tachcyardia is a more or less regular heart rate > 100 bpm that does not origin from the sinus node. The p-waves therefore have a different configuration and easily be recognized if the P waves are negative in I and/or aVF.
[[Image:DVA0237.jpg|thumb| In this
small complex tachycardia with a normal morphology of the QRS-complexes (thus no ventricular tachycardia) you can see that there is AV-association (i.e. every QRS complex is preceded by a P-wave) but the P-waves are inverted (negative in aVF), therefore this is an atrial tachycardia ]]