Ask the Clinical Instructor

A Q&A column for those new to the cath lab Questions are answered by: Todd Ginapp, EMT-P, RCIS, FSICP Todd is the Cardiology Manager for Memorial Hermann Southeast in Houston, Texas. He also teaches an online RCIS Review course for Spokane Community College, in Spokane, Washington, and regularly presents with RCIS Review Courses. I have attended 2 formal 12-lead ECG interpretation classes over the last 3 years, but I can’t seem to remember everything. How can I apply this information to my job? — RCIS Review Online student I understand your concern. When I first took 12-lead classes, I too had the same problem. I finally realized it was because I wasn’t able to fully use every day what I learned in the class. I found myself forgetting some of the things I thought I knew because I was trying to apply it only every few months. With 12-leads, unless you apply everything that you learned every day at work, you are likely to forget most of the information. I also realized that when I applied only what I needed to know for my job, that I became proficient and comfortable using a 12-lead in required circumstances. The purpose of this article is to share how you can apply the fundamental basics of 12-lead interpretation to prepare you for ST-elevation myocardial infarction (STEMI)/acute myocardial infarction (AMI) patients. Understanding where the blockage is can help prepare you with anticipating the equipment and modalities needed for your patient, which can reduce the amount of time required to accomplish the intervention. First, I would be remiss if I did not state that this article cannot substitute for a more formal education on, or a broader understanding of, 12 lead and its use in cardiology. This article will be very narrow in scope, focusing on how to apply the interpretation to your daily activities. All staff members should take a formal 12-lead interpretation class early in their career, so that they have the basic overall understanding of how 12 leads work. Regular refreshers or book reviews are necessary if you do not use the interpretation skills every day. Some recommendations for study materials are at the end of the article. Let’s refresh basic coronary artery and cardiac anatomy (Figure 1). First, the left anterior descending (LAD) artery does just that. The LAD comes off the left main (sometimes called the left main coronary artery) and descends down the intraventricular groove on the front part of the heart to the apex. The circumflex artery (sometimes abbreviated as Cx) branches off the left main and circles around the heart laterally in the atrioventricular groove. The right coronary artery (RCA) comes off the right side of the aorta and circles down the right side of the heart to the back/bottom of the heart in the atrioventricular groove. The posterior descending and posterior lateral arteries split off the RCA at the point of the “crux” and feed the bottom part of the heart. Each of these arteries covers a section of the myocardium, much as sprinkler systems cover certain areas of a lawn. There are overlaps where their distant ‘sprays’ meet, but each covers main areas. As a result, 12 leads may not indicate an infarct location as precisely as what we will present in this article, since the infarct can sometimes be spread over a large area of myocardium. Table 1 lists areas of the heart that can be affected by a blockage. Now that we know the anatomy of the heart, how do we “look” at these areas? How do we accomplish that through an ECG so that we can correlate it to the myocardium? Figure 2 represents the placement of electrodes during a 12-lead procedure. It is beyond the scope of this article to delve into the electrical concepts of each lead. Think of the 12-lead ECG as surrounding the heart with electrodes and “looking in” at the heart. The limb leads (RA, RL, LA, LL) combine to make the I, II, III, aVR, aVL and aVF leads. The “V” leads look into the heart from different vectors. Once combined on paper, they help us determine areas of trouble within the myocardium. The 12 lead is structured so that you can identify which areas show ST (see Table 1). You will have to remember these areas, but as you will discover, they are “grouped” in a logical manner. You will scan the ECG and look for obvious ST elevations. This is the area after the R wave and before the T wave. When elevated above the ECG baseline, an ‘ST elevation’ exists. Sometimes, the ST elevation is so pronounced that you cannot differentiate between the R wave and the T wave. We call these “tombstones” (Figure 3) and once you see it in an acutely ill patient, you will likely not forget it. “Tombstones” require that the staff move into the next gear to speed up the treatment process. How does ST elevation occur? Getting into specific electrical theory is beyond the limitations of this article. However, in general, ST elevations occur when there is a section of the myocardium injured as a result of oxygen deprivation. Due to this lack of oxygen, the injured myocardium cannot repolarize as it should, and as the cells around it continue to do. Since the injured myocardium stays slightly more positive than the surrounding tissue, an ST elevation occurs. When you receive a 12-lead, scan it and look for elevations. You will see them in groupings. Figure 4 shows groupings which have been overlaid on a normal 12-lead ECG. Be sure to compare the color-coded shadings to the colored text in Table 1. Once you notice how the groupings apply to the surface areas of the heart, you can tell the difference. Figure 5 demonstrates how one student first began to memorize where a blockage might be occurring based upon the 12 lead. In Figure 6, the elevations are in leads II, III and aVF. This is due to a blockage in the RCA. In these cases, anticipate bradycardic rhythms due to the SA and AV nodes being supplied by the RCA. Atropine and transvenous pacing equipment should be ready. More often than not, it may also require aggressive use of the AngioJet or other thrombectomy device. You would also know that you need a right-sided guiding catheter, such as a JR4. In these cases, opening a JR4 guide instead of a JR4 diagnostic catheter should be considered, as it can result in door-to-balloon time savings. In Figure 7, the elevations are in lead V1, V2, V3 and V4. Since these leads “look” at the front part of the heart, we can deduce that the blockage is in the LAD. You would know in this case that you need a left-sided guiding catheter, such as a JL4 or XB, etc. These patients may also need an intra-aortic balloon pump (IABP) due to the large involvement of the supply to the left ventricle. Having it ready and in the room based upon what you see from the initial 12-lead ECG will only save time. In Figure 8, there are elevations in the lateral, or V5 and V6, leads, indicating a circumflex blockage. In these cases, you still need a left-sided guiding catheter. It is important to revisit the fact that not all occlusions result in such clear-cut 12-lead changes. Many times, the occlusions affect blood flow to numerous regions of the heart, so that the ST elevations cross over to other ‘territories.’ With experience and the application of these principles, you will be able to decipher the ECG in order to determine the likelihood of the blockage. You may see situations in which you see elevations in leads II, III and aVF, only to find out it is a circumflex blockage because the patient is in the left-dominant group (a discussion for another article). There are times that the ECG cannot be relied upon to tell you what is happening. In cases of left bundle branch block (LBBB), the conduction defect through the left bundle and resultant widening of the QRS complex can hide any ST elevation that might be present (Figure 9). American Heart Association (AHA)/ American College of Cardiology (ACC) standards indicate that someone arriving with the signs and symptoms of an AMI and a new onset of LBBB will be treated as a STEMI until proven otherwise. As we mentioned, there are times when the blockage will cover 2 (or more) territories. Figure 10 shows an inferior-lateral infarction. Note that the ST elevations appear in II and III (RCA territory) as well as V4, V5 and V6 (Cx territory). In this case, the blockage was in the circumflex in a left-dominant patient (again, a discussion for another article) so that the posterior descending and posterior lateral arteries were also occluded, resulting in the inferior ischemic changes. In this case, anticipate a blockage on the “left” side and pull the appropriate equipment. Finally, sometimes there is just too much going on to isolate the specific location of the problem (Figure 11). If you are an expert in 12-lead interpretation, please help your co-workers apply these principles to start their development. If you struggle with 12-lead interpretation, use these principles as a means to prepare your room and equipment for what you might face. There are also many, many books available that assist in the understanding and application of 12 leads. My favorite reading of late has been 12-Lead ECG — The Art of Interpretation from Thomas B. Garcia, MD, FACEP and Neil Holtz, BS, EMT-P, RCIS (ISBN 0-7637-1284-1). While their materials are available on CD and print, I prefer the book. I have it sitting on my table and I can read a few pages at a time, get a quick refresher, and put it down. The next time I pick it up, I don’t feel as if I have to re-read the previous material. It is broken up into sections that make it easy to read a few pages or a few chapters at a time. The authors also have an excellent website for materials and self-assessment at www.12lead ECG.com. Acknowledgements: I need to thank my 17-year-old son, Matt, for not being too modest in helping with one of the pictures in this article. Next month, we’ll address a question about anticoagulants in the cath lab.

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