I took Tizandine (Zanaflex) about 12 years ago. I saved the bottle, and wrote on the bottle that this drug slowed my HR way down, and also caused an erratic Heartbeat. I remember it was a bad experience, although I took it at 6mg over 4 hours. I am reading that the type of IR Heartbeat caused can be QT prolongation, which can be very serious.
The dosage of a drug that impacts the heart is important, so it it sounds wise to cut the dose down low enough that repeated EKG's are normal, and you don't have noticeable side effects relating to HR and BP.