Dr.S.Venkatesan MD

It is  a well proven concept   beta adrenergic blockers have a useful role in controlling   the  frequency, and intensity  of  vaso- vagal syncope .

One may wonder how an anti adrenergic drug help to counter hyper vagotonia syndrome !

This is because  during  vaso -vagal  syncope ,  the  inital trigger is  sympathetic . A   sudden hyper adrenergic  surge occurs   that stimulate the vagus, ( Which  overshoots the   initial  quantum of adrenergic signal)   and  cause a systemic vasodilatation ,  hypotension and bradycardia.

How does adrenergic surge stimulate the  vagus?

By two ways

• Brain stem spill over effect in medulla (Vasomotor to tractus solitarius)
• Cardiac  stretch caused  by hyperadrenergic activity . This stretch initiates a  vagal reflex  especially from  the base of the heart (Similar to Bazold Zarish reflex ). This  mechanism is  thought to be more important than brain stem spill over  , that’s why  it is referred to as  neuro-cardiogenic syncope .

How does beta blocker help?

1.  It   sedates  the  adrenergic centre which  modulates the trigger  .It  also blocks the  sympathetic  afferent limb of the syncope circuit.
2.  Anxiety  and panic reactions are close associate’s of vaso- vagal syncope. They are  not only  considered as  prodrome for syncope  but also act as  important triggers.This is effectively tackled by beta blockers .
3. Finally , beta blockers  soothes the mycardial  stretch  receptors by reducing the  ventricular shear stress (Reduced contractility and wall stress )  hence neuro-cardiogenic  axis is  pacified.

It is important to remember beta blcokers can only  prevent/  reduce  episodes  of  vaso vagal syncope. It  may aggravate  the situation   if administered  shortly  after the event , as bradycardia and hypotension  is dominant  in the recovery phases.

*During an episode of vaso vagal syncope atropine group  of drugs is most useful .

Which beta blocker ?

Propronolol is the prototype  as it has non selectivity and good penetrance  of  blood brain  barrier ,  which is  the most appropriate site for suppressing hyper adrenergic drive.

Cardio selective beta blockers  do have a role as cardiac stretch  receptors is  one of the two target sites .

Final message

Ironically ,   in the long term management of  vaso-vagal syncope , anti adrenergic drugs  have a major role  rather than atropine like drugs .