Why is Afib irregular rhythm?

[voicesinmyhead]

Doesnt the ventricular beat in Afib depend on the AV node refractory period,

Isnt Av node refractory period always same?

Doesnt this mean that even if atrial contraction is irregular, a beat will only be transmitted depending on the refractory period which is constant? - Hence shouldnt ventricular rhythm be regular?

Kinda overkill question. But thought I'd ask,

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[thehundredthone]

A constant refractory period (which is not exactly true) would only result in a fixed minimum value for the conduction delay, and thereby the conduction ratio. If the gap between two generated impulses is higher than that refractory period then the rhythm can still be irregular. So for example if your refractory period was 100 ms, but the impulses were generated at 0, 110, 240 and 340 ms respectively, you would still end up with an irregular heart rate.

The ventricular rate does depend on the refractory period, that's why even with an atrial rate of 5-600 the ventricle only goes up to about 120-150.

http://cardiovascres.oxfordjournals.org/content/44/2/249.long
http://circres.ahajournals.org/content/35/2/204.full.pdf
http://www.ncbi.nlm.nih.gov/pubmed/2229766

 

[DeeJay2728]

Yes, you're right... that's why afib is considered an Atrial arrhythmia. The irregular ectopic beats are coming from the atria.

 

[DeeJay2728]

Also, the R-R intervals of afib are not equal and are therefore considered irregular.

 

[starstarie]

The way I think about a. fib is that there are very irregular impulses coming from different parts of the artium (not the SA node like normal), which hit the AV node at different and irregular times (some before the refractory period, some after). Thus the impulses that do get conducted through are not happening in any set pattern.

In contrast, for example, A. flutter is a reentry circuit that is going around and around. It is going very fast and so not all the impulses that hit the AV node are getting conducted either (due to the refractory period in this case) BUT in this case they are regular because the atrial impulse is regular (albeit too fast to conduct all of them)