Myocardial Ischemia

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Myocardial Ischemia
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Ischemia

• Is when blood flow to the myocardium is
  insufficient to maintain the metabolic demand of
  the myocytes.
• Transmural Ischemia
• The hallmark of acute transmural ischemia (across
  the heart wall from endocardium to epicardium )
  is the elevation of the ST segment of the ECG.
• This is visualized by the ST-segment being
  raised above the isoelectric baseline.
• This is due to a higher (more positive) resting
  voltage of ischemic cells.

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• Which cause the ST-segment baseline to be more
  positive (an upwards deflection).
• This change in the ST segment is mostly localized
  in the lead most directly overlying the ischemic
  myocardial area.
• This may lead to unstable angina, and should be
  taken very seriously, as this is the condition
  that progresses to an acute MI.
• As the ischemia becomes more extensive the ST
  segment elevation becomes more pronounced.

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ST-Segment Elevation
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• Sub-Endocardial Ischemia
• Relative decreased flow in the subendocardial
  regions is a normal consequence of the squeezing
  of the myocardium, which compresses the blood
  supply to the endocardium during ventricular
  systole.
• However, this can also be a pathological
  condition, expressed in stable-angina. This is a
  condition where the myocardial demands exceed the
  coronary artery blood supply .

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• This is represented on the ECG as ST-segment
  depression.
• The ischemic cells become more positive in their
  resting voltage (due to channel leakage) and this
  makes the subendocardium more positive since the
  endocardium is further away from the precordial
  leads than the more negative mycardium we see
  this as a depression .

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ST-Segment Depression in subendocardial ischemia
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ECG showing depressed S-T segment (ischemia)
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• After the ischemia has progressed to an infarct,
  and the tissue has scarred, the ECG will show
• Q wave Necrosis (significant Qs only)
• Significant Q wave is one millimeter (one small
  square) wide, which is .04 sec. in duration or
  more or is a Q wave gt1/4the amplitude (or more)
  of the QRS complex.
• Old infarcts significant Q waves (like infarct
  damage) remain for a lifetime.

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• ST (segment) elevation (acute) Injury)
• Signifies an acute process, ST segment returns to
  baseline with time.
• T wave inversion Ischemia
• Inverted T wave (of ischemia) is symmetrical
  (left half and right half are mirror images).
  Normally T wave is upright when QRS is upright,
  and vice versa.
• Usually in the same leads that demonstrate signs
  of acute infarction (Q waves and ST elevation).

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Q-waves with Inverted T-Waves
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• Infarction Location
• Lateral
• Qs in lateral leads I and AVL (Circumflex
  Coronary Artery).
• Anterior
• Qs in V1, V2, V3, and V4 (Anterior Descending
• Coronary Artery)
• Inferior (diaphragmatic)
• Qs in inferior leads II, III, and AVF (R. or L.
  Coronary Artery).

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