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Why sinus node dysfunction rarely kills a patient ?

Posted Apr 22 2009 11:57pm

Sick sinus syndrome  or sinus node dysfunction (SSS, SND ) is one of the common cause of  symptomatic bradycardia .The other cause for  pathological bradycardia is complete heart block.Together , these two entities share 99% of indications for permanentpacemaker implantation.

The sinus node can get affected in various diseases . The commonest cause for SND is age related.This is manifested  as inappropriate bradycardia .The  other common presentation of  SND is exaggerated bradycardia to betablockers and calcium blockers.In fact , some consider drug  induced bradycardia  is  nothing but  , unmasking of underlying SND.Pathological states that result in SND include  hypothyrodism , infiltrative   and inflammatory diseases . (Surprisingly ,  ischemic  SND  is a lesser  clinical problem when considering  the  rampant CAD in our population )

What is  is a fundamental difference between SND and complete  heart block* ?

Sinus node is the proximal most pacemaker of the heart. When it fails the chances of  a  subsidiary pacemaker coming to the rescue is far greater than  a complete AV block. Further the quality and stability of the escape pacemaker is better in SND. In fact , in pure SND  ( With out AV nodal disease)  a sinus arrest is rarely fatal as escape rhythm  occur without fail.

* It should be emphasised  ,  there can be associated AV nodal disease in  significant (10%)  number of patients with SND .This may be present either at the  time of diagnosis or it can develop later in the course .This has important implication in the selection of   pacemaker .The discussion here is confined to isolated SND .

How common is ventricular escape rhythm in SND ?

It is very rare. the ventricle never gets a chance to come to the rescue as invariably junctional pacemaker takes over at times of extreme sinus pause/arrest.For the same reason , pause dependent VT (Brady dependent ) is also less common in SND .

What is  stokes Adam’s attack ? How  common it is  seen in SND  ?

It is the cardiogenic  syncope due  to extreme bradycardia. This classically occurs in complete heart block , when

the the escape rhythm becomes either very slow or temporarily goes for sleep .This results in a huge  pause (unlike sinus pause  of   , the pause  here is  ventricular pause  , this is  actually an  asystole  )  it  can  immediately trigger an VT or VF .

If  SND is not life threatening why pace maker is indicated in them ?

The pacemaker is primarily indicated for prevention of dizziness , near syncope or syncope.So primary impact is on improving quality of life  , not reduction in mortality. While in CHB  pacemakers improve  symptoms and survival.

Which form of SND can be dangerous ?

When SND is associated with rapid atrial fibrillation  some times it can trigger a VT/VT if ,  these patients also have

a fast accessory pathway with short refractory period. (<250msec)

Final message

If you have only one pacemaker at your disposal , but there are  two patients ,  one with SND and other with CHB please put the pacemaker to  the patient with CHB , even if the later has insurance coverage and the former is not .You are justified in  diverting  the pacemaker !

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