What is the importance of recogising slow -slow AVNRT ?
Posted Jun 03 2010 10:08am
AV nodal reentrant tacycadia(AVNRT) is the commonest mechanism of SVT. It is divided into slow-fast, fast-low, slow-slow , representing the two limbs of he circuit.
Slow -Slow circuit is the rarest type of AVNRT. It should be appreciated , the scientific validity of slow-slow circuit is applicable only in relative terms . A virtually similar antegrade and retrograde limbs with identical conduction velocity and refractory properties , can neither initiate nor sustain an AVNRT.
Caveat in the definition of slow -slow AVNRT.
Even though , we call it a slow-slow tachycardia , one of the limbs need to be faster than the other. So , every slow -Slow AVNRT in reality will have two types
Slow- Slow ( Still , faster than antegrade slow) mimic a slow-fast physiology
Slow( Faster than retograde slow ) -Slow closely mimic typical fast slow .
Implication for electrophysiologists and points of contention for the ablationist !
In Slow -Slow AVNRT ablation we do not know exactly , which of the slow pathway is being ablated , unless we specifically analyse the post ablative data.
Very often it is not done.Every one in the lab is happy , for breaking the tachycardia circuit. Only after the procedure is over , we may realise the tachycardia is not really killed as it finds an alternate highway to complete the short circuiting of heart.
We need to suspect this type of AVNRT prior to the procedure .Electrophysiologist shall spend little more time and a wide area ablation done , in the vicinity of coronary sinus ostium can be attempted. .
It is not a smart practice to advocate wide area ablation as a routine protocol in all AVNRT
as it directly increase the rate of complication >
A hurriedly done slow pathway ablation which may temporarily terminate the AVNRT ,only to recur later as the retrograde slow pathway may again form a substrate .The area of slow conduction acts as a turnaround gateway and capture the retrograde fast pathway which could be available in plenty in the anterior aspects of AV node . (Note : The unablated slow pathway now form the antegrade circuit )