2Essential 12-Lead ECG Interpretation GoalsRecognize and localize AMI on the ECGFeel comfortable with 12-lead interpretationNote to the speaker: This module has been structured to instill confidence and dispel concerns regarding the difficulty of 12-lead ECG interpretation. While many aspects of 12-lead interpretation do require much study and practice, it is a simple matter to identify the pattern of ST segment elevation produced by AMI.After a brief review of essential terminology, participants move directly into AMI recognition and localization. In fact, participants will be able to recognize and localize myocardial infarction approximately 30 minutes into this module! A fact that may be worthy of mention at the onset of your presentation.
312-Lead ECGThis is an example an “in-hospital” 12-lead ECG. Diagnostic 12-lead ECGs taken in the field obtain the same information, but are often in a slightly different format. Let’s compare a typical “in-hospital” 12-lead to a field 12-lead ECG.
412-Lead ECGAt the bottom of this 12-lead are the rhythm strips (highlighted). In this case three leads are seen, sometimes there are fewer. Any of the 12-leads can be shown as rhythm strips, the users define which leads they want to see.In the field you probably will obtain the rhythm strips first, and will see three leads simultaneously. You can configure the device to show you any of the six limb leads on the rhythm strip (I, II, III, aVR, aVL or aVF).
512-Lead ECGThe format of the 12-lead ECG is very standard. While there are a few exceptions, the format you see here is typical of what you will see in most 12-lead ECGs done in North America.
612-Lead ECGThe 12-lead can provide a computer generated interpretation. The computer’s interpretive algorithm is designed to favor “specificity”. In other words, when the machine says “ACUTE MI SUSPECTED” it needs to always be right. The program is almost perfect in that regard, and when you see “ACUTE MI SUSPECTED” the machine is right about 98% of the time.However, in order to attain that specificity, if the computer isn’t absolutely sure that an AMI is present, it will not say anything about it. Depending on the version of the software in your 12-lead machine, the computer may miss as many as one third of the cases where AMI could be suspected on the ECG.In other words YOU are the primary interpreter, the computer is your backup.
712-lead ECGThe computer is very good at measuring intervals and durations. For example, it is actually much better than we are at measuring the PR-interval and the QRS width.This information is always provided, however, it may be expressed in unfamiliar terms.While we typically express these intervals and durations in seconds, the 12-lead expresses them in milliseconds. It is a simple matter to convert milliseconds to seconds.
880 milliseconds = 0.08 seconds 12-Lead ECG080.00.08080 milliseconds = 0.08 secondsTo convert seconds to milliseconds simply move the decimal three places to the left.If the QRS is measured by the 12-lead as 80 milliseconds, that equals 0.08 seconds.Question: If the 12-lead measures the PR interval at 160 milliseconds is that within normal limits?Answer: Yes, it’s the same as 0.16 seconds.
912-Lead ECGSome of you may be wondering how anyone can make a sound interpretation with only 2.5 seconds shown in each lead.Ask the group to look at the highlighted beat in the lead two rhythm strip, ask them to determine the rate and rhythm. Of course it cannot be done.As you well know, when you use an ECG to determine the cardiac rate and rhythm, certain sampling time is required. Usually, at least a six second tracing is necessary, and complex rhythms may require even more sampling time.However, do not be intimidated by the short sampling time in each of the 12-leads. What is different about 12-lead interpretation is this: Only one beat from each lead is needed to make an interpretation. The 2.5 seconds in each lead is usually long enough to capture one good, representative beat.Recognition of AMI involves analyzing the shape of one beat in each lead. With that in mind let’s look at the shape of the waveforms in each lead.
10R Wave R wave: The first positive deflection. No matter where it occurs in the complex, the first positive deflection is called the R wave.The R wave includes not only the upstroke of the positive deflection, but the downstroke returning to the baseline as well.
11Q Wave Q Wave: A negative deflection preceding the R wave. If there is any negative deflection in front of the R wave, it is labeled the Q wave.The Q wave includes the negative down stroke and the return to baseline.
12S Wave S wave: A negative deflection following the R wave. Like the Q wave and the R wave, the S wave includes both the departure from and return to the baseline.
13QRS Q waves Physiologic Q waves Pathologic Q < .04 sec (40ms) Q waves can occur normally in several leads (I, III, aVL, aVF, V5, V6). These Q waves are called physiologic Q waves.Physiologic Q waves normally do not exceed 30ms.Pathology (including myocardial infarction) can place a Q wave in any lead.It is possible to examine the Q wave and, based upon its width, speculate whether it is pathologic or physiologic.
14QRSQ waveWhen a Q wave is noted in any lead, always measure its width. Red dots indicate where the Q wave is measured.A physiologic Q wave is very narrow, usually less than 30ms (0.03 seconds).A Q wave is considered pathologic when it equals or exceeds 0.04 seconds. (one small box on the ECG grid)
15QS ComplexWhen the entire complex is negatively deflected, it is called a QS complex.A QS complex is considered equivalent to a wide Q wave.NOTE: When an rS complex is present, students often overlook the small R wave and erroneously label the complex a QS. It will be helpful to point out the difference while reviewing some of the early ECGs.
16J-PointJ-Point: The junction between the end of the QRS and the beginning of the ST segment.The J-point is found by looking for the point where the QRS stops and makes a sudden sharp change of direction.You may opt to point out to the students that they have already found hundreds of J-points. Every time they have ever measured the width of a QRS, they have found the J-point. While they may not have labeled it as such, the end of the QRS complex is the J-point.
17ST SegmentST segment: The ECG segment between the J-point and the beginning of the T wave.The ST segment is probably the single most important element to identify on the ECG when looking for evidence of AMI.
18Practice Find J-points and ST segments EXERCISE: Have participants turn to of the course guide and locate the J-Point and ST segment in each of the first three complexes. They are not yet concerned with ST elevation or depression, simply identifying the J-point and ST segment.Review the first three complexes with the group.Continue the exercise with the next three complexes, then review them with the group.
19Practice Find J-points and ST segments Review J-points and ST segment with group with the group.
20ST Segment Compare to TP segment ST TP In order to determine if the ST segment is elevated it is necessary to have a reference point.The TP segment is the best reference to the isoelectric line.Do not compare the ST segment to the PR segment because the PR can be depressed, giving the illusion of ST segment elevation.STTP
21ST Segment AnalysisThis time find not only the J-point, but determine if the ST segment is elevated one millimeter or more above the TP segment.#1 No#2 Yes#3 Yes#4 No#5 Yes#6 No
2212-Lead ECG AMI recognition Two things to know What to look for Where you are lookingThe first level of 12-lead interpretation is simply a matter of knowing two facts:1. What changes an AMI can place on the 12-lead, and2. Knowing which part of the heart that each lead “sees.”Lets look at each in more detail.
23AMI Recognition What to look for ST segment elevation One millimeter or more (one small box)Present in two anatomically contiguous leadsST segment elevation, measured at the J-point, of 1mm or more is considered an abnormal finding.When that elevation is found in at least two anatomically contiguous leads, it is considered presumptive evidence of AMI.NOTE: The concept of anatomically contiguous leads is simple, but may be difficult to explain. Essentially it means two leads looking at adjoining areas of tissue. The difficulty comes when trying to determine which leads are contiguous with other leads. Here is one explanation:If two leads have the same name (i.e., lateral or inferior) they are contiguous. Also, in the chest leads, if they are numerically consecutive, they are also contiguous. For example V2 is called a septal lead, and V3 an anterior lead, but they are anatomically contiguous.
24ST Segment Elevation Presumptive evidence of AMI Indication for acute reperfusion therapyWith an appropriate clinical presentation, ST segment elevation is presumptive evidence of acute myocardial infarction.These patients benefit from immediate reperfusion, usually in the form of a thrombolytic drug or PTCA (Percutaneous Transluminal Coronary Angioplasty).NOTE: In later modules of this program the participant will be introduced to the full spectrum of Acute Coronary Syndromes (ACS). You may opt to acknowledge that other criteria for reperfusion will be added in later in the program. However, it is appropriate to begin with ST segment elevation.
25Practice EXERCISE: approximately 2 minutes Instructions: Review the 12-lead ECG.Go lead by lead, and pick one good complex in each lead.Find the J-point and ST segment.Compare the ST to the TP segment, looking for 1mm (one small box) of elevation (ignore ST depression for now).Place a checkmark next to any lead with 1mm of ST segment elevation.Review findings with group, pointing out every J-point and ST segment. Note leads II, III and aVF display elevation.Remember ST segment elevation is presumptive evidence for AMI. Knowing which part of the heart leads II, III and aVF “sees” would tell you where the infarct is located.
26Lead “Views”Each lead has one, and only one, positive electrode. We can think of the positive electrode as a camera or an eye. The view is from the positive electrode toward the negative electrode. The portion of the left ventricle that each leads “sees” is determined by the location of that positive electrode on the patient’s body.Different placements of the electrodes will yield different viewpoints.There are six positive electrodes on the chest, yielding six leads.There are four electrodes on the limbs from which the ECG machine makes another six leads.
27Lead Groups I aVR V1 V4 II aVL V2 V5 III aVF V3 V6 Limb Leads Chest Leads
28Lead “Views” Here’s what they look like on a patient. The ECG machine simultaneously derives the 12-leads from the various positive electrodes.Let’s discuss which part of the heart each lead “looks” at.
29Inferior Wall II, III, aVF Left Leg I II III aVR aVL aVF V1 V2 V3 V4 The positive electrode for leads II, III, and aVF is attached to the left leg. The ECG monitor uses this one electrode as the positive electrode for all three leads.From that perspective, these leads “look up” and “see” the inferior wall of the left ventricle.NOTE: A heart model is helpful at this juncture, particularly to remind students that the heart does not sit “straight up” in the chest.
30Inferior Wall I II III aVR aVL aVF V1 V2 V3 V4 V5 V6 Inferior Wall NOTE: This is a posterior view of the heart.The portion of the heart that rests on the diaphragm is called the “inferior wall”.Leads II, III, and aVF, “look” up and see the inferior wall.When ST segment elevation is noted in II, III and aVF, suspect an inferior infarction.Inferior Wall
31Lateral Wall I and aVL Left Arm I II III aVR aVL aVF V1 V2 V3 V4 V5 V6 Leads I and aVL share the positive electrode on the left arm.From the perspective of the left arm, these leads “see” the lateral wall of the left ventricle.
32Lateral Wall V5 and V6 Left lateral chest I II III aVR aVL aVF V1 V2 V5 and V6 are positioned on the lateral wall of the left chest which is why these two leads also “see” the lateral wall of the left ventricle.
33Lateral I, aVL, V5, V6 I II III aVR aVL aVF V1 V2 V3 V4 V5 V6 Lateral WallIIIIIIaVRaVLaVFV1V2V3V4V5V6Portions of the lateral wall are shown here from both the anterior and posterior perspective.Leads I, aVL, V5 and V6 “see” the lateral wall. When ST segment elevation is seen in these leads, consider a lateral wall infarction.
34Anterior Wall V3, V4 Left anterior chest I II III aVR aVL aVF V1 V2 V3 The positive electrode for these two leads is placed on the anterior wall of the left chest. This correlates to their designation as anterior leads.
35Anterior Wall V3, V4 I II III aVR aVL aVF V1 V2 V3 V4 V5 V6 Of course, ST segment elevation in V3 and V4 implies an anterior wall infarction.
36Septal Wall V1, V2 Along sternal borders I II III aVR aVL aVF V1 V2 V3 These leads are positioned one on each side of the sternum. From that placement they “look through” the right ventricle and “see” the septal wall.NOTE: The septum is left ventricular tissue.
37Septal V1,V2 I II III aVR aVL aVF V1 V2 V3 V4 V5 V6 V1 and V2 “look through” the right ventricle to “see” the septum.
38AMI Localization I II III aVR aVL aVF V1 V2 V3 V4 V5 V6 Anterior: V3, V4Septal: V1, V2Inferior: II, III, AVFLateral: I, AVL, V5, V6This represents the 3x4 format of the 12-lead ECG.Each box represents one lead, and the viewpoint of that lead is indicated.
39AMI Recognition I Lateral aVR II Inferior aVL Lateral III Inferior V1 SeptalV4 AnteriorII InferioraVL LateralV2 SeptalV5 LateralIII InferioraVF InferiorEach box represents one lead, and the viewpoint of that lead is indicated.NOTE: Refer participants to their pocket card where this information is summarized as well.V3 AnteriorV6 Lateral
40AMI Recognition Know what to look for Know where you are looking ST elevation> 1mmTwo contiguous leadsKnow where you are lookingUse pocket card as a referenceYou will soon have this memorized
41Practice Instructions: Review the 12-lead ECG . Go lead by lead, and pick one good complex in each lead.Find the J-point and ST segment.Compare the ST to the TP segment, looking for 1mm (one small box) of elevation (ignore ST depression for now).Place a check mark next to any lead with 1mm of ST segment elevation.Localize the area of infarction.Acute inferior wall infarction
42Practice Instructions: Review the 12-lead ECG. Go lead by lead, and pick one good complex in each lead.Find the J-point and ST segment.Compare the ST to the TP segment, looking for 1mm (1 small box) of elevation (ignore ST depression for now).Place a check mark next to any lead with 1mm of ST segment elevation.Localize the area of infarction.Acute antero-septal wall infarctionAfter reviewing this ECG, check in with the group. Press to see if everyone feels comfortable with the information presented to this point. If so, proceed to the next topic. The broader pattern of ECG changes are produced by infarction.
43Evolution of AMI Hyperacute While ST segment elevation is presumptive evidence of an AMI, there is a whole spectrum of ECG changes associated with AMI.If one were to monitor the heart as a coronary artery became occluded, the earliest ECG change suggestive of AMI would be the T wave becoming tall and peaked. This is referred to as a HYPERacute change, because it may occur so early as to actually precede clinical symptoms.Of course hyperacute T waves are seen only in leads “looking” at the infarcting area.Note: True hyperacute T waves are identified not only by their height, but by their shape as well. However, participants often wonder “how tall is tall”?As a rule of thumb T wave height can normally be up to five millimeters in the limb leads and ten millimeters in the chest leads.
44Evolution of AMIAcuteST segment elevation is the next probable ECG change.ST segment elevation implies at least three things:1. Myocardial tissue injury is presently occurring.2. This injury is probably due to an occluded coronary artery.3. Unless corrected, this condition will lead to tissue necrosis.Therefore, even though necrosis has not yet occurred, we say that ST segment elevation is “presumptive evidence” of AMI.Final point: when the ST segment is elevated we assume that the infarct is acute rather than old.
45Evolution of AMIAcuteNote that the ST segment is elevated. We therefore assume that the infarct is acute (occurring right now).However, notice the presence of a Q wave that is at least 40 milliseconds wide. This pathologic Q wave is associated with cellular necrosis.ECG evidence now suggests that some myocardium is only injured and can still be saved (ST elevation) while some other portions of the myocardium may have already become necrotic (Q wave).It is critical to recognize and consider this pattern as acute and target this patient for reperfusion therapy.NOTE: When Q waves first form, the tissue may not yet be necrotic and, with prompt reperfusion, the Q waves may disappear.
46Evolution of AMI Age undetermined Note the presence of a wide Q, and the absence of ST segment elevation. This ECG pattern is associated with a previous MI.It is not possible to determine when this infarct may have occurred, so it is described as “age undetermined” rather than “an old MI”.Look for these additional changes associated with AMI as we practice infarct recognition and localization, but remember, the most important finding is ST elevation in two contiguous leads.
47A normal 12-lead ECG DOES NOT rule out AMI AMI RecognitionA normal 12-lead ECG DOES NOT rule out AMIIt is critical to remember that a patient may indeed be experiencing an acute myocardial infarction even though ST segment elevation is not present on the ECG.Never make the mistake of presuming that a single 12-lead rules out AMI.
48Practice Instructions: Review the 12-lead ECG. Go lead by lead, and pick one good complex in each lead.Find the J-point and ST segment.Compare the ST to the TP segment, looking for 1mm (one small box) of elevation (ignore ST depression for now).Place a check mark next to any lead with 1mm of ST segment elevation.Localize the area of infarction.Antero-septal
49Practice Instructions: Review the 12-lead ECG. Go lead by lead, and pick one good complex in each lead.Find the J-point and ST segment.Compare the ST to the TP segment, looking for 1mm (one small box) of elevation (ignore ST depression for now).Place a check mark next to any lead with 1mm of ST segment elevation.Localize the area of infarction.Extensive anterior (septal + anterior + lateral)
50Practice Instructions: Review the 12-lead ECG. Go lead by lead, and pick one good complex in each lead.Find the J-point and ST segment.Compare the ST to the TP segment, looking for 1mm (one small box) of elevation (ignore ST depression for now).Place a check mark next to any lead with 1mm of ST segment elevation.Localize the area of infarction.Inferior wall infarctionNote ST depression in I, AVL and V1-V3.Lets talk about one cause of ST depression, known as reciprocal ST depression.
51Reciprocal ChangesWe have been looking for infarct based upon the presence of ST elevation.As mentioned, not every lead is elevated when AMI is present, only the leads looking at the infarct site.In fact, those leads which look at the infarct site from the opposite perspective tend to produce the opposite changes.When a lead “sees” the AMI directly, the segment becomes elevated in that lead.However, when a lead “sees” the infarct from the opposite perspective, the ST segment may be depressed in that lead.
52Reciprocal Changes II, III, aVF I, aVL, V leads Because of the way the leads are oriented on the patients body, II, III and aVF are on the bottom looking up.All the other leads are on the top, looking in.Therefore, when AMI produces elevation in II, III, and aVF, it also tends to produce depression in the opposing leads.
53Practice Instructions: Determine which leads show ST elevation. Which leads show ST depression.Localize the area of infarction.Determine if a reciprocal pattern exists.ST elevation exists in II, III and aVF. ST depression in I and aVLDoes it fit the reciprocal pattern? Yes.NOTE: Not every lead on each side of the seesaw must be elevated or depressed in order to assume reciprocal changes. Rather it is more a matter of at least some leads on one end of the seesaw being elevated and some being depressed.
54Practice Instructions: Determine which leads show ST elevation. Which show ST depression.Localize the area of infarction.Determine if a reciprocal pattern exists.Here the elevation is in leads I, aVL, V1-V5And the depression is in leads II, III and aVFExtensive anterior infarction, with reciprocal depression
55AMI Recognition Reciprocal changes Not necessary to presume infarction Strong confirming evidence when presentNot all AMIs with ST elevation produce reciprocal depression.Quite simply… some do and some don’t.When reciprocal depression is noted, the likelihood of AMI is dramatically increased.
56AMI RecognitionObvious ST elevation is seen in multiple leads. However, in this patient no infarct was present.This demonstrates the fact that conditions other than AMI can place ST segment elevation on the ECG.In later portions of the program we will learn to identify these conditions and discuss their significance in treatment decision making.Realize that we are still “treating” the clinical presentation. If you suspect AMI based upon clinical presentation, get a 12-lead. If that 12-lead has ST elevation the patient is a potential candidate for early reperfusion.Understand that some of the patients you think should receive immediate reperfusion may later be determined not to be infarcting at all. Don’t let that discourage you. We will address this issue in more detail later in the program.
57AMI Recognition Imitators of infarct LVH BBB Ventricular beats PericarditisEarly RepolarizationOthersHere are some of the imitators that can produce ST elevation.The 12-lead computer interpretation will often identify their presence. However,THE PRESENCE OF AN IMITATOR ON THE ECG DOES NOT RULE OUT A SIMULTANEOUS MYOCARDIAL INFARCTION.Therefore, any patient with a clinical presentation suggestive of AMI and ST elevation should be rapidly evaluated for early reperfusion therapy.In other words, take care to avoid the mistake of mentally ruling out infarct based upon the presence of an imitator.
58Summary AMI recognition Know what you are looking for 1mm of ST elevationTwo contiguous leadsKnow where you are lookingPositive electrode as an “eye”Pocket card
59Summary Reciprocal changes Not necessary to presume infarction Strong confirming evidence when present
60Summary ST segment elevation is presumptive evidence for AMI Other conditions may also cause ST elevationWhen clinical presentation suggests AMI and the 12-lead shows ST segment elevation, proceed as if the patient were infarcting.However, providers must be aware that there may be ST segment elevations and may not be diagnosed with AMI.
61A normal 12-Lead ECG DOES NOT rule out AMI SummaryA normal 12-Lead ECG DOES NOT rule out AMIA patient may indeed be experiencing an acute myocardial infarction even when ST segment elevation is not present on the ECG.Never make the mistake of presuming that a single 12-lead rules out AMI.
62ACS AMI is part of a spectrum of disease know as the Acute Coronary SyndromesIn future modules the full spectrum of disease know as the Acute Coronary Syndromes (ACS) is discussed in detail. For now, awareness of ACS as an entity is sufficient.Continue in the lab section for review and practice of the topics discussed in Module 1.