Background: The diagnostic dilemma of ECG rhythm diagnosis is very common for patients presenting in ED and Critical Care Units. A widely practiced method for diagnosing ECG rhythms of narrow-complex tachycardia is the use of double-speed (50mm/sec) ECGs in addition to the 25mm/sec speed ECG. Though widely practiced, accuracy of diagnosis by this method has not been evaluated adequately. Method: This single-center, single-blinded, comparative, questionnaire-based study was done amongst Emergency and Critical Care Physicians to assess whether adding a double speed ECG help in making a correct diagnosis of narrow-complex tachycardia. Results: The study did not show any significant improvement in diagnostic accuracy with use of 50mm/sec speed ECG. 26 ECGs were interpreted by 35 observers, each ECG set was diagnosed twice, once with 25mm speed only and once 25mm and 50mm speed together. Correct diagnosis was made in 534/910 (58.68%) in the standard group and 537/910 (59.01%) with the addition of the 50mm/s ECG. None of the narrow complex tachycardia revealed any improvement in diagnostic accuracy with the aid of double-speed ECG. Proportion of correct diagnosis by physicians of different years of experience with the help of double-speed ECG is not greater than that with standard speed ECG. The proportion of correct diagnosis by physicians with opinion that double-speed ECGs are helpful is also not better than physicians who do not find double-speed ECGs helpful. Conclusion: Though widely practiced in diagnosing narrow complex tachycardia, double speed ECG is not a very accurate tool. Perhaps physicians need more training and practice in interpreting double speed ECGs for more accurate information of rhythm analysis.