Lesson 8 of 21 · Waves & Intervals

The ST Segment

The ST segment runs from the J point (end of QRS) to the start of the T wave. It should be flat at the level of the PR/TP baseline. Elevation or depression is the central question of ischemia.

01Measuring

Use the PR segment (or TP) as baseline
Measure at the J point
STEMI: new STE in ≥ 2 contiguous leads: ≥ 1 mm in all leads except V2–V3
V2–V3: ≥ 2 mm men ≥ 40 y, ≥ 2.5 mm men < 40 y, ≥ 1.5 mm women

02Shapes matter

Convex ('tombstone') STE with Q waves: STEMI.

Diffuse concave STE with PR depression: pericarditis.

Horizontal/downsloping ST depression: subendocardial ischemia.

Clinical pearl. ST depression localizes poorly: it does not tell you which artery. ST elevation does. Always look for reciprocal changes.

03Mimics of STEMI

Benign early repolarization (concave, notched J point, young)
Pericarditis (diffuse, PR depression, no reciprocal except aVR/V1)
LBBB and paced rhythms
LVH with strain
Brugada pattern
Hyperkalemia
LV aneurysm (persistent STE weeks after MI)

Key takeaways

Measure at the J point vs the PR baseline
Reciprocal depression supports STEMI
Concave + diffuse + PR depression = think pericarditis