It seems everyone is discussing ECG wave forms at the moment , The simplest way to determine normal sinus rhythm is as shown by LITFL 

Characteristics of normal sinus rhythm

• Regular rhythm at a rate of 60-100 bpm (or age-appropriate rate in children).
• Each QRS complex is preceded by a normal P wave.
• Normal P wave axis: P waves should be upright in leads I and II, inverted in aVR.
• The PR interval remains constant. 
• QRS complexes are < 100 ms wide (unless a co-existent interventricular conduction delay is present).

       Source :  https://litfl.com/normal-sinus-rhythm-ecg-library/

                       Don't we all need a little dancing in our lives                                                                                     
 

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