AV node is the “Go slow” region in the cardiac highway .Every impulse is delayed for about 120ms and then pursue its onward journey to depolarize the ventricle.
Since AV node has inherently slow conduction properties , it is not surprising this zone is vulnerable for developing AV block .We know AV junction and the adjacent his bundle is the site for many types of AV block. In classical Mobitz type 2 AV block , for every two or three supra ventricular impulse only one is conducted and we call this as 2:1 or 3: 1 AV block ( More appropriately AV conduction )
Can we have reverse of the above situation ? That is , for each supra ventricular impulse can ventricles fire twice or thrice ?
Yes it can , what looks like a funny situation , could be more common .We are not recognising it often.
How is this possible ?
This can happen only if there are two different tracts of conduction from atrium to ventricle and both of them conducting fully to reach ventricle and complete the depolarisation.
This situation can occur in
- Dual AV nodal pathway*
- Triple nodal pathway**
- Multiple AV accessory pathways (All contributing AV conduction )
* Exact incidence in general population is not known ,but it could be higher than what we believe !
** Very rare
Some what related phenomenon , never the less , it mimics 1:2 or 1 : 3 AV conduction
- AV nodal echo beats
- Non sustained AVNRTs
How is simultaneous conduction possible in dual AV nodal physiology ? Will ( it not ! ) the first impulse make the ventricle refractory to the following impulse ?
Under normal physiological conditions simultaneous conduction* is not possible .It happen if . . .
- The first impulse goes relatively fast and activate the ventricles .
- The second component of the first impulse, ie through the slow path conduction is sufficiently slow , it reaches the ventricle and able to depolarize it , well after the first beat’s refractory period .
- A Further requirement is , the initial fast response fails to block the incoming slow response by a retrograde slow path block .
* It need to be further clarified , even in physiology , simultaneous conduction is possible , but it is often incomplete . At best it can result in ventricular fusion beat as in pre -excitation beat or it can be a concealed one travelling halfway through the AV node or the bundle.
Why recognising this 1:2 conduction is important ?
- It is traditional to think , an unexpected beat occurring prematurely in a given strip of ECG is always thought to be an ectopic beat .This is not the case. An unexpected premature narrow QRS complex with out a p wave , should make us suspect dual AV nodal conduction .
- If this dual AV nodal pathway is intermittently conducting or conducting with varying velocities , it becomes an irregular narrow QRS rhythm .This can , very well be confused with atrial fibrillation.
- If one of the paths in the dual AV pathway is conducted aberrantly it mimics a VPD.
1:2 AV conduction may not be rare . Cardiac physicians are encouraged to look for this phenomenon whenever they encounter an abnormal early narrow QRS beat without preceding P waves. Apart from academic curiosity , it can solve many mysteries in CCUs and EP labs .