ECG Ischemia & Infarction
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Ischemic ST Patterns
ST elevation MI (STEMI) is one of the easiest and well known patterns of infarction to identify.
ST depression patterns include reciprocal changes, or widespread ST depression with ST elevation in aVR.
ST measurements (mm)
Classical ST Elevation MI (STEMI) is defined as clinical symptoms of an acute coronary syndrome and ≥ 2 contiguous leads showing:
- ≥ 2.5 mm in V2-3 if male <40 yrs
- ≥ 2 mm in V2-3 if male >40 yrs
- ≥ 1.5 mm in V2-3 if female
- ≥ 1 mm in other leads
Classic Anterior, Inferior or Lateral MI
Two of the simplest patterns to start with are anterior and inferior STEMI patterns.
A classic anterior MI is caused by occlusion of the left anterior descending artery. This can cause ST elevation in the anterior leads V1-4.
A classic inferior MI can be caused by an occlusion of the RCA (80%), LCx (18%) or LAD. It is classically associated with ST elevation in the inferior leads II, III and aVF.
A classic lateral MI is associated with ST elevation in the lateral leads (I, aVL, V5-6). It can be more difficult to see because these leads are quite spread out on a standard 12-lead ECG. This pattern can be combined with anterior (anterolateral MI) or inferior (inferolateral MI) infarction patterns.
Posterior MI
Posterior MIs can be a little harder to see.
A posterior MI does not produce ST elevation because there are no true posterior leads on a standard 12-lead ECG. Instead, it may show ST depression in the anterior leads V1-3, which is a form of reciprocal changes. This pattern is like a mirror image of ST elevation in posterior leads. It is rare on its own, and uaually accompanies an inferior or lateral STEMI.
It is also possible to move the electrodes V4-6 to record posterior leads V7-9. If there is at least 0.5mm ST elevation in one or more posterior leads, this suggests infarction. However, the absence of ST elevation in posterior leads does not exclude a MI.
An isolated posterior MI can be caused by occlusion of a branch from the LCX or RCA. Posterior infarction can also occur along with inferior (inferoposterior), lateral (posterolateral) or all of these territories (inferoposterolateral).
Reciprocal Changes
Reciprocal ST depression occurs in the leads that are electrically opposite from those showing ST elevation during an acute infarction.
Inferior infarction is associated with reciprocal ST depression in aVL.
Lateral infarction is associated with ST depression in leads III and aVF.
Posterior infarction is associated with reciprocal ST depression in V1-3.
LAD MI variations
A high lateral MI results from occlusion of the first diagonal branch (D1) of the Left Anterior Descending artery (LAD). It may show ST elevation in leads I, aVL and V2 along with ST depression in lead III. This pattern is also known as the South African Flag sign, and it can also be described as a mid anterior MI.
A proximal LAD occlusion can have several different patterns. If the occlusion is proximal to the first septal branch S1 it may show ST elevation in aVR and V1 along with a complete RBBB and ST depression in V5. If the occlusion is proximal to the first diagonal branch (D1) it may show ST elevation in I and aVL with ST depression in the inferior leads. An extensive proximal LAD occlusion may also produce enormous ST elevations that are known as tombstones because of the associated high mortality, or a bifascicular block.
A wraparound LAD occlusion may occur if the LAD 'wraps around' the apex of the heart to supply the inferior wall as well as anterior. This occlusion may produce an MI with widespread anterior and inferior ST elevation, and reciprocal ST depression in I & aVL.
Right ventricular MI
Right ventricular infarction can be subtle but clinically important to detect, particularly because it is associated with increased risk of hypotension and sensitivity to nitrates.
A right ventricular MI is associated with ST elevation in V1 > V2, and III > II. Right sided leads may also show ST elevation.
Widespread ST depression: Left main stenosis and occlusion
A left main stenosis is associated with widespread ST depression with ST elevation in aVR. This pattern can also be caused by any global mismatch between supply and demand, such as valvular disease, anaemia, hypoxia, hypotension, hypertrophy, extreme tachycardia or hypertension. It may occur more easily if there is coronary stenosis present as well. It is commonly mistaken for a true left main occlusion.
A true left main occlusion is a disaster with a high risk of cardiac arrest. Many will not survive to have an ECG recorded, and only 0.42% - 3% of patients with an anterior STEMI have a left main occlusion found at the time of catheterisation. If there is a true left main occlusion, it may produce signs of a large proximal LAD occlusion including ventricular conduction blocks along with a circumflex occlusion (posterolateral).
ECG Ischemia & Infarction
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