Noninvasive discrimination of right atrial ectopic tachycardia from sinus tachycardia in "dilated cardiomyopathy". Right atrial ectopic tachycardia (RAET) with secondary cardiac dysfunction can be difficult to differentiate from primary dilated cardiomyopathy (CMP) with sinus tachycardia. In an attempt to separate RAET from CMP by noninvasive testing, routine surface electrocardiograms (EGGs), 24-hour ambulatory ECGs (Holter monitors), and echocardiograms of patients with RAET (n = 34) and CMP (n = 33) were reviewed. RAET atrial rates were significantly faster than CMP rates on the resting ECG and on Holter monitoring; 12 of 33 patients with RAET had resting ECG rates greater than 150% of predicted normal values for age but none of 32 patients with CMPs had resting ECG rates in this range. Mean P wave axis in the horizontal plane was more posterior in patients with RAET and was less than 0 degrees (negative in lead V2) in 8 of 29 patients with RAET but in 1 of 33 patients with CMP. Second-degree atrioventricular (AV) block was observed in 12 of 33 patients with RAET but in none of 33 with CMP. Shortening fraction less than 10% was found in 13 of 33 individuals with CMP but in only 1 of 27 with RAET. We conclude that noninvasive studies can help identify RAET among patients with poor functioning hearts and right atrial tachycardia. 