The J point
The J point is the the junction between the termination of the QRS complex and the beginning of the ST segment.
The J (junction) point marks the end of the QRS complex, and is often situated above the baseline, particularly in healthy young males.
On most ECGs the determination of the J point as a demarcation between QRS and the start of the ST is clear. However with the advance of electrophysiological studies and scrutiny of the cellular/ionic mechanisms at each stage of the ECG – these lines become blurred.
- J point is present in all ECGs and marks the transition of QRS complex to ST segment
- J wave deflection occurs before the J point
- The position of the J point in relation to a slurred terminal QRS is still debated.
Note: The letter J on the ECG defines 2 totally different and unrelated events. The J point is a point in time marking the end of the QRS and the onset of the ST segment present on all ECG’s; the J wave is a much less common long slow deflection of uncertain origin originally described in relation to hypothermia.
Abnormalities of the J point
- Elevation or depression of the J point is seen with the various causes of ST segment abnormality. It may be elevated as a result of injury currents during acute myocardial ischemia and pericarditis, as well as in various other patterns of both normal and abnormal ECGs
- Elevation of the J point occurs with benign early repolarisation
- A positive deflection prior to the J point is termed a J wave (Osborn wave) and is characteristically seen with hypothermia.
- Gussak I, Antzelevitch C. Early repolarization syndrome: clinical characteristics and possible cellular and ionic mechanisms. J Electrocardiol. 2000;33:299-309. [PMID 11099355]
- Tikkanen JT, Anttonen O, Junttila MJ, et al. Long-term outcome associated with early repolarization on electrocardiography. N Engl J Med. 2009;361:2529-2537. [PMID 19917913]
- Surawicz B, Macfarlane PW. Inappropriate and confusing electrocardiographic terms: J-wave syndromes and early repolarization. J Am Coll Cardiol. 2011;57:1584-1586. [PMID 21474037]
- Perez MV, Friday K, Froelicher V. Semantic confusion: the case of early repolarization and the J point. Am J Med. 2012 Sep;125(9):843-4. [PMID 22340816]
- Haissaguerre M, Derval N, Sacher F. et al. Sudden cardiac arrest associated with early repolarization. N Engl J Med. 2008;358: 2016–2023. [PMID 18463377]
ECG Library Basics
LITFL Further Reading
- ECG Library Basics – Waves, Intervals, Segments and Clinical Interpretation
- ECG A to Z by diagnosis – ECG interpretation in clinical context
- ECG Exigency and Cardiovascular Curveball – ECG Clinical Cases
- 100 ECG Quiz – Self-assessment tool for examination practice
- ECG Reference SITES and BOOKS – the best of the rest
- Brady WJ, Truwit JD. Critical Decisions in Emergency and Acute Care Electrocardiography
- Surawicz B, Knilans T. Chou’s Electrocardiography in Clinical Practice: Adult and Pediatric
- Wagner GS. Marriott’s Practical Electrocardiography 12e
- Chan TC. ECG in Emergency Medicine and Acute Care
- Rawshani A. Clinical ECG Interpretation
- Mattu A. ECG’s for the Emergency Physician
- Hampton JR. The ECG In Practice, 6e