Ectopic beats or premature depolarisations are the commonest cardiac arrhythmia encountered . Human heart , is a non stop electro mechanical organ , and it is not surprising ectopic beats are so common and can literally originate from every cell of heart. But , generally it emanates primarily from the special conducting cells . At times , even other cells (Myocytes, interstitial cells ) can generate abnormal electrical potential.These ectopic electrical potentials can be compared to electrical load shedding when there is excess electrical strain .
Vast majority of ectopic are benign in human population. When this occurs , in the milieu of underlying heart disease or during ischemic episodes they become clinically important and initiate a sustained arrhythmia.
Classically and traditionally ectopic beats are described in the
A.Ventricle : Ventricular premature beats, (VPD)
B.Atrium: Atrial premature beats(APD)
C.AV junction : Junctional premature beats.(JPD)
If you note , one important structure is missing from the list.
Yes , it is SA node. Can it result in premature depolarisation ?
When do you suspect a SPD(Sinus premature depolarisation)
- It manifests a an sudden unexpected , sinus beat exactly as the previous sinus beat. Followed by a pause.
- The P wave morphology exactly is similar to prior p wave.
- Many times we miss this entity as we tend to over diagnose APD than SPD.
- SPDs tend to occur in bigeminy rhythm.
Differential diagnosis
- Sinus arrhythmia and pause
- APD
- SA node echo beats (Part of SA node reentry)
- SA blocks
How do differentiate a sinus arrhythmia from sinus premature depolarisation (SPD ) ?
Sinus arrhythmia occurs in a baseline bradycardia environment.
It does not not come as “on -off ” pattern . It has a gradual onset offset dynamics.
Clinical significance
This is a clinically unimportant arrhythmia* .This is probably the reason , it is not a popular concept .
*But it can confound in the diagnosis of , other important rhythm disorders.it could be a expression of sinus node dysfunction and a precursor of inappropriate sinus tachycardia The significance could be substantial in atrial triggered based pace maker rhythm
Final message
When you confront an unexpected , early , sinus beat not accountable to sinus arrhythmia or APD
suspect SPD.It is not rare , it is a grossly under diagnosed entity.
Reference
Sinus premature systole http://www.chestjournal.org/content/64/1/111.full.pdf?ck=nck
Dr.S.Venkatesan MD 