Wednesday, 28 July 2021

 

Myocardial Ischaemia

This page covers the ECG signs of myocardial ischaemia seen with non-ST-elevation acute coronary syndromes (NSTEACS). ST-elevation and Q-wave myocardial infarction patterns are covered elsewhere: LMCA occlusion, Anterior STEMI, Lateral STEMI, Inferior STEMI, Right Ventricular Infarction, Posterior Infarction and Wellens syndrome

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Myocardial Ischaemia Background

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

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

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
Two main ECG patterns associated 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.

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 segment depression upsloping downsloping horizontal

Examples of ST segment morphology in myocardial ischaemia

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 of ST depression failing to localise is further discussed on Dr Smiths blog.
LMCA 2
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.

Lateral-TWI 2
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 A – Biphasic, with initial positivity and terminal negativity (25% of cases)
    Type B – Deeply and symmetrically inverted (75% of cases)
Biphasic T Waves (Type A)
Wellens Pattern A Type 1 T wave 2
Wellens Pattern A Type 1 T wave
Deeply Inverted T Waves (Type B)
Wellens Pattern B Type 2 T wave
Wellens Pattern B Type 2 T wave 2

NB. There is confusion in the literature regarding the naming of the T wave patterns, with some authors using Type 1 (Type A) for biphasic T waves and Type 2 (Type B) for inverted. It may be better to just describe the T wave pattern!

Wellens wave evolution

T wave changes can evolve over time from Type A to Type B pattern (Smith et al).

Evolution of T-wave inversion [A-D] after coronary reperfusion in STEMI reperfusion and in Wellens syndrome (NSTEMI)
Evolution of T-wave inversion [A-D] after coronary reperfusion in STEMI reperfusion and in Wellens syndrome (NSTEMI). Modified from Smith et al. Evolution of T-wave inversion. The ECG in acute MI, 2002
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 Myocardial Ischaemia ECG Examples
Example 1
Ischaemia1 2

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
inferior-ST-depression 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!
Example 3
Biphasic-T-waves 2

Wellens Syndrome:

  • There are abnormal T waves in V1-4 — biphasic in V1-3 and inverted in V4.
  • This pattern is known as Type A Wellens Syndrome and is highly specific for a critical stenosis of the proximal LAD artery.
Example 4a
nstemi1

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
nstemi2

ECG of the same patient after treatment with oxygen, nitrates, heparin and anti-platelets:

  • 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_U_wave-inversion_only2

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.

Want to find out the full story behind this ECG?.

References
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Online

Textbooks

LITFL Further Reading
ECG LIBRARY 700

ECG LIBRARY

Electrocardiogram

EKG Library

Emergency Physician in Prehospital and Retrieval Medicine in Sydney, Australia. He has a passion for ECG interpretation and medical education | ECG Library |

Emergency physician MA (Oxon) MBChB (Edin) FACEM FFSEM with a passion for rugby; medical history; medical education; and asynchronous learning #FOAMed evangelist. Co-founder and CTO of Life in the Fast lane | Eponyms | Books |

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