When horses were premedicated with dantrolene , CO at 25 , 35 , and 45 minutes after induction of anesthesia was significantly lower than that when horses were premedicated with water after which time difficulty in obtaining valid measurements suggested a continued decrease in CO ; plasma potassium concentration progressively increased during anesthesia , whereas serum creatine kinase activity remained fairly stable and within reference limits through 12 hours after anesthesia ; and 2 of 6 horses developed cardiac arrhythmias that required medical intervention .