In sinus arrhythmia, the rate usually gradually slows and then gradually speeds up. This is due to breathing and the effects of the sympathetic and parasympathetic system. In sinus arrhythmia, there is a normal, extremely minimal increase in HR during inspiration and an extremely minimal decrease in HR during expiration. The slight increase in heart rate is due to inspiration-activated sympathetic stimulation of the SA Node. The slight decrease in HR is due to expiration-activated parasympathetic inhibition of the SA Node. The sympathetic NS stimulate when you suck in air and parasympathetic stimulate when you push out air.
Mnemonic: Inspiration inspires the heart and increases heart rate. Expiration causes the heart to “expire” and slow down.