Sinus arrhythmia is a variant of sinus rhythm. During sinus arrhtyhmia there is much (>0.16 sec) variation in the consecutive RR intervals.
An example of an ECG with sinus arrhythmia. This was a young patient without any symptoms. This sinus arrhythmia is often observed in young people and is related to in and expiration
Another example of sinus arrhythmia