Myocardial Ischaemia

This page covers the ECG signs of myocardial ischaemia seen with non-ST-elevation acute coronary syndromes. ST-elevation / Q-wave myocardial infarction patterns are covered elsewhere (see links at bottom of page). 

Background

Non-ST-elevation acute coronary syndrome (NSTEACS) encompasses two main entities:

  • Non-ST-elevation myocardial infarction (NSTEMI).
  • Unstable angina (UA).

The differentiation between these two conditions is usually retrospective, based on the presence/absence of raised cardiac enzymes at 8-12 hours after the onset of chest pain. Both produce the same spectrum of ECG changes and symptoms and are managed identically in the Emergency Department. 

Patterns of Myocardial Ischaemia

There are two main ECG abnormalities seen with NSTEACS:

While there are numerous conditions that may simulate myocardial ischaemia (e.g. left ventricular hypertrophy, digoxin effect), dynamic ST segment and T wave changes (i.e. different from baseline ECG or changing over time) are strongly suggestive of myocardial ischaemia.

Click on the links above to read more about the different causes of ST segment and T wave abnormalities. 

Other ECG patterns of ischaemia

Morphology Of ST Depression

  • ST depression can be either upsloping, downsloping, or horizontal (see diagram below).
  • Horizontal or downsloping ST depression ≥ 0.5 mm at the J-point in ≥ 2 contiguous leads indicates myocardial ischaemia (according to the 2007 Task Force Criteria).
  • ST depression ≥ 1 mm is more specific and conveys a worse prognosis.
  • ST depression ≥ 2 mm in ≥ 3 leads is associated with a high probability of NSTEMI and predicts significant mortality (35% mortality at 30 days).
  • Upsloping ST depression is non-specific for myocardial ischaemia.

ST depression: upsloping (A), downsloping (B), horizontal (C)

 

ST segment morphology in myocardial ischaemia

 

Distribution of ST segment depression

ST depression due to myocardial ischaemia may be present in a variable number of leads and with variable morphology:

  • ST depression due to subendocardial ischaemia is usually widespread — typically present in leads I, II, V4-6 and a variable number of additional leads.
  • A pattern of widespread ST depression plus ST elevation in aVR > 1 mm is suggestive of left main coronary artery occlusion.
  • ST depression localised to a particular territory (esp. inferior or high lateral leads only) is more likely to represent reciprocal change due to STEMI. The corresponding ST elevation may be subtle and difficult to see, but should be sought. This concept is discussed further here.

Widespread subendocardial ischaemia due to LMCA occlusion

T wave inversion

T wave inversion may be considered to be evidence of myocardial ischaemia if:

  • At least 1 mm deep
  • Present in ≥ 2 continuous leads that have dominant R waves (R/S ratio > 1)
  • Dynamic — not present on old ECG or changing over time

NB. T wave inversion is only significant if seen in leads with upright QRS complexes (dominant R waves). T wave inversion is a normal variant in leads III, aVR and V1.

Widespread T wave inversion due to myocardial ischaemia (most prominent in the lateral leads)

Wellens’ Syndrome

  • Wellens’ syndrome is a pattern of inverted or biphasic T waves in V2-4 (in patients presenting with ischaemic chest pain) that is highly specific for critical stenosis of the left anterior descending artery.
  • Patients may be pain free by the time the ECG is taken and have normally or minimally elevated cardiac enzymes; however, they are at extremely high risk for extensive anterior wall MI within the next 2-3 weeks.

There are two patterns of T-wave abnormality in Wellens’ syndrome:

  • Type 1 Wellens’ T-waves are deeply and symmetrically inverted
  • Type 2 Wellens’ T-waves are biphasic, with the initial deflection positive and the terminal deflection negative

Wellens’ Type 1

Wellens 1 274x300 T wave wellens1 1 256x300 T wave

Wellens’ Type 2

Wellens 2 2 T wave Wellens 2 T wave

Read more about Wellens’ syndrome here.

Non-specific ST segment and T wave changes

The following changes may occur with myocardial ischaemia but are relatively non-specific:

  • ST depression < 0.5 mm
  • T wave inversion < 1 mm
  • T wave flattening
  • Upsloping ST depression

More ECG Examples

Example 1

Subendocardial ischaemia:

  • The most striking abnormality is the widespread ST depression, seen in leads I, II and V5-6. This is consistent with widespread subendocardial ischaemia.
  • There is also some subtle ST elevation in V1-2 and aVR with small Q waves in V1-2, suggesting that the cause of the widespread ischaemia is a proximal LAD occlusion.

 

Example 2

Reciprocal change:

  • The most obvious abnormality is the horizontal ST depression in III and aVF.
  • This could be misinterpreted as “inferior ischaemia” — however, subendocardial ischaemia does not localise.
  • Regional ST depression should prompt you to scrutinise the ECG for signs of reciprocal ST elevation… In this case there is subtle ST elevation in aVL.
  • This is a high lateral STEMI!

Dr Stephen Smith covers two similar cases on his excellent ECG blog. 

 

Example 3

Wellens’ Syndrome:

  • There are abnormal T waves in V1-4 — biphasic in V1-3 and inverted in V4.
  • This pattern is known as Type 2 Wellens’ Syndrome and is highly specific for a critical stenosis of the proximal LAD artery.

 

Example 4a

Dynamic ST depression in a patient with chest pain:

  • Widespread ST depression (leads I, II, V5-6) indicates subendocardial ischaemia.
  • Q wave in lead III with slightly elevated ST segment suggests the possibility of early inferior STEMI.

 

Example 4b

ECG of the same patient after treatment with oxygen, nitrates, heparin and antiplatelets:

  • The ST changes have now resolved.
  • Inferior ST segments and Q waves are stable — this patient had a history of prior inferior MI.
  • Troponin was raised, confirming that the initial ST depression was due to NSTEMI.

 

Example 5

NSTEMI presenting with isolated U wave inversion:

  • There are inverted U waves, most prominent in leads V5-6.
  • This is an infrequently recognised but very specific sign of myocardial ischaemia — this patient had a 12-hour troponin of 4.0 ng/mL.

To find out the full story behind this ECG, click here.

 

Related Topics

Further Reading

Author Credits

References

  • Chan TC, Brady WJ, Harrigan RA, Ornato JP, Rosen P. ECG in Emergency Medicine and Acute Care. Elsevier Mosby 2005.
  • Surawicz B, Knilans T. Chou’s Electrocardiography in Clinical Practice (6th edition), Saunders 2008.
  • Thompson, Peter L. Coronary Care Manual, 2nd edition.  Elsevier, 2011.
  • Thygesen K, Alpert JS, White HD; Joint ESC/ACCF/AHA/WHF Task Force for the Redefinition of Myocardial Infarction. Universal definition of myocardial infarction. Eur Heart J. 2007 Oct;28(20):2525-38. PubMed PMID: 17951287. Full text.
  • Wagner, GS. Marriott’s Practical Electrocardiography (11th edition), Lippincott Williams & Wilkins 2007.
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