Posterior Myocardial Infarction

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Clinical Significance of Posterior MI

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.

ECG posterior infarction 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.

ECG Posterior AMI flip image V2

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.
Posterior leads V7 V8 V9 ECG placement
Posterior lead placement V7, V8, V9

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 ECG
Example 1a
ECG Posterior AMI 1

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

The same patient, with posterior leads recorded:

ECG Posterior AMI 1b V789
  • 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
ECG Posterior AMI 2

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

The same patient, with posterior leads recorded:

ECG Posterior AMI 2a V789
  • 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

Patient presenting with chest pain:

ECG Posterior AMI 3
  • 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

An ECG of the same patient taken 30 minutes later:

ECG Posterior AMI 3b
  • 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

The same patient with posterior leads recorded:

ECG Posterior AMI 3 V789
  • Posterior infarction is confirmed by the presence of ST elevation >0.5mm in leads V7-9.
Example 4a

Patient presenting with central chest pain

ECG Posterior AMI 4a
  • Inferior STEMI with posterior extension. Extensive territory
ECG Posterior AMI flip image V2 2
Flip ECG, confirm V2 STEMI changes of posterior AMI
Example 4b

The same patient with posterior leads (V8,9) recorded:

ECG Posterior AMI 4a V789
References
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Emergency Physician in Prehospital and Retrieval Medicine in Sydney, Australia. He has a passion for ECG interpretation and medical education | ECG Library |

2 Comments

  1. Can someone please clarify something for me. So St depression in v2 and v3. Swap leads v4, 5 and 6, and place them on the posterior aspect of the thorax as per diagram. When the ecg is recorded how come in some of the examples above leads v4, 5 and 6 are crossed out and replaced by leads v7, 8 and 9. On contrast some examples have v1, 2 and 3 cross out and are replaced by v7, 8 and 9.
    Can someone please tell me the correct was to record a posterior ecg or are both ways acceptable?

    Many thanks

  2. Hi Mr.Lee,
    1. Take conventional ECG
    2 take out v4,v5,V6 and place them posteriorly. Mark as v7,v8v9.
    You gotta keep two ecgs and read it.

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