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 in the ECG is the point where the QRS complex joins the ST segment. It represents the approximate end of depolarization and the beginning of repolarization as determined by the surface ECG. There is an overlap of around 10ms.

The J point marks the end of the QRS complex, and is often situated above the baseline, particularly in healthy young males. The J point may deviate from the baseline in early repolarization, epicardial or endocardial ischaemia or injury, pericarditis, RBBB, LBBB, RVH, LVH or digitalis effect.

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.

For simplicity:

  • 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.
ECG J point J wave J point elevation
J point in a) normal; b) c) J point elevation; d) J point depression; e) with J wave (Osborn wave)

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.
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Chris is an Intensivist and ECMO specialist at the Alfred ICU in Melbourne. He is also a Clinical Adjunct Associate Professor at Monash University. He is a co-founder of the Australia and New Zealand Clinician Educator Network (ANZCEN) and is the Lead for the ANZCEN Clinician Educator Incubator programme. He is on the Board of Directors for the Intensive Care Foundation and is a First Part Examiner for the College of Intensive Care Medicine. He is an internationally recognised Clinician Educator with a passion for helping clinicians learn and for improving the clinical performance of individuals and collectives.

After finishing his medical degree at the University of Auckland, he continued post-graduate training in New Zealand as well as Australia’s Northern Territory, Perth and Melbourne. He has completed fellowship training in both intensive care medicine and emergency medicine, as well as post-graduate training in biochemistry, clinical toxicology, clinical epidemiology, and health professional education.

He is actively involved in in using translational simulation to improve patient care and the design of processes and systems at Alfred Health. He coordinates the Alfred ICU’s education and simulation programmes and runs the unit’s education website, INTENSIVE.  He created the ‘Critically Ill Airway’ course and teaches on numerous courses around the world. He is one of the founders of the FOAM movement (Free Open-Access Medical education) and is co-creator of litfl.com, the RAGE podcast, the Resuscitology course, and the SMACC conference.

His one great achievement is being the father of three amazing children.

On Twitter, he is @precordialthump.

| INTENSIVE | RAGE | Resuscitology | SMACC

BA MA (Oxon) MBChB (Edin) FACEM FFSEM. Emergency physician, Sir Charles Gairdner Hospital.  Passion for rugby; medical history; medical education; and asynchronous learning #FOAMed evangelist. Co-founder and CTO of Life in the Fast lane | Eponyms | Books | Twitter |

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