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ECG Basics: Junctional Rhythm

Sat, 12/28/2013 - 18:59 -- Dawn

A basic rhythm strip showing junctional rhythm in Lead II.  The junctional pacemaker is located between the atria and the ventricles, and the resulting P wave is caused by retrograde conduction through the atria.  This causes the P wave to be negatively deflected in Lead II.  In junctional rhythms, the P wave can occur just before the QRS, during the QRS, or after the QRS, or may not be seen at all.  If the P wave occurs before the QRS, the PR interval is usually short, reflecting the fact that the atria and the ventricles are depolarized almost simultaneously.  In this example, the PRI is .12, on the short side of normal.

The junctional pacemakers have a slow intrinsic rate so that the sinus node can remain in control of the heart's rate under normal circumstances.  If the sinus rate drops below the intrinsic rate of the junctional pacemaker, the junction will take over control of the heart.  An idiojunctional rhythm is generally between 40 and 60 bpm.  In this example, it is about 63 bpm.

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Comments's picture

As per Dawn - the rhythm in the Figure is regular at a rate just over 60/minute - has a narrow QRS - and is preceded by a negative P wave with fixed PR interval. This rhythm is often presumed to be one of the 3 varieties of a junctional rhythm (the other two being absence of any P wave in lead II - and presence of a negative P wave in lead II that only appears after the QRS complex). I'll add a few additional clinical points:

  • The rate of idiojunctional escape may vary with age. In children, rather than between 40-to-60/minute - the AV nodal escape rate typically varies between 50-80/minute (depending on the age of the child). Thus, it is important for us to know the age of the patient - since the finding of an "accelerated" junctional rhythm should prompt a different set of diagnostic entities (ie, Dig toxicity if taking the drug; post-operative state, hypotension, etc.). In this case - although 63/minute is a bit over "60/minute" - it is not enough over this upper limit to prompt a search for other underlying cause.
  • This is NOT necessarily a junctional rhythm. You can see a negative P wave with fixed PR interval in low atrial rhythms positioned in fashion such that the direction of atrial activation is seen as moving away from lead II. There is really NO WAY clinically to distinguish between low atrial rhythm (with negative P preceding the QRS) vs junctional rhythm. Although junctional rhythms typically have a shorter PR (since their site of focus is generallly closer to the AV node than it is for a low atrial focus) - other factors (including speed of conduction) may alter this relationship. Therefore - we really have NO IDEA if the above rhythm is low atrial or junctional.
  • The "good news" - is that clinically, it really does NOT matter whether this rhythm is low atrial or junction - as clinical implications are essentially the same for these 2 rhythms (ie, benign if the patient is asymptomatic and otherwise healthy - or potentially pathologic if there is underlying disease or metabolic derangement).

Ken Grauer, MD   [email protected] 

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