Primary anterior area:
ST elevation in two contiguous leads V 1-4 defines a primary anterior area of involvement. The LAD is the culprit artery. Areas contiguous to anterior (V t-4) include lateral (I & aVL) and apical (V5 & V6). ST elevation in these leads suggests more myocardium at risk and conveys more adverse outcomes.

Primary inferior area:
ST segment elevation in two contiguous leads (II, aVF or III) defines a primary inferior area of involvement and a probable RCA lesion. Areas contiguous to inferior (I1, aVF & III) include apical (V5 & V6), posterior (V1-3 or V7-9) and right ventricular (V4R). ST elevation in these contiguous leads suggests more myocardium at risk and conveys more adverse outcomes.

Identification of the culprit artery:
Prediction of the culprit artery involved in the acute infarction can be determined by extrapolating results of data in the table below, taken from a subset of 2,128 patients enrolled in the GUSTO study.

Location of ST segment elevation (2 contiguous leads)	Total (N)	LAD (%)	RCA (%)	LCx (%)
Anterior ONLY	341	97.9	0.9	1.2
Ant + Ap ONLY	143	98.6	0.7	0.7
Ant + Lat ONLY	118	97.5	0.8	1.7
Ant + Ap + Lat	173	99.4	0.6	0.0
Ant + Inf + Ap	28	78.6	21.4	0.0
Ant + Inf + Ap + Lat	8	100.0	0.0	0.0
Ant+ Inf ONLY	34	47.1	52.9	0.0
Inferior ONLY	897	0.7	85.7	13.6
Inf + Ap ONLY	302	2.3	64.6	33.1
Inf + Lat ONLY	11	0.0	63.6	36.4
Apical ONLY	8	25.0	62.5	12.5
Lateral ONLY	30	63.3	0.0	36.7
Ap + Lat ONLY	24	41.7	0.0	58.3
Inf + Ap + Lat	11	18.2	9.1	72.7

Therefore, patients with ST segment elevation in any 2 contiguous leads V 1-4, either alone or with associated changes in leads V5-6 and/or I and aVL, had LAD obstruction in 98.3%. In patients with ST segment elevation only in leads II, aVF and III, there was RCA obstruction in 85.7%.

The anatomy of the coronary arteries:
The following diagram, adapted from Selvester's chapter in MacFarlane and Veitch Lawrie, shows a Mercator projection of the arteries to the left ventricle. Section A includes the septal branches of the left anterior descending (LAD) coronary artery; Section B includes the diagonal branches of the LAD; Section C includes the branches of the circumflex coronary artery and Section D includes the branches of the right coronary artery supplying the LV. Curved bold lines identify the distribution of each artery.

PRIMARY ANTERIOR PROCESS: acute occlusion of the LAD coronary artery, producing changes in the anterior leads (V1-4).

Earliest findings of occlusion:

• "Hyperacute" changes: (may not be seen in the E.D. setting)
  ST elevation (injury) with loss of normal ST segment concavity, commonly with tall peaked T waves.
  

  

• Acute injury: ST elevation
  The ST segment commonly appears as though a thumb has been pushed up into it.
  

  

Evolutionary changes of anterior infarction:

• Development of pathologic Q waves (infarction): Pathologic Q waves may occur within the first hour after symptom onset in at least 30% of patients.
  

  

• ST segment elevation decreases. T wave inversion, usually occurs, in the 2nd 24 hour period of the infarction.
  

  

• Fully evolved pattern: pathologic Q waves, rounded upward ST segment, inverted T waves.
  

  

A patient who presents to the E.D. with cardiac-type chest pain who hag T wave inversion in. leads with pathologic Q waves is most likely in the evolutionary or completed phase of infarct.

Successful revascularization usually causes prompt resolution of the acute signs of injury or infarction and results in the ECG signs of a Fully evolved infarction.

PRIMARY INFERIOR PROCESS: usually acute occlusion of the right coronary artery, producing changes in the inferior leads (II, aVF & III)

• Earliest findings:
  Acute injury: (ST segment elevation) The J point may "climb up the back" of the R wave (a), or the ST segment may rise up into the T wave (b).
  

  

• Evolutionary changes:
  ST segment elevation decreases and pathologic Q waves develop. T wave inversion may occur int he 1st 12 hours of an infeior MI, in contrast to that in anterior MI.
  

  

• Right ventricular involvement:
  There is ST segment elevation, best seen in lead V4R, either with or without a QS complex signifying infarction.
  

  

• Normal morphology of the QRS complex in lead V4R:
  The normal morphology of the QRS complex in lead V4R is shown below. The normal J point averages + 0.2 mm.
  

  

Identification of the culprit artery in patients with inferior injury based upon the QRS-ST-T morphology in lead V4R:
Bract studied patients with ST segment elevation in leads II, aVF, & III, all of whom had an acute MI. They determined the culprit artery and site of occlusion and related the ECG waveforms to the anatomy, as follows, with the performances cited.

POSTERIOR INJURY OR INFARCTION: commonly due to acute occlusion of the left circumflex coronary artery, producing changes in the posterior leads (V7, V8, V9), or reciprocal ST segment depression in leads V1-3.

• Acute patterns: ST segment depression in V1-3, and possibly also V4, usually with upright, often prominent T waves.
  

  

• Chronic pattern: Pathologic R waves with prominent tall T waves in leads V1-3.
  

  

Indications for obtaining posterior chest ECG leads
Whenever a patient presents with cardiac pain and only ST depression on the standard 12-lead ECG, especially in leads V 1-4, you should obtain posterior leads V7-9. These may reveal ST segment elevation reflective of an occluded artery, for which the treatment is relief of the occlusion. Diffuse ST segment depression due to subendocardial injury, shown below, will not produce ST segment elevation in„ the, posterior leads, but will show the expected ST segment depression.

Placement of the posterior lead electrodes:
All electrodes are on the same horizontal plane as lead V4: Place V7 at the posterior axillary line, V8 under the scapular tip, and V9 in the left paraspinous recess.

Normal QRS morphology in the posterior leads (V8 illustrated):
The normal morphology of V8 resembles that of lead V6, but its amplitude is diminished because of the attenuating effects of lung tissue. Examples of waveforms are shown below.

Subendocardial injury: As illustrated by a positive exercise stress test
Lead aVR is the only lead that consistently views the endocardial surface of the left ventricle. Subendocardial injury produces ST segment elevation in aVR. On initial inspection of the 12-lead ECG, however, the finding that one should notice is diffuse ST segment depression, a reciprocal change. Other conditions, namely LVH, digitalis and hypokalemia, may produce ST elevation in aVR but are not examples of subendocardial injury. One of the most potent causes of significant depression of the ST segment is acute anemia.

Pathophysiology of subendocardial injury:
Usually a transient supply-demand mismatch caused by elevated blood pressure and/or heart rate in a patient with a fixed coronary artery lesion. Clinical presentation: typical angina.

Reciprocal changes in the setting of acute MI:
A recent study of patients who had continuous ST segment monitoring in the setting of acute MI showed that in 66% of patients, there was ST depression in leads remote from the primary site of injury. With successful reperfusion, the ST depression resolved. This was felt to be a purely reciprocal change. In another 23% of patients, there were no reciprocal changes seen on the 12lead ECG. In another 11% of patients, when the ST segment elevation resolved, there was persistent ST segment depression in leads remote from the acute injury. In this latter group, the mortality was significantly increased, compared with the other two groups. These patients had more significant three-vessel disease, and had so-called "ischemia at a distance."

MI - Overview | Go to Steps 1 & 2 | Go to Step 4 | Back to Index