Case 1: Inferior STEMI: Syndrome of RCA occlusion
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EKG: The ST segment is positive in leads III and AVF (Figure 14.10).
Visualization: The ST is pointing to the patient’s right side, which is toward
the inferior wall (Figure 14.11).
Critical Thinking: Since the ST segment is pointing toward the inferior wall
(Figure 14.10), we locate the area of extreme or transmural ischemia here
(Figure 14.11). The right coronary artery (RCA) supplies blood to the AV node
and inferior wall through its posterior descending artery branch (PDA). We
expect a significant obstruction with a ruptured plaque and a superimposed
thombus in the RCA or PDA.
We apply the Third Rule of the T Waves. We ignore the T wave changes and ST depression in leads I and AVl, and diagnose ST elevation myocardial infarction (NSTEMI). The right coronary artery supplies the right ventricle with blood. Right ventricular infarction may be
present. Right ventricular infarction can cause right-sided heart failure and
hypovolemic shock without pulmonary edema, since it is too weak to pump
sufficient blood to the left ventricle.
The RCA supplies the AV node 90% of
the time, therefore heart block such as 1° AV block, Wenckebach, or complete
heart block may be associated as well. A vagal response, with sinus bradycardia,
nausea, and vomiting may also be associated. The posterior part of the
ventricular septum may rarely rupture (producing an acquired ventricular
septal defect) as a complication of RCA occlusion.
Reciprocal changes: Transmural ischemia of the inferior wall attracts the ST segment toward the
inferior wall. This produces ST segment elevation in the inferior leads, leads
III and AVF, and usually lead II as well (Figure 14.11). The observers or leads
on the other side of the heart also see the transmural process, but they see it as going away from them. Therefore, leads I and AVL show ST segment depression (Figure 14.12).
Reciprocal changes continued: This ST segment depression does not represent additional subendocardial
ischemia of the lateral wall. There is no way for the ST segment to point
towards the inferior wall without also pointing away from the lateral lead
observers. This geometric fact of life is called reciprocal changes. It occurs in
lateral transmural ischemia, and anterior transmural ischemia as well.
Involvement of the Posterior Wall: Transmural ischemia of the inferior wall attracts the ST segment toward the inferior wall. This produces ST segment elevation in the inferior leads, leads III and AVF, and usually lead II as well. The ST depression in leads I and AVL are the result of reciprocal changes from the inferior process. The anterior leads in inferior transmural ischemia sometimes demonstrate ST segment depression.
A side view of the heart from a hypothetical lateral view (from the patient’s left side) shows inferior transmural ischemia. The sensor in lead V2 sees this process as going away from it, and this can explain the ST segment depression in lead V2. The transmural
ischemia in this example is not just inferior, but posterior as well. (There is also a chance that the ST segment in V2 is pointing away from subendocardial ischemia of the anterior wall, but this adds another disease. “No one test answers all the questions.”)
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Case 2: STEMI: Syndrome of LAD occlusion
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EKG: The ST segment is elevated in two consecutive leads from V1,
V2, V3, or V4 (Figure 14.14).
Visualization: The ST is pointing anteriorly toward the patient’s
septum, anterior wall, or inferior wall.
Critical Thinking: Since the ST segment is pointing toward the septal and anterior walls, we locate the area of extreme or transmural ischemia here. The left anterior descending coronary artery (LAD) supplies blood (Figure 14.19) to the septum through its septal
perforators and the anterolateral wall through its diagonal branches. We expect a significant obstruction with a ruptured plaque and a superimposed thombus in the LAD.
We apply the Third Rule of the T Waves. We ignore the T wave changes and ST depression and diagnose ST elevation myocardial
infarction (NSTEMI). The LAD supplies nearly half the myocardium (Figure 14.18) with blood and so pump failure may result. This can lead to shock, pulmonary edema, or congestive heart failure. The LAD supplies the anterior part of the septum, so bundle branch
block, hemi-block, or complete heart block may occur.
LBBB as an ST Elevation Equivalent: In the setting of symptoms suggestive of an acute coronary syndrome, a new left bundle branch block is equivalent to diagnosis of ST segment elevation myocardial infarction (STEMI). If the LBBB is not known to be old, it should be considered clinically identical to a new STEMI.
LBBB as an ST Elevation Equivalent
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Case 3: STEMI: Syndrome of circumflex artery occlusion
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EKG: The ST segment is positive in leads I and AVL (Figure 14.16).
Visualization: The ST is pointing to the patient’s left side toward the lateral
wall (Figure 14.17).
Critical Thinking: Since the ST segment is pointing toward the lateral wall
(Figure 14.16), we locate the area of extreme or transmural ischemia here
(Figure 14.17). The left circumflex coronary artery (LCX) supplies blood to the
lateral wall through its obtuse marginal branches. In 10% of patients it supplies
a branch to the AV node, and then the inferior wall as well.
We expect a significant obstruction with a ruptured plaque and a superimposed thombus in the LCX. We apply the Third Rule of the T Waves. We ignore the T wave changes and ST depression and diagnose ST elevation myocardial infarction (NSTEMI). The LCX supplies the AV node only 10% of the time, therefore heart block such as 1° AV block, Wenckebach, or complete heart block are much less common than with an RCA occlusion. The circumflex artery supplies a relatively smaller portion of the LV than does the LAD, so pump failure, shock, and systolic heart failure are less common.
Pattern to memorize:The ST waves point toward (0° to −75°) in leads I and AVL, producing ST elevation in these leads.
Complications of STEMI: Because of the total deprivation of oxygen supply many myocardial cells quickly become severely hypoxic and “stunned.” Although possibly not yet
irreversibly damaged, they can lose contractile function to the point of shock, pulmonary edema, or congestive heart failure, as the cells become functionally incapable without oxygen for energy. Many cells die, and they die quickly. Mechanical complications such as rupture of the ventricular septum, rupture of the free wall of the left ventricle, rupture of the mitral valve papillary muscle, and aneurysm formation can occur. Infarction of the septum can damage the right and left bundle branches leading to heart block.
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Image credits
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