A method is disclosed for treating heart and vascular tissue with cryotreatment. A medical instrument, such as a catheter is positioned to contact a target region of cardiac tissue such as the epicardial tissue. The instrument or catheter provided includes a cryotreatment element that has thermally-transmissive properties. The cryotreatment element may be a cryochamber for enclosing the flow of a fluid refrigerant therein. The cryotreatment element is disposed at the situs of heart or vascular tissue to be treated, usually by piercing the epicardium sac via an opening in the patient's body. A refrigerant flow within the cryochamber creates endothermic cooling with respect to the targeted heart or vascular tissue, inducing hypothermia and forming iceballs proximate the tissue. The cooling may be reversible and non-permanent, or may be permanent leading to cell death, necrosis, apoptosis and/or surgical excision or ablation of tissue.