ECG Basics · Lesson 12 · Leads, axis and conduction
ST segment and QT
The ST segment should sit on the baseline; the QT should fit the heart rate. Both are where the ECG most directly warns of danger.
ST elevation in a territory
New ST elevation at the J point in two contiguous leads suggests acute injury. Common thresholds are ≥ 1 mm in most leads. V2–V3 use higher cut-offs: ≥ 2 mm in men 40 or older, ≥ 2.5 mm in men under 40, and ≥ 1.5 mm in women. Elevation that respects one coronary territory, with reciprocal depression in the opposite leads, is more convincing than diffuse elevation.
This engine example shows inferior ST elevation (II, III) with reciprocal depression in aVL.
Diffuse ST elevation with PR depression points toward pericarditis. Other mimics include early repolarisation, LVH, LBBB, ventricular pacing and Brugada pattern. ST depression can mean ischaemia, a reciprocal change, a secondary change, or a drug effect such as digoxin.
The QT interval
Measure from QRS onset to the end of the T wave, then correct for rate. A long QTc delays repolarisation and favours early afterdepolarisations, which can trigger torsades de pointes. Common causes and risk factors are QT-prolonging drugs, low potassium and low magnesium, low calcium (which lengthens the QT), slow rates and pauses, and congenital long-QT syndromes.
Key points
- Territorial ST elevation with reciprocal depression suggests acute injury; time matters.
- Always measure QT and correct it for rate; QTc over 500 ms is high risk.
- Look for causes you can fix: drugs and electrolytes.
References
- Thygesen K, et al. Fourth universal definition of myocardial infarction (2018). Circulation 2018;138:e618–e651.
- Rautaharju PM, et al. Recommendations for the standardization and interpretation of the electrocardiogram, Part IV: the ST segment, T and U waves, and the QT interval. J Am Coll Cardiol 2009;53:982–991.