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?
- It sedates the adrenergic centre which modulates the trigger .It also blocks the sympathetic afferent limb of the syncope circuit.
- 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 .
- 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 .
Ironically , in the long term management of vaso-vagal syncope , anti adrenergic drugs have a major role rather than atropine like drugs .