When horses were premedicated with dantrolene , CO at @ , @ , and @ 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 @ hours after anesthesia ; and @ of @ horses developed cardiac arrhythmias that required medical intervention .