Posterior Myocardial Infarction

 Clinical Significance

  • Posterior infarction accompanies 15-20% of STEMIs, usually occurring in the context of an inferior or lateral infarction.
  • Isolated posterior MI is less common (3-11% of infarcts).
  • Posterior extension of an inferior or lateral infarct implies a much larger area of myocardial damage, with an increased risk of left ventricular dysfunction and death.
  • Isolated posterior infarction is an indication for emergent coronary reperfusion. However, the lack of obvious ST elevation in this condition means that the diagnosis is often missed.

Be vigilant for evidence of posterior MI in any patient with an inferior or lateral STEMI.

How to spot posterior infarction

As the posterior myocardium is not directly visualised by the standard 12-lead ECG, reciprocal changes of STEMI are sought in the anteroseptal leads V1-3. 

Posterior MI is suggested by the following changes in V1-3:

  • Horizontal ST depression
  • Tall, broad R waves (>30ms)
  • Upright T waves
  • Dominant R wave (R/S ratio > 1) in V2

In patients presenting with ischaemic symptoms, horizontal ST depression in the anteroseptal leads (V1-3) should raise the suspicion of posterior MI.

Posterior MI appearance in V2

  • Typical appearance of posterior infarction in V2

Posterior infarction is confirmed by the presence of ST elevation and Q waves in the posterior leads (V7-9).

Explanation of the ECG changes in V1-3

The anteroseptal leads are directed from the anterior precordium towards the internal surface of the posterior myocardium. Because posterior electrical activity is recorded from the anterior side of the heart, the typical injury pattern of ST elevation and Q waves becomes inverted:

  • ST elevation becomes ST depression
  • Q waves become R waves
  • Terminal T-wave inversion becomes an upright T wave

The progressive development of pathological R waves in posterior infarction (the “Q wave equivalent”) mirrors the development of Q waves in anteroseptal STEMI. 

posterior MI - V2 inverted

  • This picture illustrates the reciprocal relationship between the ECG changes seen in STEMI and those seen with posterior infarction. The previous image (depicting posterior infarction in V2) has been inverted. See how the ECG now resembles a typical STEMI!

Posterior leads

Leads V7-9 are placed on the posterior chest wall in the following positions (see diagram below):

  • V7 – Left posterior axillary line, in the same horizontal plane as V6.
  • V8 – Tip of the left scapula, in the same horizontal plane as V6.
  • V9 – Left paraspinal region, in the same horizontal plane as V6.

Reproduced from Morris and Brady, 2002. Click image for link to original reference.

The degree of ST elevation seen in V7-9 is typically modest – note that only 0.5 mm of ST elevation is required to make the diagnosis of posterior MI!

Example ECGs

Example 1a

Posterior MI

Inferolateral STEMI. Posterior extension is suggested by:

  • Horizontal ST depression in V1-3
  • Tall, broad R waves (> 30ms) in V2-3
  • Dominant R wave (R/S ratio > 1) in V2
  • Upright T waves in V2-3

 

Example 1b

Posterior MI V789

The same patient, with posterior leads recorded:

  • Marked ST elevation in V7-9 with Q-wave formation confirms involvement of the posterior wall, making this an inferior-lateral-posterior STEMI (= big territory infarct!).

Example 2a

Posterior MI standard leads

In this ECG, posterior MI is suggested by the presence of:

  • ST depression in V2-3
  • Tall, broad R waves (> 30ms) in V2-3
  • Dominant R wave (R/S ratio > 1) in V2
  • Upright terminal portions of the T waves in V2-3

(The ECG changes extend out as far as V4, which may reflect superior-medial misplacement of the V4 electrode from its usual position).

 

Example 2b

posterior MI leads V7-9

The same patient, with posterior leads recorded:

  • Posterior infarction is diagnosed based on the presence of ST segment elevation >0.5mm in leads V7-9.
  • Note that there is also some inferior STE in leads III and aVF (but no Q wave formation) suggesting early inferior involvement.

 

Example 3a

posterior MI

Patient presenting with chest pain:

  • The ST depression and upright T waves in V2-3 suggest posterior MI.
  • There are no dominant R waves in V1-2, but it is possible that this ECG was taken early in the course of the infarct, prior to pathological R-wave formation.
  • There are also some features suggestive of early inferior infarction, with hyperacute T waves in II, III and aVF.

Example 3b 

posterior MI

 An ECG of the same patient taken 30 minutes later:

  • There is now some ST elevation developing in V6.
  • With the eye of faith there is perhaps also some early ST elevation in the inferior leads (lead III looks particularly abnormal).

Example 3c 

posterior MI - V789

The same patient with posterior leads recorded:

  • Posterior infarction is confirmed by the presence of ST elevation >0.5mm in leads V7-9.

 

Related Topics

Further Reading

Author Credits

References

  • Edhouse J, Brady WJ, Morris F. ABC of clinical electrocardiography: Acute myocardial infarction-Part II. BMJ. 2002; 324: 963-6. [full text]
  • Mattu A, Brady W. ECGs for the Emergency Physician 2, BMJ Books 2008.
  • Morris F, Brady WJ. ABC of clinical electrocardiography: Acute myocardial infarction-Part I. BMJ. 2002; 324: 831-4. [full text]
  • Phibbs BP. Advanced ECG: Boards and Beyond (second edition). Elsevier 2006.
  • Surawicz B, Knilans T. Chou’s Electrocardiography in Clinical Practice (6th edition), Saunders 2008.
  • Van Gorselen EO, Verheugt FW, Meursing BT, Oude Ophuis AJ. Posterior myocardial infarction: the dark side of the moon. Neth Heart J. 2007; 15: 16-21.
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Comments

    • Justin says

      I agree, as an EMS educator/ mentor I am always asked about posterior MI’s as it is covered for like 10 seconds in most medic programs and that’s it. This will allow me to explain the pathophysiology of Posterior MI’s better, thanks a million.

  1. er@gmail.com says

    I appreciate your work, in precise explanation we are learning each day while taking a cup of tea.
    Thanks, for updating.

  2. James Higgs says

    Great Article! Can someone please clarify why you would do a right-sided ECG vs a posterior lead ECG (ie V7 posterior axillary, V8 scapula, etc). Thanks

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