updated information and services, is located on the World Wide Web at: http://circ.ahajournals.org/content/46/1/206.2.citation Permissions: Requests for permissions to reproduce figures, tables, or portions of articles originally published in Circulation can be obtained via RightsLink, a service of the Copyright Clearance Center, not the Editorial Office. 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