Lead orientation and posterior myocardial infarction

  Each precordial lead views the heart (and its electrical acitivity) from a different view.

      This provides the oppurtunity that if one electrode misses the activity, another electrode may record it.

  In standard ECG however, no chest electrode is orientated directly to the posterior wall.

      Therefore, the anterior chest leads are expected to show mirror-image (inverse) changes

      during posterior myocardial infarction.

  Generally leads V1 to V3 (sometimes extending to V4) are looked for these changes.



The expected mirror-image changes in leads V1 to V3 during posterior myocardial infarction are

  Tall and widened R waves       (mirror-image of deep Q or QS waves).

      This also results in a rightward shift of the R wave transition zone.

  Tall and upright T waves         (mirror-image of negative T waves).

  ST depression                         (mirror-image of ST elevation).



Click here to see other causes of tall R wave in lead V1.



Alternative lead placement to detect posterior myocardial infarction

  Since no chest electrode is orientated directly to the posterior wall in standard ECG,

      alternative electrode placement may be done to observe the posterior wall directly.

      This will obviate the need to search for mirror-image signs. Instead you may now look for typical

      electrocardiograhic signs of acute myocardial infarction (ST elevation, T negativity, Q wave).

  You can use V7, V8, and V9 leads to observe the posterior wall directly.

      They are somewhat the extension of anterior chest leads to the posterior chest.

  Lead V7 : posterior midaxillary line.

  Lead V8 : mid-scapular line.

  Lead V9 : between the mid-scapular line and the spine.

Click here and see the Figure 1 on Page 1 as example for this type of electrode placement.



References

  Tex Heart Inst J 2012;39:292-293.

  Can Med Assoc J. 1978;119:745-748.




ECG 1. Acute posterior myocardial infarction due to proximal Circumflex (CX) coronary artery occlusion.
Observation of positive T waves in leads C1 and C2 is generally an expected (but not a necessary) finding in posterior MI.

Click here for a more detailed ECG




ECG 2a. The ECG above belongs to a patient who is experiencing ACUTE posterior myocardial infarction due to occlusion of the
Circumflex (Cx) coronary artery. Generally, the ECG in these patients show tall and upright (positive) T waves in lead C2.
Also, the R wave is widened in lead C2.

Click here for a more detailed ECG



ECG 2b. The same patient's ECG, 2 years later. He is asymptomatic. High R waves and high upright T waves in C2 persist.

Click here for a more detailed ECG




ECG 3a. Above is the ECG of a 69-year-old woman with severe obstructive lung disease who developed substernal chest pressure
and progressively worsening dyspnea. Her ECG shows 2 mm of horizontal ST depressions with upright T waves (*) in leads V3
through V4. On the basis of the above ECG findings and clinical suspicion, additional posterior leads were placed and
recorded (ECG 3b, below).

The ECG above has been used with the permission of Texas Heart Institute Journal.

Click here for a more detailed ECG

Click here to go to the relevant article of the Texas Heart Institute Journal



ECG 3b. On the basis of ECG 3a and clinical suspicion, additional posterior leads were placed and recorded (above).
This ECG shows 1 mm of ST elevation in leads V7 through V9 : acute posterior myocardial infarction.
Subsequent urgent cardiac catheterization revealed an occluded proximal left circumflex artery which was successfully
opened with placement of a drug-eluting stent.

The ECG above has been used with the permission of Texas Heart Institute Journal.

Click here for a more detailed ECG

Click here to go to the relevant article of the Texas Heart Institute Journal