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The heart has four chambers, two upper chambers—the left and right atrium, or together the atria, and two lower chambers—the left and right ventricles.
Well there’s a wave of muscle contraction that flows through the atria that looks like its flapping or fluttering, hence the name.
Usually, that signal moves in one direction from the atria to the ventricles through the AV node, it then moves down to the ventricles, and causes them to contract shortly after.
Reentrant signals loop back on themselves, overriding the sinus node and setting up an endless cycle that causes the atria to contract again and again and again—at really fast rates.
There are actually two types, type 1 or typical atrial flutter is more common and is caused by a single reentrant circuit that moves around the annulus, or the ring of the tricuspid valve of the right atrium, usually in a counterclockwise direction when viewed looking up through the tricuspid valve.
In this case, a stretch of tissue along the pathway called the cavotricuspid isthmus propagates the signal more slowly than the surrounding tissue.
Tissue that was just activated can’t be activated again until a certain amount of time has passed, which is called the refractory period; so that slow propagation gives the tissue enough time to be out of refractory, and therefore the circuit can loop on itself.
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