Explain Sinus Arrhythmia - electrocardiogram

In few adults and most children, there is raise in heart rate during inspiration and a decrease during expiration. This rhythm is most generally observed with breathing due to fluctuations in parasympathetic vagal tone. Throughout inspiration, the chest expands making a suction force that results in greater backflow of blood into the atria from the veins.

Greater filling of right atrium results in higher heart rate, probably by stretching the SA node.