STi
The ST interval is measured from the end of the QRS complex (the J point) to the end of the T wave. It therefore includes the ST segment and the whole T wave, and is also called the JT interval. It reflects the duration of ventricular repolarisation alone, without the time needed for depolarisation (the QRS). This makes it useful when the QRS is wide, for example in bundle branch block or ventricular pacing, because a wide QRS lengthens the QT interval even if repolarisation itself is normal.
How it is used
ST (JT) interval = QT interval − QRS duration
Like QT, the ST interval shortens as heart rate rises and can be corrected for heart rate. The AHA/ACCF/HRS recommendations note that in patients with ventricular conduction defects, QT should be interpreted with allowance for the QRS duration, and the JT interval is one way to do this. A long ST interval has the same causes and risks as a long QT: medicines, low potassium, magnesium or calcium, congenital long QT syndrome, ischaemia and bradycardia. A short ST interval occurs with high calcium and digoxin.
ST interval and ST segment
- The ST segment ends at the start of the T wave; its level (elevation or depression) is the key sign of ischaemia
- The ST interval ends at the end of the T wave; its duration reflects how long repolarisation takes
- Both start at the J point
A prolonged ST or QT interval should be reviewed by a doctor, together with all medicines being taken and blood levels of potassium and magnesium. Fainting or palpitations need urgent assessment.
References
- Rautaharju PM, Surawicz B, Gettes LS. AHA/ACCF/HRS recommendations for the standardization and interpretation of the electrocardiogram: part IV: the ST segment, T and U waves, and the QT interval. Circulation. 2009;119(10):e241–e250. doi:10.1161/CIRCULATIONAHA.108.191096
- Drew BJ, Ackerman MJ, Funk M, et al. Prevention of torsade de pointes in hospital settings: a scientific statement from the American Heart Association and the American College of Cardiology Foundation. Circulation. 2010;121(8):1047–1060. doi:10.1161/CIRCULATIONAHA.109.192704
This article is for general information and does not replace a consultation with a doctor. ECG findings must be interpreted by a physician together with symptoms and other tests.
Published on 7 July 2019