Precooled cryogenic ablation system

A cryosurgical instrument for ablation of endocardiac tissue, comprising:

• a source of a gaseous primary refrigerant said source providing said primary refrigerant at a temperature above the critical temperature of said primary refrigerant;
• a source of a liquid secondary refrigerant, said secondary refrigerant having a critical temperature higher than said critical temperature of said primary refrigerant;
• a secondary expansion element connected to receive said liquid secondary refrigerant, said secondary expansion element being constructed to vaporize and expand said secondary refrigerant to a temperature below said critical temperature of said primary refrigerant;
• a primary-to-secondary heat exchanger having a primary refrigerant flow path connected to receive said gaseous primary refrigerant, and a secondary refrigerant flow path connected to receive said vaporized and expanded secondary refrigerant from said secondary expansion element, said heat exchanger being constructed to cool and liquefy said primary refrigerant;
• a primary expansion element connected to receive said liquid primary refrigerant from said heat exchanger, said primary expansion element being constructed to vaporize and expand said primary refrigerant to a selected cryogenic temperature;
• and
• a cryoablation heat transfer element connected to receive said vaporized and expanded primary refrigerant;
• wherein said primary refrigerant comprises SUVA-95, and said secondary refrigerant comprises AZ-20.