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The Nuts and Bolts of Paced ECG Interpretation The Nuts and Bolts of Paced ECG Interpretation Tom Kenny © 2009 St. Jude Medical. ISBN: 978-1-405-18404-5 Commissioning Editor: Thomas V. Hartman Development Editor: Kate Newell Production Editor: Cathryn Gates The Nuts and Bolts of Paced ECG Interpretation Tom Kenny St Jude Medical Austin, Texas USA A John Wiley & Sons, Ltd., Publication This edition fi rst published 2009, © 2009 by St. Jude Medical Blackwell Publishing was acquired by John Wiley & Sons in February 2007. Blackwell’s publishing program has been merged with Wiley’s global Scientifi c, Technical and Medical business to form Wiley-Blackwell. 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No warranty may be created or extended by any promotional statements for this work. Neither the publisher nor the author shall be liable for any damages arising herefrom. Library of Congress Cataloging-in-Publication Data Kenny, Tom, 1954- The nuts and bolts of paced ECG interpretation / Tom Kenny. p. ; cm. ISBN 978-1-4051-8404-5 1. Electrocardiography. 2. Cardiac pacing. I. Title. [DNLM: 1. Electrocardiography--methods. 2. Cardiac Pacing, Artifi cial. WG 140 K365n 2009] RC683.5.E5K46 2009 616.1’207547--dc22 2008055897 ISBN: 9781405184045 A catalogue record for this book is available from the British Library. Set in 9.5/12 pt Minion by Sparks, Oxford – www.sparkspublishing.com Printed and bound in the US by Sheridan 1 2009 Contents Preface, vii 22 States of Dual-Chamber Pacing, 55 Before We Start …, 1 23 Maximum Tracking Rate, 57 24 Pacemaker Multiblock, 60 Part I Timing Cycles and Troubleshooting 25 Pacemaker Wenckebach, 63 Review 26 Pacemaker-Mediated Tachycardia, 66 1 Calculating Rates and Intervals, 5 27 Mode Switching, 68 2 Pacing Spikes, 8 28 In Depth: Upper-Rate Behavior in Dual- 3 The Basics of Capture and Sensing, 11 Chamber Pacemakers, 71 4 In Depth: Modes, 14 29 Troubleshooting the Paced ECG, 73 5 Ventricular Sensing, 17 30 More Troubleshooting the Paced ECG, 75 6 Pacing Intervals, 19 31 Automatic Capture Algorithms, 77 7 Loss of Capture, 22 32 Capture Testing, 80 8 Oversensing, 24 33 In Depth: Basic Troubleshooting Guide, 83 9 In Depth: Sensing, 26 34 Navigating the Intracardiac Electrogram, 86 10 QRS Morphologies, 28 35 Tracings from a Programmer (Combining 11 Fusion and Pseudofusion, 31 E-grams with the ECG), 93 12 In Depth: Single-Chamber Timing Cycles, 34 36 Stored Electrograms, 96 13 Intermittent Oversensing, 36 37 In Depth: Electrograms, 99 14 Undersensing, 38 38 Conclusion, 101 15 Hysteresis Intervals, 40 Part II Workbook 16 In Depth: Hysteresis, 42 Introduction to Workbook, 105 17 Rate Response, 44 Easy, 107 18 Annotated ECGs, 46 Moderate, 237 19 AV Synchrony, 49 Tough, 313 20 Atrial Tracking, 51 Scramble, 409 21 AV Conduction, 53 Index, 475 v Preface It seems that every time I start to talk about cardiac Early in my career, I was given a great piece of ad- pacing and how devices work and how clinicians vice with respect to paced ECGs. I was told, “You see can better care for the device patients they see, it only what you look for, you recognize only what you always comes back to the paced ECG. As much as know!” I heard it from one of my fi rst mentors, Dr. we like to talk about advanced device features or Michael Chizner, but the original quotation comes describe the wonders of downloadable diagnostic from W. Proctor Harvey, MD, a pioneer of cardi- reports, I am increasingly convinced that the key to ology and professor of medicine at Georgetown really understanding device behavior is contained University in Washington, D.C. I suspect that many in the paced ECGs. young device specialists struggling to interpret a And yet paced ECGs are surprisingly intimidat- paced ECG heard that same bit of advice. ing, even to clinicians who navigate around non- It is my privilege, as author of this book, to hand paced ECGs all day long. Pacemakers can behave in down this sage bit of advice. Don’t let the simplic- unusual and unexpected ways and even when they ity of the saying deceive you. It’s a powerful insight are behaving themselves appropriately, they can that will help you very well as you master the art and leave a mysterious paced ECG behind for clinicians science of paced ECGs. “You see only what you look to deal with. for, you recognize only what you know!” I wrote this book to supplement the fi rst book By the way, paced ECGs (or any tracing) can be in The Nuts and Bolts series, The Nuts and Bolts of surprisingly controversial. That’s one reason they Cardiac Pacing. That book, recently revised and up- call it ECG interpretation! I have frequently observed dated, contains a lot of good and useful information formal and informal sessions where respected, about how pacemakers work, but it does not take knowledgeable, expert colleagues have vigorously the time to delve thoroughly into the mysteries of disagreed with each other about what a paced ECG the paced ECGs. might actually mean. The truth is, sometimes we do Rather than tack this onto the pacing book, I decid- not know for sure what is going on from an ECG ed to make it a stand-alone volume. Much of the ma- (particularly just one short section of a tracing and terial in this book applies to ICDs with pacing func- viewed apart from the patient). As we developed the tion as well as pacemakers. And because it is its own workbook section of this book, I saw many ECGs volume, it can be as thorough as I wanted it to be. that my colleagues might interpret differently than While some clinicians may cringe at seeing a I do. In the event that you disagree with my inter- paced ECG, I always like to see them. I like them be- pretations, that’s not entirely unexpected! For my cause most of the time, they tell me that a person is part, I tried to provide evidence for my conclusions getting improved quality of life and possibly better so that you can at least follow my thinking. In a few functional capacity because of a tiny implanted de- instances, my interpretation is based on “the most vice. I also like them because they can sometimes be likely” interpretation, which, as we all know, may a bit of a puzzle. Yet by knowing the basics, keeping not always be the true one. it as simple as possible (when in doubt, look at the I must thank my very wonderful editor at Black- most obvious solutions), and evaluating everything well, Gina Almond, for her patience and willingness in a steady, systematic way, most clinicians can learn to share my passion for this admittedly unusual sub- to interpret tracings accurately. ject, and to Kate Newell at Blackwell who does the vii viii Preface magic of putting these books together. This is the to hear about how useful these volumes have been in fi rst time Kate and I have worked together, and it has your own practice. been a pleasure. As usual, thanks to my creative team As always, I am indebted to my family for their who helps translate my ideas into book form. I am patience as they provided encouragement for me indebted to Jo Ann LeQuang (who helps guide the and “another book.” Much of the material for this project from my fi rst phone call when I announce, book has been taken from work done here at St. Jude “hey, I’ve got an idea” through the fi nal printing) Medical and I thank my many colleagues for their and Belinda Kinkade (who is one of the few profes- support and suggestions to make this the best pos- sional artists in the world who can now read paced sible book. ECGs). I would also like to thank all of my colleagues Tom Kenny at St. Jude Medical who have been overwhelmingly May, 2009 supportive of these books and who are always glad Austin, Texas Before We Start … I wanted to start out this book at ground zero! quite clear-cut. Because looks can be deceiving, the Whether you are an old hand at ECG interpretation systematic approach is invaluable. It will help you or brand new to the clinical world, ECG interpre- uncover troublespots in ECGs that look – at a glance tation is a challenging practice that can keep you – to be quite normal and it will give you confi dence studying – and learning – for an entire career! Paced to interpret unusual tracings. ECGs are even more of a challenge since the electri- The fi rst portion of this book is going to train you cal representation of what’s going on in the heart to recognize some of the key landmarks on paced refl ects not only intrinsic activity (which can be dif- rhythm strips. We’ll go over pacing spikes, paced fi cult enough to interpret at times) but also device events, timing cycles, fusion and pseudofusion, and activity. Device activity changes intrinsic activity how to determine pacing rates. This entire fi rst sec- and intrinsic activity changes device activity. It can tion of the book is my attempt to show you the many be exciting to see how devices can restore a heart things that you are going to need to be able to fi nd rhythm, but it does not make interpreting paced and distinguish on a paced ECG. tracings any easier. You’ll learn with visual examples. These fi rst Your job in evaluating a paced ECG is to deter- chapters are roughly arranged in order of diffi culty, mine not only what’s going on but to offer a clini- but it is my experience as a teacher that those rank- cal assessment as to the value of what’s going on. Is ings are subjective. If you fi nd a topic particularly it helping? Could the device be programmed more challenging, stay with it until you learn the basics. appropriately? Is the patient benefi ting from the And with paced ECGs, it always comes back to the therapy as well as he might? Is the device not “see- basics! It’s all about pacing spikes, sensing, capture, ing” all that it should see? and timing cycles. Just keep those four principles in The fundamental rule in paced ECG interpreta- mind. From there, we’ll build up to rates and special tion is this: you see only what you look for, you rec- features. It’s a matter of building from the ground ognize only what you know. up – because you can’t really begin to understand A secondary rule is that you need to approach hysteresis behavior until you understand the basics. any paced ECG systematically. Over the years, I have So please do not rush through the fi rst part of this developed my own system and I am going to infl ict book, and do not hesitate to revisit it periodically as that on my readers in this book. But the actual sys- a refresher. tem itself is not as important as the fact that you take You’ll also see examples of pacemaker features a careful, point-by-point approach to looking at any in action that can complicate the paced ECG. For ECG. Don’t skip over things or look at any ECG hap- instance, it’s not unusual in paced patients to see hazardly. Too much is at stake for you to approach a paced rhythm at a rate above or below the pro- a paced tracing casually. It can be very tempting to grammed base rate! Oversensing and undersensing home in on some unusual aspect of a tracing and are not uncommon on paced rhythm strips. Fusion miss the big picture. That’s why you can’t beat the and pseudofusion have puzzled many clinicians! system when it comes to paced ECGs. And if the patient has high-rate intrinsic atrial ac- Many of the seemingly simple ECGs you encoun- tivity, the pacemaker can demonstrate what we call ter in the clinical setting can turn out to be very “upper-rate behavior” or mode switching, all of challenging, and sometimes an odd-looking ECG is which show up on the ECG. The Nuts and Bolts of Paced ECG Interpretation, 1st edition. By Tom Kenny. 1 Published 2009 by Blackwell Publishing, ISBN: 978-1-4501-8404-5. 2 The Nuts and Bolts of Paced ECG Interpretation Once you know what to look for, you can evaluate by the fact that the paced ECGs in this book look a most paced ECGs without much trouble, because little bit too polished or too “perfect.” We did this so you are going to recognize what you know. you can focus on the basic principles and then apply The second part of the book is my favorite. I’ve them to your real-world clinical practice. found about 100 paced ECGs that I have collected Finally, I want to say that I called this book The over the years. Most of them are not particularly Nuts and Bolts of Paced ECG Interpretation because unusual ECGs. In fact, I’d venture to say that if you “interpretation” is all you can do with an ECG. The worked full-time at a pacing clinic, you’d run into clues or evidence that appears on a tracing has to be the things illustrated in these strips routinely over put together with other factors in order to arrive at the course of a year. I saved these particular strips a “likely explanation” for what appears on the trac- because they illustrate the very things that clini- ing. Tracings don’t lie, but they aren’t always outspo- cians ought to look for (and that means these are ken with the truth, either. I fully expect that you or the things clinicians must know!). They’re assem- other colleagues may occasionally disagree with my bled in a workbook format so you can try your hand interpretation or at least consider a possible ECG at evaluating the paced ECG yourself and then read interpretation that I dismiss. My interpretation is through my systematic analysis on the next pages. just that – my interpretation. But it’s based on many I divided the workbook into sections, rather sub- years of clinical and industry experience. Over that jectively classifi ed as easy, moderate, and tough. Then time, primarily as an educator, I gained a keen ap- at the very end, I offer a section I called “scramble,” preciation of what seems to trip clinicians up when which takes strips of all diffi culty levels and offers confronting a paced ECG. Many of these strips were them in random order. That’s actually the way they selected with that in mind. will be handed to you if you work in a clinic! You just An ECG is just one piece of the puzzle in the care never know what you will see next. of a pacemaker patient and most ECGs can only tell One last caution. For the purposes of this book, us about how the patient and device interacted over we had an artist re-draw all of our ECGs. We did this a few moments of time. Patient symptoms, history, mainly for the sake of allowing you to focus on the and a routine physical exam are all important in principles of paced ECG interpretation rather than the care of a pacemaker patient. Thus, this book is a the vagaries of real-world ECG equipment. In prac- bit lopsided in that you will get my interpretations tice, you’ll see strips at different speeds, in different based solely on a strip – something I would not do sizes, and you’ll see stuff that can be grainy or blurry. in the clinic if the patient was available! In the real world, you’ll see device artifacts and ECGs And remember: you see only what you look for, that seem to want to fl y off the page. Don’t be put off you recognize only what you know!