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 !
Dr.S.Venkatesan MD 