Ectopic (unifocal) atrial tachycardia arises from an independent focus within the atria, distinguished by a consistent P-wave of abnormal shape and/or size that falls before a narrow, regular QRS complex. It can be caused by automaticity, which means that some cardiac muscle cells, which have the primordial (primitive, inborn, inherent) ability to generate electrical impulses that are common to all cardiac muscle cells, have established themselves as a rhythm center with a natural rate of electrical discharge that is faster than the normal SA node. Some atrial tachycardias, rather than being a result of increased automaticity may be a result of a micro-reentrant circuit (defined by some as less than 2 cm in longest diameter to distinguish it from macro-reentrant atrial flutter). Still other atrial tachycardias may be due to triggered activity caused by after-depolarizations. Multifocal atrial tachycardia (MAT) is tachycardia arising from at least three ectopic foci within the atria, distinguished by P-waves of at least three different morphologies that all fall before irregular, narrow QRS complexes. This rhythm is most commonly seen in elderly people with COPD. Atrial fibrillation meets the definition of SVT when associated with a ventricular response greater than 100 beats per minute. It is characterized as an "irregularly, irregular rhythm" both in its atrial and ventricular depolarizations and is distinguished by its fibrillatory atrial waves that, at some point in their chaos, stimulate a response from the ventricles in the form of irregular, narrow QRS complexes.
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References
External links

Cardiac Disorders – Open Directory Project Archived 2017-03-16 at the Wayback Machine