14. Totally Occluded Artery: ST Elevation MI

Headshot of Chris Anderson, MD · Clinical Education Director, Pediatrics
Chris Anderson
MD · Clinical Education Director, Pediatrics
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Headshot of Kevin Hodges, Vice Chair, Emergency Medicine
Kevin Hodges
Vice Chair, Emergency Medicine
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Table of Contents
  • Define and identify the following:
    • Normal ST segment.
    • Normal ST segment amplitude.
  • Define the two EKG criteria for abnormal ST segment elevation.
  • List four clinical causes of ST elevation.
  • Describe the expected EKG changes in ST elevation MI (STEMI).
  • Describe the pathophysiology of STEMI.
  • Define the Third Rule of the T Waves.
  • Describe the clinical approach to treatment of STEMI.
  • Identify the ST changes for each of the major coronary artery occlusions.
  • Define and Identify reciprocal changes in the setting of ST segment elevation.
  • Identify ST changes associated with the posterior wall.
  • Identify the circumstances that make LBBB an ST elevation equivalent.
  • Describe conduction abnormalities associated with STEMI.

ST segment elevation: An overview

The ST segment represents electrical activity produced during ventricular repolarization. Normally, the ST segment is at the baseline because only an even exchange of charges is taking place, as ions inside and outside the cell exchange places to be ready for the next QRS. Ions normally move only through specialized ion doors (channels) in the otherwise impermeable cell membrane. These channels completely control the entry and exit of each ion. The ion channels require energy to function, and loss of energy production from ischemia affects the cell’s ability to control overall ion flow, direction, and balance. Cell death from severe ischemia generates holes in the cells, and ions can flow freely in and out bypassing the channels completely. In addition to ischemia, inflammation, drugs, electrolyte abnormalities, and genetic variation can each affect the normal function of these cardiac channels. ST elevation can also be seen in bundle branch block, hypertrophy, and pericarditis.

ST elevation: Pathophysiology

ST segment elevation in two neighboring (contiguous) leads, in the setting of appropriate symptoms, typically represents a 100% blockage in a coronary artery. The occlusion has two components.

The first is an underlying atherosclerotic plaque. This plaque accumulated slowly, over a period of years, on the inside wall of the coronary artery. At this point (Figure 14.3), the lesion may not have been large enough to obstruct blood flow either at rest or with stress. The patient may have had no previous symptoms, and a stress test would have been negative.

This all changes with the development of an acute coronary syndrome. It begins with an unpredictable rupture of that atherosclerotic plaque, which pours highly thrombogenic material on top of the plaque (Figure 14.4).

This immediately attracts and activates the coagulation system. A thrombus forms at the site of the ruptured plaque and forms the second component of the blockage (figure 14.5). When the thrombus combines with the underlying ruptured plaque, it can completely occlude 100% of the arterial lumen. This reduces the oxygen supply to zero producing extreme ischemia across a large layer of myocardium. This is called transmural ischemia, or ST segment elevation MI (STEMI). Although the terminology seems irreversible, immediate opening of the artery can salvage much of the myocardium. The disruption in ionic control produces the classic ST segment finding, which is attracted to the part of the ventricle supplied by the blocked artery.

Location of ischemia

Complete occlusion of a coronary artery represents the extreme case of supply and demand mismatch. The region of ischemia involves not just the subendocardium, but the entire thickness of myocardium. Therefore, it is called “transmural” ischemia. Clinically, it is called ST elevation MI (STEMI).

The third rule of T waves

The Third Rule of T Waves states: On a single EKG, if ST elevation is present in two contiguous leads, ignore the presence of T wave inversion and even ST depression. ST elevation is the primary process. In our previous analogy, overheating myocardium produced the smell of smoke (T wave inversion), or visible smoke (ST depression). ST elevation is seeing the myocardium in flames. There is no longer any question as to the next action. Diagnose Acute STEMI. Call the appropriate “Code MI” in your hospital. If you have a capable, experienced interventional lab, notify the personnel immediately. The clock has started on rescuing the myocardium trapped without oxygen behind the 100% occlusion.

Treatment of STEMI: Primary angioplasty vs. thrombolysis

Immediate recognition of ST segment elevation on an EKG allows for surgical intervention to open the closed artery.

In the absence of an immediately available interventional lab, administration of thrombolytic drugs can dissolve the thrombus that overlies the ruptured atherosclerotic plaque. Intravenous thrombolytic therapy takes less time to administer than arranging for emergency angioplasty, but has a greater risk of severe hemorrhage. Figure 14.9a demonstrates an occlusion of the LAD artery, which opens up in Figure 14.9b after therapy.

memorize this

Table 14.1 Location of the ST Segments.
Patterns of ST segment elevation Location of ischemia/infarction
III, AVF (+/-II)
Inferior
I, AVL
Lateral
I, II, AVF
Apical
V1, V2
Septal
V3, V4
Anterior
V5, V6
Lateral

STEMI: Summary