Acute Myocardial Infarction on ECG
February 15th, 2021
Acute myocardial infarction (MI) may be diagnosed using a 12 lead ECG.
ST elevation MI (STEMI) requires immediate coronary intervention and as such rapid assessment of the patient and ECG is imperative. Non-ST elevation MI may present with many features of STEMI, without ST elevation.
ECG Findings in Acute Myocardial Infarction
- Hyperacute T waves - tall, symmetrical T waves in contiguous leads
- ST elevation at the J point
- Reciprocal ST depression - in contiguous leads
- T wave inversion - compare with old ECGs
- Q waves - broad (>1mm in duration) or deep (>2mm or >1/3 of R wave height)
- New left bundle branch block
Diagnostic Criteria for ST Elevation Myocardial Infarction
ST elevation in at least two contiguous leads:
- >2.5mm in V2-V3 (males <40 years)
- >2mm in V2-V3 (males >40 years)
- >1.5mm in V2-V3 (females)
- >1mm in all other leads
- The first ECG finding in acute myocardial infarction is hyperacute T waves, which are tall and symetrical and occur within the first few minutes.
- These then resolve and ST elevation develops in affected leads over minutes to hours, with ST depression in reciprocal leads. The ST elevation associated with myocardial infarction tends to be convex, and may become elevated above the T wave (tombstoning).
- As ST elevation improves, pathologic Q waves and T wave inversion develop. Pathologic Q waves may persist for life.
New left bundle branch block is also a feature of ST elevation myocardial infarction, because the presence of LBBB makes interpretation of an ECG for STEMI impossible. Right bundle branch block causes no such issue.
Localising an MI
The ECG leads may be grouped into contiguous lead complexes, and findings in specific lead complexes can be used to suggest the location of an ST elevation myocardial infarction.
The location of the hyperacute T waves, ST elevation and Q waves suggest the location of an infarct.
- Lateral - I, aVL, V5, V6
- Inferior - II, III, aVF
- Septal - V1, V2
- Anterior - V3, V4
- Right ventricular (requires extra leads) - V1R-V6R
- Posterior (requires extra leads) - V7-V9
Anterior Myocardial Infarction
Anterior and anteroseptal infections tend to occur due to occlusion of the left anterior descending (LAD) artery.
On an electrocardiogram, leads V3 and V4 are the anterior leads and V1 and V2 are the septal leads. These leads tend to show signs of infarction in anterior MI.
ECG Findings in Anterior Myocardial Infarction
- ST elevation in leads V1-V4
- ST depression in the inferior leads (II, III, aVF) or aVL
- Poor R wave progression
- De Winter waves - up-sloping ST depression followed by tall, symmetrical T waves in leads V1-V6 (suggests proximal LAD occlusion)
- Wellen's syndrome - deeply inverted or biphasic T waves in leads V2-V3 (suggests proximal LAD occlusion)
Inferior Myocardial Infarction
Inferior myocardial infarctions tend to occur due to occlusion of the right coronary (RCA) or, less commonly, the distal left circumflex (LCx) artery.
Inferior MIs may be associated with lateral, posterior or right ventricular infarction.
On an electrocardiogram, leads II, III and aVF are the inferior leads and these tend show signs of infarction in inferior MI.
ECG Findings in Inferior Myocardial Infarction
- ST elevation in at least 2 inferior leads (II, III, aVF)
- ST depression in aVL
- Q waves in II, III and aVF
Lateral Myocardial Infarction
The lateral left ventricle is supplied by branches of the left anterior descending (LAD) and left circumflex (LCx) artery.
Isolated lateral MI is uncommon, and the lateral wall tends to be involved in anterolateral, posterolateral of inferolateral infarcts.
On an electrocardiogram, leads I, aVL, V5 and V6 are the lateral leads.
ECG Findings in Lateral Myocardial Infarction
- ST elevation in the lateral leads (I, aVL, V5, V6)
- ST depression in leads III and aVF
Patterns of ECG Changes
- Isolated lateral MI - ST elevation in the lateral leads (I, aVL, V5, V6)
- High lateral MI - isolated ST elevation in leads I and aVL [Suggests occlusion of the first diagonal branch of the LAD]
- Anterolateral MI - ST elevation in the lateral leads (I, aVL, V5, V6) and anterior leads (V3-V4) [Suggests occlusion of the proximal LAD]
- Inferolateral MI - ST elevation in the lateral leads (I, aVL, V5, V6) and inferior leads (II, III, aVF) [Suggests occlusion of the proximal left circumflex artery]
Right Ventricular Infarction
Right ventricular infarcts tend to occur due to occlusion of the proximal right coronary artery (RCA).
In order to assess for right ventricular infarction using an electrocardiogram, added leads are required on the right side of the chest (V1R to V6R).
Right ventricular infarction should be suspected in patients with isolated ST elevation in lead V1. It should also be suspected in patients with inferior MIs.
ECG Findings in Right Ventricular Infarction
- ST elevation in leads V1-V2
- ST elevation in leads V3R to V6R
Posterior Myocardial Infarction
Posterior MI can be difficult to diagnose, as it may not be evident on a 12-lead ECG. Posterior infarcts are associated with inferior MIs and should be considered in patients with this.
Using an electrocardiogram, extra leads on the back (V7 to V9) are required for diagnosis of posterior myocardial infarction.
ECG Findings in Posterior Myocardial Infarction
- ST depression in leads V1-V3
- ST elevation in posterior leads (V7-V9) of at least 0.5mm
De Winter Waves
De Winter waves are an ECG finding that suggest acute occlusion of the proximal left anterior descending (LAD) artery. These are a STEMI equivalent, and should be treated immediately.
ECG Features of De Winter Waves
- Up-sloping ST depression at the J point (1-3mm) in the precordial leads (V1-V6)
- Tall, positive, symmetrical T waves
Wellen's syndrome, also known as the 'widowmaker', is an ECG finding that is highly suggestive of proximal left anterior descending (LAD) artery occlusion and imprending anterior myocardial infarction.
This may be associated with unstable angina, however the finding often occurs while asymptomatic.
The presence of Wellen's syndrome is life-threatening and requires rapid intervention, though has the potential to be missed if an ECG is not examined carefully.
Signs of Wellen's Syndrome
- Type A - biphasic T waves in leads V2-V3
- Type B - deep T wave inversion in leads V2-V3