There is one pitfall in patients with atrial fibrillation and severe tachycardia. A so-called pseudo-regularity may be found, starting at heart rates of above 160-180 bpm, which is usually attributable to measurement inaccuracies. When just looking at the ECG, the R-R-intervals appear to be regular. Particularly in the case of very high ventricular rates, the R-R intervals must be measured precisely. In the case of tachycardia, fibrillatory waves are rather difficult to distinguish, because the T wave fills almost the entire period of diastole. The most important differential diagnoses for the regular narrow complex tachycardias are atrial tachycardias, an AVNRT, and an AVRT. We will talk about these extensively in a later chapter. For now, just remember that apart from atrial fibrillation, when assessing a patient with tachycardia, you must keep in mind these three important differentials: atrial tachycardia, AVNRT, and AVRT.