Dr.S.Venkatesan MD

Atrial fibrillation is one of  the common tachycardias encountered in cardiology practice.In this condition even though atria fibrillates  up to 600 times a  minute, only a fraction of that reach the ventricles. Thanks to the AV node.It acts like an electrical sink . Hence in  most  episodes of AF ,  the ventricular rate will  be   manageable and hovers   between 150-220 .We  also know ,  in the presence of  accessory AV nodal pathway there is a risk of 1:1  conduction and  result in  ventricular fibrillation and risk of sudden death. (Sudden death in WPW syndrome)

A case scenerio

The other  day  my resident called  me  to inform about a  patient with   atrial fibrillation and  hypotension    .

I told him  , to  control  the heart  rate with Amiodarone  and  I  shall come in shortly ,

He replied ,  the rhythm looked  to him  like a bradycardia  !   I  asked him to wait  , when  I went there  , it  turned out to be  an  interesting  ECG . 

This was  a  73 year old man  in our  ER with  a  syncope  .

1. 

   Atrial fibrillation with ventricular bardycardia

Magnified view of lead V 1 and V 2. Note the ventricular rate of 40 /mt even a the coarse f wave are recorded > 300 mt .He had a structurally normal heart .This patient has been adviced a VVI pacemaker .

While atrial  fibrillation is primarily a tachycardia , occasionally  like the  above patient  it   may present as bradycardia ! 

 How this happens ?

As mentioned before  AV node  acts like  an electrical sink .

How AV node is able to  filter out much of the  incoming impulses is not clear. This property of AV node is actually the major physiological property of AV node .This is  refered to as decremental conduction (The  faster it is bombarded with electical stmuli the longer it will take rest !)  When this filtering function  of  AV nodal tissue  is too much   we call it  pathological  AV nodal response.Some believe  ,  it is an expression of  associated   pathological  high-grade AV nodal  block .  Others belive it is simple vagotonia.

Another possibility  is  it is a sequale to  complete  AV block and what you witness is nothing but  a  junctional escape rhythm ( But here RR interval  would be fairly regular )

Excessive AV nodal blockers (Digoxin/Verapamil )  can mimic the same picture .

What is  the  relationship between sinus  node dysfunction(SND) and atrial fibrillation  ?

AF with slow ventricular response is common in elderly population with sinus node dysfunction.

AF can be associated with SND in two ways

1. Atrial disease  and sinus node dysfunction is known  to occur  together   .  This is not surprising,  considering the close proximity they live. SA node is surrounded by  atrial tissue in its entire length and  breadth .When degenerative and infiltrative  disease of atria occur it  may trigger a simultaneous SND  ,  as well as  atrial fibrillation .
2. While another possibility is that   AF is a default electrical response to SND . There is  some evidence  to suggest  the atria may  release a   ectopic escape rhythm  which  may  either degenerate into AF  or  mainfest a  primary  AF .

What is controlled ventricular rate and what is slow ventricular rate in AF ?

• This aspect is not well-defined  in literature.
• Controlled response generally  means  HR  70- 90/mt
• Slow  ventricular response would be <60 /mt
• Pathological bradycardia is diagnosed with  HR < 50  or at  any  symptomatic slow rhythm .
• Holter or event monitors would help in these situations.

What is the incidence of AV nodal disease in SND ?

AV nodal disease is seen in significant population of SND.(Some series show up to 30 %) .Further ,  the incidence of  new  onset  AV block  increase   with every year of follow up ) Reversible forms are commnly due to drugs and electrolyte disorders. The AV nodal disease has another importance as they determine the selction of pacemaker  mode .SND with intact AV node function can be  managed with atrial based pacemaker ,while  ventricle must be  paced in patients with AV block or in whom the risk of AV block is high.

  Is there a clinical advantage of   having  some AV nodal disease in AF ?

It may seem so , as long as the AV nodal disease  do not lead to severe  symptomatic  CHB.A slow ventricular rate is a desirable response in patients with  angina and cardiomyopathy (especially tachycardic ). While we continue to  debate for years  about the superiority of   rhythm  control  over rate   control  ,  if the AV node  chooses to slow down by natural means  , ventricles would   welcome it with pleasure !  

Final message

Atrial fibrillation is  primarily a  tachyarrhytmia ,  occasionally it may present as  bradyarrhytmia .In this scenario one has to suspect  hidden AV nodal as well as sinus nodal dysfunction. ( This entity was also refered to as Tachy brady syndrome )   It is important to recognise this entity  because many times  dangerous bradycardias   have occurred with a single dose of  Amiodarone  bolus or  DC shock . These episodes  represent   “unmasking  effect”   of   occult AV nodal  disease.