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The ECG and Myocardial Infarction

The ECG leads and orientation are shown in Figure M4. The typical ECG changes of acute myocardial infarction (AMI) with pathological Q waves, S-T segment elevation and T wave inversion are highlighted in leads III and AVL of acute inferior infarction (Fig. M5). Lead AVL facing the opposite side of the heart shows reciprocal S-T depression.

Figure M4

ECG leads and orientation (based on Einthoven triangle)

An illustration represents ECG leads and orientation. The illustration shows the right arm on the left, the left arm on the right, and ECG leads and orientation at the bottom centre. An arrow labelled lead 1 from the negative end of the right arm points to the positive end of the left arm. An arrow labelled lead 2 from the negative end of the right arm points to the positive end of ECG leads and orientation. An arrow labelled lead 3 from the negative end of the right arm points to the positive end of ECG leads and orientation. Within the arrows, a circle is present and it is divided into 12 parts labelled in the clockwise direction as follows: 0 degrees (left-centre), 30 degrees, 60 degrees, 90 degrees, 120 degrees, 150 degrees, 180 degrees, negative 150 degrees, negative 120 degrees, negative 90 degrees, negative 60 degrees, and negative 30 degrees. Four parts are shaded in a different colour. A dashed line from the right arm reads, positive aVR negative. A dashed line from the left arm reads, negative aVL positive. A dashed line from ECG leads and orientation reads, positive aVF negative.
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Figure M5

Two leads from ECG of AMI (inferior infarction)

Two graphs represent two leads from ECG of AMI (inferior infarction). The first graph representing lead 3 shows a P wave, followed by a deep broad Q wave, a high rise R wave, S elevation, and T wave inversion. The second graph representing AVL shows a P wave, followed by a small Q wave, an R wave that rises a little, a deep S wave, and a rising T wave.
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From Figure M6 it is apparent that:

Figure M6

Areas of heart wall affected by AMI

Two diagrams represent the areas of the heart wall affected by AMI. First diagram: the left lateral view shows anterior infarction, posterior infarction, and inferior infarction. Second diagram: the transverse anterior section shows RV, LV, lateral infarction, and anterior infarction.
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  1. The leads overlying the anterior surface of the left ventricle will be V2–V5 and these will be the leads giving evidence of anterior infarction.

  2. The leads overlying the lateral surface will be lateral chest leads V5–V6.

  3. No leads directly overlie the inferior or diaphramatic surface. However, the left leg leads, although distant, are in line with this surface and will show evidence of infarction in this area.

  4. There are no leads directly over the posterior surface.

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Table M9

Region of heart wall

Artery occluded

Leads showing ECG changes

Anterior

L anterior descending

V2–V5, I, aVL

Lateral

Circumflex, branch of LAD

V5–V6, I, aVL

Anteroseptal

LAD

V1–V4

Inferior

R coronary

II, III, aVF, aVL (reciprocal)

Posterior

RCA or circumflex

V1–V2 (unclear)

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