Cis atracurium is better then atracurium because:-

Atracurium Cis-atracurium Mixture of cis-isomers Pure cis-isomer Histamine release No / minimal release CVS unstable CVS stable Less potent Dose - 0.5-0.6mh/kg More potent Dose - 0.1-0.2 mg/kg - Cis-atracurium and atracurium are dependent on Hoffman elimination - Cis-atracurium's duration of action is more predictable because of spontaneous breakdown. Although atracurium undergoes similar elimination, it is less potent than cis-atracurium, so its breakdown product, laudanosine, is found in higher concentrations