Ectopic beats , other wise called premature depolarisaton are one of the common ECG abnormalities diagnosed by physicians.
- Atrial premature beats (APDs)
- Ventricular premature beats(VPDs)
APDs and VPDs form the bulk of all clinically important ectopic beats.
Heart has a specialised electrical conducting system , every cell in this system is capable of firing on it’s own. But why then only the atrium and ventricle produce ectopic beats .Other structures like AV node, His bundle , purkinje are relatively rare to produce ectopic beats .
Is the AV junction relatively immune to develop JPDs?
The answer to this question would be “May be yes” . Yet, we need to recognise they may not be as rare as we think , many times we fail to diagnose it or rather recognise it !
Certain observation about Junctional premature depolarisation are made .AV junction has unique properties than any other parts of the heart.The basic purpose of AV junction ( AV node is not a preferred word as it has no anatomically distinct demarcation) is to apply a electrical break on the incoming electrical signal .Nature does this with a purpose . It is essential for the ventricles to fill adequately . We call it as PR interval.
So, when the basic purpose of AV junction is slow down the conduction it is logical to expect it won’t get irritated that easily and result in ectopic beats. So JPDs are less common than other forms of ectopic beats.
What is invisible JPD and HIS ectopics ?
We should realise many of the JPDs & his bundle ectopics are not conducted , the impulses simply dissipate down hill . Unlike the atrium and ventricle the junctional and his tissue has no associated chambers to depolarise , hence they are not often visible in the surface ECG.The only evidence in the surface ECG may be an unexpected pause which represents concealed conduction. A EP study of the bundle ECG often unmask these silent JPDs and His VPDs.
JPDs are less common , while junctional escape beats are the hall mark of any severe supraventrcualr bradycardia . How does that occur ?
AV junctional cells have an unique behavior in that , it comes to the rescue of the heart whenever the native SA node becomes too slow . This happens as a passive response .We call this as junctional escape beat.The major difference between a JPD and Junctional escape beat (JEP or JED ) is in the initial timing of the beat . Escape beat comes late .The coupling interval of escape beat (We generally use coupling interval for ectopic beats only , but it helps to understand ) will be longer than the previous sinus cycle. So escape beat is never premature (Rather a post mature beat !) .Ectopic beats are always premature ,( except Interpolated ) and occurs earlier than the next anticipated beat.
The other difference is escape beats are tolerated well as the primary purpose is to rescue back up.Their rate is generally equal to the intrinsic rate of AV junction ie around 40-50.
General characters of Junctional premature beats and tachycardia
- Fortunately rare, fires at a higher rate.(Unlike junctional escape beats )
- Enhanced automaticity is a common mechanism
- Reentrant JPD is rare , unless the patient has AVNRT or it’s variant physiology.
- Manifest as narrow qrs complex . JPD with aberrancy is distinctly possible .In that case differentiation from VPD may be difficult.Retograde P wave morphology may help.But it is non specific as VPDs also have varied atrial capture depending upon the VA conduction .
- Causes include Hypoxia, (Rarely ischemic junctional tachycardia. ) common causes include digoxin induced , post operative states, incessant JT
- JTs are Difficult to control.Overdrive pacing may be needed. May lead onto tachycardic cardiomyopathy.
- A benign form of junctional ectopic tachycardia is also reported .
Importance of Junctional escape rhythm
The role of AV junctional escape is vital in extreme bradycardia , as if the junction fails to escape the dangerous ventricular cells take over electrical control and that’s bad news for the heart with sinister consequence.The situation can rapidly degenerate to VT , what we call as phase dependent or brady dependent VT. The treatment for which is increasing the proximal heart rate. By isoprenaline or pacing. So the AV junction does a delicate balancing act .At times of tachycardia it blocks unnecessary impulses.At times of extreme bradycardia it assists the heart as escape rhythm . The problem here is many of the disorders that affect SA node , affect the AV node as well .So , AV node may not be able to help the SA node always.That is the reason many extreme myocardial end up with VT straightaway.
Final message
JPDs are not very uncommon as one would believe.It has some unique properties. There are vital difference between JPDs and junctional escape beats.JPDs can trnasform into JTs in local pathological milleu and as a rule they are difficult to control.
AVNRT is also a type of junctional tachycardia but, it is delinked from the ( unofficial ! ) classification of JT , not with any academic purpose but by tradition.
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Dr.S.Venkatesan MD 