Atrial flutter usually begins with a premature beat and may be supported with a reentry mechanism or a single ectopic focus, creating a regular rhythm of 240-350 bpm. Not all atrial contractions are propagated through to the ventricles due to a variable block within the junctional area (this is due to the refractory nature of the conduction pathway at the higher rate). The second degree block is symptomatic. When conduction through the ventricles does occur, the morphology is normal but RR interval is irregular.
The atrial waves give a sawtoothed morphology, which is said to be comprised of F waves and not P waves. There is no PR interval. The complex is described as irregularly irregular.
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