What is Fibrillation ?

Fibrillation is an arrhythmia that affects either the atria as a pair, or the ventricles as a pair, producing “a-fib”, or “v-fib”, respectively. (Come to think of it, if a person is in VF, do their atria fibrillate as well? Does it matter?) Most cardiac rhythms are organized – they’re regular in some way, producing some sort of regular (as opposed to disorganized), rhythmic motion of the chambers, hopefully producing a blood pressure. In fibrillation, the cardiac tissue of the chambers involved wiggles about like (classic phrase) “a bag of worms”. Does a chamber wiggling like a bag of worms pump any blood, produce a cardiac output, eject any fraction of its contents? No, it does not!

As I always try to point out, all the waves that you see on EKG strips actually represent some kind of physical motion of one or the other set of cardiac chambers, and the trick is to try to visualize what those chambers are doing in any given rhythm situation. Let’s see if a quick review of some strips helps the visualization process. Can I have the first slide please?

Here we are: look familiar? Sinus rhythm. Organized, rhythmic, producing stable contraction of the chambers – first the atria, then the ventricles. So - visualizing on the mental screen, that’s what I see: nice orderly motion, first above, then below.

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Okay so far? Right – next slide, please. OK: atrial flutter. Still organized: the atria are contracting rapidly, sure, at about 300 bpm, and the ventricles are responding to every third or fourth impulse, slowly enough that the ventricular chambers have time to fill up nicely between beats, fast enough to probably maintain a good blood pressure. So I visualize the atria clipping along, with the ventricles contracting every third or fourth time.

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This one? Well – is it organized? Actually it is: see the pattern of doubles? It’s a little easier to figure out by looking at the lower part of the strip – this is a sinus rhythm, and after every sinus beat comes a PAC, followed by a compensatory pause. So yes, still organized. “Regularly irregular”.

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How about this one? Yup, VT. Ugly, scary, but still organized, regular – the chambers (which ones?) are moving in a steady manner. On your mental screen you should see the ventricular walls contracting very rapidly – do they have time to fill? Should we shock this rhythm? It depends…

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