A 12-year old Warmblood mare has been sedated with a combina…

A 12-year old Warmblood mare has been sedated with a combination of xylazine and butophanol for dental evaluation because she has been dropping feed.  You listen to the heart immediately after sedation and note a heart rate of 20 beats per minute (normal = 28-44) with an occasional “dropped” beat; the horse is standing quietly.  What is the most appropriate action to take next?