Ischemia

 

                                    Ischemia  Ischemia occurs when part of the heart muscle, the myocardium, is deprived of oxygen and nutrients. Common causes of ischemia are:  - Narrowing or obstruction of a coronary artery. - A rapid arrhythmia or hypertension, causing an imbalance in supply and demand for energy. A short period of mild ischemia causes reversible effects: The heart cells will be able to recover. When the episode of ischemia lasts for a longer period of time, heart muscle cells die. This is called a heart attack or myocardial infarction. That is why it is critical to recognize ischemia on the ECG in an early stage.  Severe ischemia results in ECG changes within minutes. While the ischemia lasts, several ECG changes will occur and disappear again. Therefore, it may be difficult to estimate the duration of the ischemia on the ECG, which is crucial for adequate treatment.

 

time

Coronary plaque progression

0% 20% 30% 50% 70% 90%

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Diagnosis of myocardial infarction

Not only based on the ECG; enzymes are more important.Myocardial infarction is defined as:• Elevated blood levels of cardiac enzymes (CKMB or Troponin T)   • AND one of the following criteria:

o The patient has typical complaints.o The ECG shows ST elevation or depression.o Pathological Q waves develop on the ECG.o A coronary intervention had been performed (such as stent

placement).  However enzymes don't become elevated until 5-7 hours after the onset of myocardial infarction, when most of the damage has already been done. Therefore, the ECG is very important for the acute diagnosis!

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ECG infarct

 ECG dynamic during persistent ischemia

In the first hours and days after the onset of a myocardial infarction, several changes can be observed on the ECG: 1. Large peaked T waves (or hyperacute T waves)

(minutes)– ST elevation (hours) – Negative T waves (hours, can last for months)– Finally pathologic Q waves develop (days)

 

Scar formation

onset of myocardial necrosis

endocardium___________

epicardium

Q waves are a sign of absent local electrical activity. The scar is like a window through which the QRS on the opposite side of the heart is seen resulting in the Q wave (inverted R wave).

Common causes of ST shift

Evolution of an infarct on the ECG

Typical negative T waves days after an anterior myocardial infarction

ECG manifestations of ischemiaa) Normalb) ST elevation           c) Pathologic Q Waved) T wave inversion                     e) normalisation with persistent Q wave 

Also: Change in heart axis, ST depression, R wave decreaseWidened QRS complex, QTc prolongation

ECG manifestations of acute myocardial ischemia

• ST elevationNew ST elevation at the J-point in two contiguous leads with the cut-off points: ≥ 0.2 mV in men or ≥ 0.15 mV in women in leads V2-V3 and/or ≥ 0.1 mV in other leads.• ST depression and T-wave

changes New horizontal or down-sloping ST depression > 0.05 mV in two contiguous leads: and/ or T inversion ≥ 0.1 mV in two contiguous leads with prominent R-wave or R/S ratio ≥ 1. 

               Pathological Q wavesPathological Q waves are a sign of previous myocardial infarction. They are the result of absent electrical activity (scar). Absence of patholgical Q waves does not exclude a myocardial infarction.

Lead III often shows Q waves which are not pathological  as long as Q waves are absent in leads II and aVF.

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• An infarction of the inferior wall will result in ST segment elevation in leads II, III and AVF.

• A lateral wall infarct results in ST segment elevation in leads I and AVL.

• An anterior wall infarct results in ST segment elevation in the precordial leads.

The lead with ST segment elevation is closest to the infarct

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Coronary arteries

The heart is supplied with oxygen and nutrients by the right and left coronary arteries.  • The left coronary artery (the Left Main

or LM) divides itself into the left anterior descending artery (LAD) and the ramus circumflexus (RCX). 

• The right coronary artery (RCA) connects to the ramus descendens posterior (RDP). With 20% of the normal population the RDP is supplied by the RCX. This is called left dominance.

Infarct localisation• Left main artery occlusion:

o diffuse ST-depression with ST elevation in AVR

o very high risk•  Anterior wall:

o ST elevation V1-V4. LAD. (often tachycardia)

• Inferior wall: o ST elevation II, III, AVF. o 80% RCA (elevation III>II; depression

> I or in AVL), or RCX ( in 20%). (often bradycardic due to sinus node or AV node ischemia)

• Right ventricle infarct: o ST elevation in V4R.

• Posterior wall: o high R and ST-depression in V1-V3

• Lateral wall: o ST elevation in lead I, AVL, V6.o LAD (D-branch)

Congruent ECG leads 'belong together' when diagnosing an MI. You expect congruent leads to have similar ST shift.

• Inferior MI = ST segment elevation in red regions (lead II, III and AVF). • Lateral MI = ST elevation in blue leads (lead I, AVL, V5-V6).• Anterior MI = ST segment elevation in yellow region (V1-V4).• Left main stenosis: ST elevation in gray area (AVR).

V4 right helps diagnose right ventricular involvement (in RCA occlusion)

Acute inferior MI

Old inferior MI: prominent Q waves in II, III & AVF

Subendocardial ischemia (not an MI)

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ST elevation in the absence of an aMI

As discussed in the 7+2 step plan, ST elevation can be present in the absence of an acute MI. Some other conditions that can cause ST elevation are:• Pericarditis/myocarditis.• Left ventricular hypertrophy (LVH)• Physiological/benign ST elevation• Cardiac aneurysm• Hyperkalemia

Acute anterior-lateral infarct

Acute antero-septal MI

Recent (days old) anterior MI (after PCI)

Old anterior-septal MI

Acute posterior MI

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 Notice the rather typical relative absence of ST deviation.

Acute RCX occlusion

Old/recent posterior-lateral MIprominent R in V2 (a 'reciprocal Q wave')

Acute inferior-posterior-lateral MI

Acute inferior and right ventricular MIElevation of V4R

Left main diseaseDiffuse ST depression and elevation in AVR

ST elevation in LBBB

 

ST elevation not due to myocardial infarction

• During left bundle branch block• Lef ventricular hypertrophy• High potassium• Pericarditis

ST elevation in LVH

Arguments for MI:

MI diagnosis in LBBB

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ST elevation during high potassium levels

Diffuse ST elevation in pericarditis