What is sinus arrhythmia?

Normal: Sinus arrhythmia refers to a prominent, but normal, variation in heart rate. The heart beat begins in the sinus node. The sinus node is regulated by the autonomic nervous system. The system may react quickly enough to cause more noticeable changes in heart rate. This variation is usually more prominent in younger individuals.

Rate vs breathing : The pacemaker in your heart=sinus node which generates tiny amount of electricity which triggers the heart to contract/beat, average 60-100 BPM at rest. Sinus arrhythmia = sinus node is a bit sensitive to breathigng--inhaling causes it to slow a bit and exhaler speeds up a bit, thus arrhythmia. It is normal variation and commonly seen in young/healhty people. Hope that makes sense. Good luck.