The Ischemic Cascade on EKG
Key stages, criteria, and pitfalls in recognizing ischemia, injury, and infarction on the EKG.
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Questions Covered in This Set
10 cards to master
Why do ischemic changes appear in repolarization (ST/T) before depolarization (QRS)?
Repolarization is slow and depends on ATP-hungry pumps and potassium channels, so it fails first with oxygen deprivation; Q waves (a depolarization change) only appear once tissue is actually dead.
Why does subendocardial ischemia cause ST depression?
Coronaries feed from outside inward, so the subendocardium suffers first with partial occlusion; the injury current vector points away from the electrode, producing ST depression.
What are hyperacute T waves and how do they differ from hyperkalemic T waves?
Earliest change (first minutes): tall, broad, symmetric, upright T waves with a fat base. Hyperkalemic T waves are narrow and peaked.
List the five stages of the ischemic cascade in order.
1) Hyperacute T waves, 2) ST depression/T-wave inversion, 3) ST elevation, 4) Q-wave formation, 5) T-wave inversion and resolution.
What defines a pathologic Q wave?
>40 ms wide, or >25% the height of the following R wave, in two contiguous leads.
What are the ST elevation thresholds for STEMI?
≥1 mm in two contiguous leads; in V2–V3: ≥2 mm (men ≥40), ≥2.5 mm (men <40), ≥1.5 mm (women). Measure at the J point relative to TP/PR baseline.
What is the shorthand for interpreting ST elevation vs Q waves?
ST elevation = happening now; Q waves = already happened; both together = happening now with some muscle already lost.
What does persistent ST elevation with well-formed Q waves weeks later suggest?
A ventricular aneurysm rather than acute occlusion.
Describe the Wellens patterns.
Pain-free patient with biphasic (Type A) or deeply symmetric inverted (Type B) T waves in V2–V4, preserved R waves, no Q waves — signals critical LAD stenosis despite no ST elevation.
Why do reciprocal ST depressions matter?
ST depression in the opposite lead group strongly confirms that the ST elevation represents true transmural injury.
