AV nodal reentrant tachycadia(AVNRT) is the commonest mechanism of SVT. It is divided into slow-fast, fast-slow, 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 >
Final message
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 )
Dr.S.Venkatesan MD 