ST elevation myocardial infarction electrocardiogram
ST Elevation Myocardial Infarction Microchapters
ST elevation myocardial infarction electrocardiogram On the Web
A primary purpose of the electrocardiogram is to detect ischemia or acute coronary injury in broad, symptomatic emergency department populations. Common EKG findings in STEMI include ST segment elevation, new LBBB pattern and hyperacute T waves.
The characteristic ECG changes consistent with STEMI are:
- ST elevation in at least 2 contiguous leads of 2 mm (0.2 mV) in men or 1.5 mm (0.15 mV) in women in leads V2–V3 and/or of 1 mm (0.1mV) in other contiguous chest leads or the limb leads
- ST depression in at least two precordial leads V1-V4 (suggestive of posterior MI)
- ST depression in several leads plus ST elevation in lead aVR (suggestive of occlusion of the left main or proximal LAD artery)
- New left bundle branch block (LBBB)
The 12 lead ECG is used to classify patients into one of three groups:
- Those with ST segment elevation or new bundle branch block (suspicious for acute injury and a possible candidate for acute reperfusion therapy with thrombolytics or primary PCI),
- Those with ST segment depression or T wave inversion (suspicious for ischemia), and
- Those with a so-called non-diagnostic or normal ECG.
A normal ECG does not rule out the presence of acute myocardial infarction. Sometimes the earliest presentation of acute myocardial infarction is instead the presence of a hyperacute T wave. In clinical practice, hyperacute T waves are rarely seen, because they exists for only 2-30 minutes after the onset of infarction. Hyperacute T waves need to be distinguished from the peaked T waves associated with hyperkalemia.
The electrocardiographic definition of ST elevation MI requires the following: at least 1 mm (0.1 mV) of ST segment elevation in 2 or more anatomically contiguous leads. While these criteria are sensitive, they are not specific as thrombotic coronary occlusion is not the most common cause of ST segment elevation in chest pain patients.
ST depression in the anterior leads might either represent reciprocal changes on EKG or might be pathologically caused by either anterior ischemia in the context of a patent artery or posterior infarct due to the complete occlusion of a coronary artery.
Shown below is a table depicting the interpretation of ST elevation and ST depression by the involved contiguous leads.
|V1-V2|| Septal MI, or
Right ventricular MI
|Lateral MI||Left circumflex artery|
|II-III-aVF||Inferior MI|| RCA in 85% of the cases|
Left circumflex artery in 15% of the cases
|V1-V2- V4R||Right ventricular MI||RCA|
|V1-V2-V3|| Anterior ischemia, or
Posterior MI , or
| LAD |
RCA or left circumflex artery
Copyleft image obtained courtesy of ECGpedia, http://en.ecgpedia.org/wiki/File:AMI_evolutie.png
Copyleft image obtained courtesy of ECGpedia,http://en.ecgpedia.org/wiki/Main_Page
Copyleft image obtained courtesy of, http://en.ecgpedia.org/wiki/Main_Page
Copyleft image obtained courtesy of, http://en.ecgpedia.org/wiki/Main_Page
For more EKG examples of ST elevation myocardial infarction click here
Limitations of the 12 lead ECG
While clinicians are widely familar with the 12 lead ECG, there are limitations to its sensitivity and specificity in detecting myocardial injury due to thrombotic vessel occlusion. A single static ECG represents a brief sample in time. In STEMI, the culprit artery is often opening and closing or "winking and blinking" due to cyclic flow variations with the clot dissolving and reforming again. Furthermore, patients with acute coronary syndromes often have rapidly changing supply versus demand characteristics. As a clinical example, one of the authors (CM Gibson) once saw a patient with 6 hours of chest pain who had non-specific ECG changes. 15 minutes later the patient complained of worsening chest pain, and at that time the ECG showed ST elevation. Had the second ECG with ST elevation been instead the first ECG that was reviewed, it would have been concluded that the patient was infarcting for over 6 hours, when instead, the artery was occluded for 15 minutes at most during the recent episode of chest pain. Because a single ECG may not accurately represent evolving myocardial injury, it is standard practice to obtain serial 12 lead ECGs, particularly if the first ECG is obtained during a pain-free episode. Alternatively, many emergency departments and chest pain centers use computers capable of continuous ST segment monitoring. While the standard 12 lead ECG is well designed to detect anterior ischemia, by virtue of the fact that it samples only electrical vectors from the front left chest wall, the 12 lead ECG may miss right ventricular and posterior infarctions. In particular, acute myocardial infarction in the distribution of the circumflex artery is likely to produce a nondiagnostic ECG or ST segment depression in the anterior precordial leads. The use of non-standard ECG leads like right-sided lead V4R and posterior leads V7, V8, and V9 may improve sensitivity for right ventricular and posterior myocardial infarction. Newer technologies such as the 80 lead ECG have demonstrated greater sensitivity in detecting RV and posterior infarcts compared with the 12 lead ECG. The failure to identify patients with ST elevation MI delays care and has a negative effect on the quality of patient care.
Localization of the Culprit Artery Based Upon the 12 lead ECG
While the ECG leads that are involved with ST elevation and depression are often used to predict the potential location of the culprit artery, the sensitivity and specificity of these techniques are often poor. New technologies such as 80 lead ECGs may prove to be more useful in this regard. Identification of the potential culprit artery can be important in guiding patient management. In particular, the ECG should be used to identify patients with a right ventricular infarct where nitrate administration is contraindicated. The 12 lead ECG can also be used to identify the most appropriate artery to perform angiography on first when performing primary angioplasty.
|Wall Affected||Leads Showing ST Segment Elevation||Leads Showing Reciprocal ST Segment Depression||Suspected Culprit Artery|
|Septal||V1, V2||None||Left Anterior Descending (LAD)|
|Anterior||V3, V4||None||Left Anterior Descending (LAD)|
|Anteroseptal||V1, V2, V3, V4||None||Left Anterior Descending (LAD)|
|Anterolateral||V3, V4, V5, V6, I, aVL||II, III, aVF||Left Anterior Descending (LAD), Circumflex (LCX), or Obtuse Marginal|
|Extensive anterior (Sometimes called Anteroseptal with Lateral extension)||V1,V2,V3, V4, V5, V6, I, aVL||II, III, aVF||Left main coronary artery (LCA)|
|Inferior||II, III, aVF||I, aVL||Right Coronary Artery (RCA) or Circumflex (LCX)|
|Lateral||I, aVL</sub>, V5, V6||II, III, aVF||Circumflex (LCX) or Obtuse Marginal|
|Posterior (Usually associated with Inferior or Lateral but can be isolated)||V7, V8, V9||V1,V2,V3, V4||Posterior Descending (PDA) (branch of the RCA or Circumflex (LCX))|
|Right ventricular (Usually associated with Inferior)||II, III, aVF, V1, V4R||I, aVL||Right Coronary Artery (RCA)|
Evolution of ST Segment Elevation
As the myocardial infarction evolves, there may be loss of R wave height and development of pathological Q waves. T wave inversion may persist for months or even permanently following acute myocardial infarction. Typically, however, the T wave recovers, leaving a pathological Q wave as the only remaining evidence that an acute myocardial infarction has occurred. Understanding the typical time course of ST changes in acute MI is critical in distinguishing STEMI from pericarditis and other conditions.
|minutes|| hyperacute T waves (peaked T waves)
|hours|| ST-elevation, with terminal negative T wave
negative T wave (these can last for months)
|days||Pathologic Q Waves|
Measurement of the Magnitude of ST Elevation: 60 Milliseconds after the J point
The optimal time after the J point to measure ST elevation is debated. This example shows the technique of measuring the magnitude of ST elevation 60 milliseconds or 1.5 small boxes after the J point.
Distinguishing Early Repolarization and Other Normal Variants from Pathologic ST Elevation
Early repolarization is an electrocardiographic pattern that can mimic the changes of ST elevation myocardial infarction. It is often seen in younger males. As the example below shows, there is often a notch (shown in red) where the QRS and ST segments join.
Copyleft image obtained courtesy of ECGpedia, http://en.ecgpedia.org/wiki/Main_Page
Diagnosing STEMI in the Setting of Left Bundle Branch Block
Ordinarily in the setting of left bundle branch block, the T waves are inverted in the lateral leads (they are 'discordant'). If they become upright, they are called 'concordant', and concordant T waves can be observed in ST elevation MI. The EKG below was obtained in a patient with a new left bundle branch block (LBBB) and a totally occluded left anterior descending artery.
2013 Revised and 2004 ACCF/AHA Guideline for the Management of ST-Elevation Myocardial Infarction (DO NOT EDIT)
|"1. Performance of a 12-lead electrocardiogram (ECG) by emergency medical services personnel at the site of first medical contact (FMC) is recommended in patients with symptoms consistent with STEMI (Level of Evidence: B)"|
|"2. A 12-lead ECG should be performed and shown to an experienced emergency physician within 10 minutes of ED arrival on all patients with chest discomfort (or anginal equivalent) or other symptoms suggestive of STEMI.(Level of Evidence: C)"|
|"3. If the initial ECG is not diagnostic of STEMI but the patient remains symptomatic, and there is a high clinical suspicion for STEMI, serial ECGs at 5- to 10- minute intervals or continuous 12-lead ST-segment monitoring should be performed to detect the potential development of ST elevation.(Level of Evidence: C)"|
|"4. In patients with inferior STEMI, right-sided ECG leads should be obtained to screen for ST elevation suggestive of RV infarction.(Level of Evidence: B)"|
- 2013 Revised ACCF/AHA Guideline for the Management of ST-Elevation Myocardial Infarction
- The 2004 ACC/AHA Guidelines for the Management of Patients With ST-Elevation Myocardial Infarction 
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