RR interval in Atrial fibrillation is irregular because . . .
- The Atria fires irregularly
- AV node conducts irregularly
- Atria confuses the AV node with its random firing and varying penetration *
- The ventricle just reflects irregular response of atria .
The answer is all of the above. Response 3 explains best.
*Please note , the AV nodal property is predominantly responsible for the irregular RR interval in AF . Atria confuses the AV node with its random firing .The varying penetration into different depths of AV nodal structure and the resultant concealed conduction make the the AV nodal refractory period into continuous oscillation .This random delays in AV node is reflected in RR interval as irregularity )
The response we get in ventricles in AF can be summed up as “A filtered atrial rhythm”
Paradoxically, amidst the chaos in atria the rate is fairly constant within the atria (Fibrillatory wave firing at up-to 600/mt ) Of course , the FF interval in the atria will also be varying . At a rate of 450-600 this is difficult to quantitate especially in fine AF.
When does RR interval becomes regular in AF ?
- When the patient develops complete heart block.
- Digoxin toxicity
- Associated Sinus node dysfunction
For advanced readers in EP : A mystery explanation for irregular rhythm in AF in the offing ?
AV node is a physiological and electrical sink .
When atria fires at 600/mt it absorbs about 60-70 % of the atrial response .Whether it releases the original impulse or initiate a new rhythm in the junction is not clear.
There is some evidence to suggest the rhythm that control the ventricle in AF may not be filtered original rhythm from the atria .Instead it could be a fast junctional escape rhythm (Is that a junctional fibrillation ?)
Dr.S.Venkatesan MD 