Sinus arrhythmia is a common condition. There are different types--some of which are a disease. Respiratory sinus arrhythmia is a common type--an this is a normal process. This is not a disease, although not everyone has it. It is a normal variant.
Respiratory sinus arrhythmia is the natural variation of heart rate with breathing. The word sinus means that the heart rhythm is coming from the sinus node--which is the starting point of the heart. This sinus node is the heart's "pacemaker." The word arrhythmia means that the heart rate is irregular--not a constant rate.
Respiratory sinus arrhythmia normally occurs when heart rate slows during inspiration and speeds up in exhalation. This occurs because the heart rate is controlled by the brain. The brain varies its control of heart rate during the breathing cycle and uses the vagus nerve to change the heart rate. This can happen in everyone, but in most people you cannot detect this because it is so small. In some people it is more exaggerated for an unknown reason. In general, people who have good cardiovascular endurance--like marathon runners, are more likely to have this. So this may mean you are in good shape!
Talk with your doctor
if you have more problems.