System and method for assessing ice ball formation during a cryoablation procedure

A method for assessing an ice ball formation during the cryoablation of a target tissue of a patient, said method comprising the steps of:

• contacting the patient with a reference electrode;
• providing a cryocatheter having a cryotip wherein said cryotip includes an expansion chamber;
• cooling said cryotip by expanding a refrigerant in said expansion chamber to create an ice ball and cryoablate said target tissue;
• generating a measurement signal having a frequency in the range of 15 to 25 khz and an RMS voltage of less than 1.0V;
• using said measurement signal to measure a current between said cryotip and said reference electrode to assess the formation of said ice ball;
• and
• expanding said refrigerant in said expansion chamber until said current is substantially zero.

A system for assessing ice ball formation during the cryoablation of a target tissue of a patient, said system comprising:

• a reference electrode for contacting said patient;
• a cryocatheter having a cryotip wherein said cryotip includes an expansion chamber;
• a means for positioning said cryotip proximate said target tissue;
• a means for cooling said cryotip by expanding a refrigerant in said expansion chamber to create an ice ball and cryoablate said target tissue;
• and
• an electronic means connected to said cryotip and said reference electrode to measure an impedance therebetween to assess formation of said ice ball and to allow for expanding said refrigerant in said expansion chamber until a ratio of two measured impedances is substantially zero.

A method for assessing contact between a cryotip of a cryocatheter and a target tissue in the vasculature of a patient, said method comprising the steps of:

• contacting said patient with a reference electrode;
• positioning said cryotip proximate said target tissue;
• measuring a first impedance between said cryotip and said reference electrode;
• moving said cryotip relative to said target tissue;
• measuring a second impedance between said cryotip and said reference electrode after said moving step;
• and
• determining a ratio of said first impedance to said second impedance wherein said first and second impedance are measured using a signal having a frequency of approximately 20 khz and an RMS voltage of approximately 0.5V and said signal is produced by generating a square wave, then converting said square wave to a sine wave using a four pole, low pass, active filter, and rectifying said sine wave using a plurality of analog switches driven by a 20 khz signal that is phase shifted relative to said sine wave by approximately 90 decrees to assess contact between said cryotip and said target tissue.