Sinus bradycardia is treated depending on the underlying cause. Treating that cause primarily will usually lead to the heart rate increasing to normal again.
Examples include warming up a cat that is hypothermic, or addressing an underlying GI tract, urinary, or respiratory disease that is leading to high vagal tone.
During anesthesia, sinus bradycardia is addressed by adjusting the anesthetic medication being used or by using a medication that increases heart rate.