The 2013 ACCF/AHA ST-segment elevation myocardial infarction guidelines state, for example, to improve recognition of symptoms of a cardiac event episode; ambulance personnel are advised to perform a live ECG on patients for faster (risk) stratification and more rapid treatment.
1. Three major features of AMI ECG
The ECG features of AMI are the presence of necrotic Q waves, injury-type ST-segment elevation and ischemic T-wave evolution. In the clinical electrocardiographic diagnosis, the diagnosis of AMI is often made based on these three changes.
(1) Presence of necrotic Q waves
AMI mainly occurs in the subendocardial myocardium, so that the initial 30-40ms QRS vector deviates from the necrotic area, and abnormal Q waves appear in the leads of the infarcted area. Generally speaking, the width and depth of the Q wave reflect the depth of AMI; the wider and deeper the Q wave, the deeper the myocardial necrosis; the smaller the Q wave, the more superficial the myocardial necrosis.
(2) Injury-type ST-segment elevation
In the leads of the necrotic area, ST-segment elevation is injury-type, forming a “one-way curve”, which is the most diagnostic feature of AMI. Clinically, ST-segment depression of ≥50% after thrombolysis is used as one of the indicators of coronary revascularization.
(3) Ischemic T-wave evolution
The typical T-wave evolution process shows that the T-wave is upright and elevated, and after the formation of necrotic Q-wave, the T-wave amplitude gradually decreases and becomes bidirectional and inverted. The degree of inversion gradually deepens, and after a few weeks, the T-wave inversion gradually becomes shallow. However, there are some AMI patients who never have T-wave inversion.
2, MI ECG localization diagnosis
The 2009 AHA/ACC/HRS ECG Standardization and Analysis Recommendations: Advanced imaging techniques including ultrasound and MRI have demonstrated that the existing descriptive terminology for MI sites needs further improvement. The International Society of Ambulatory and Noninvasive Cardiology recommends the use of new diagnostic terminology for the six different MI regions (anterior interstitial, anterior, anterolateral, high lateral, extensive anterior, and inferior) whose descriptions have been confirmed by enhanced magnetic resonance. At this time, the writing group believes that there are not enough new data to abolish the existing terminology.
Relationship of ECG localization diagnosis to AMI-related vessels
The ECG presentation of AMI is dependent on the size, length, direction of travel, degree of obstruction, collateral circulation, pre-existing myocardial necrosis, intraventricular differential conduction, and cardiothoracic location of the infarcted vessel. Therefore, the accuracy of 12-lead ECG in diagnosing the infarct site and associated infarct vessels is only relative. Retrospective studies have shown that the classification of AMI infarct sites as anterior, inferior, and lateral is most appropriate.
Anterior wall MI
The most frequent lead with ST-segment elevation is V2, followed by V3, V4, V5, aVL, V1, and V6, with the most pronounced ST-segment elevation in leads V2 and V3, and the most reliable ECG diagnosis of acute anterior wall MI.
Acute inferior wall MI
The best criterion to diagnose acute inferior wall infarction by ST-segment elevation is ST-segment elevation of ≥1 mm in one or more inferior wall leads (II, III, aVF), which is usually caused by occlusion of the right coronary artery or left ileal branch, with 80% to 90% of lesions in the right coronary artery, the rest in the left ileal branch, and very rarely in the left anterior descending branch. In a large retrospective controlled study, acute inferior wall infarction with aVL-lead ST-segment depression was almost always associated with right coronary occlusion (sensitivity 94%, specificity 71%), whereas those without aVL-lead ST-segment depression were mostly associated with left circumflex lesions.
Acute collateral MI
The best diagnostic criterion for ST-segment elevation in acute lateral wall infarction is ST-segment elevation in one or more lateral wall leads (V5, V6, aVL, I) ≥1 mm. acute lateral wall infarction is mostly caused by left gyrus occlusion, and 12-lead ECG is poor for the diagnosis of left gyrus branch lesion, therefore, additional leads often have a better adjunctive effect, increasing the diagnostic rate by 6% to 14%.
Acute posterior wall MI
The criteria for ST-segment elevation in leads V7-V9 in acute posterior wall infarction are more commonly used, with ≥1 mm. Posterior wall infarction due to left gyral branch occlusion. ST-segment elevation in leads V7-V9 was more specific for the diagnosis of posterior wall infarction than in the anterior chest leads (84% versus 57%). Posterior wall infarction in lead V1 has an increased R wave, depressed ST and inverted T wave.
Acute right ventricular MI
Right ventricular infarction rarely occurs alone, occasionally in patients with right ventricular hypertrophy, but mostly in combination with inferior and posterior left ventricular infarction. Right ventricular infarction is almost always a right coronary artery lesion. Occlusions occur in the proximal segment of the right coronary artery near the sharp-edge branch, resulting in a large infarct in the posterior right ventricular wall.
Left main occlusion
In acute left main stem occlusion, the following ST segment changes may occur: ST segment elevation in aVR leads, ST segment elevation in V1 leads (ST segment elevation of STV6 < STaVR), ST segment depression in II and aVF leads (left ventricular basal ischemia), ST segment depression in V2 left anterior thoracic leads (left ventricular posterior wall ischemia), and right bundle branch block may be present.