Use of a moratorium to achieve consistent liquid nitrogen cryoprobe performance.

BACKGROUND AND OBJECTIVES

During liver cryosurgical procedures, the authors observed seriously inconsistent rates of iceball growth implying inconsistent probe cooling rates. This inconsistency can lead to several problems, most importantly, loss of precision and reliability in freezing the chosen volume of tissue. The observation led to investigation of the performance of the cryosurgery machine.

METHODS

The performance of the Cryotech LCS 3000 with different moratorium periods was investigated. Freezing was performed in a gelatin phantom with thermocouple monitoring at the probe tip.

RESULTS

It was determined that a moratorium period between unit filling and clinical use essentially eliminates the uncertainty in probe performance. The need for this arises from the design of the system. A full liquid nitrogen tank does not have the necessary pressure required to induce effective freezing. A partially filled tank increases vapor pressure in the storage dewar driving more liquid nitrogen through the probes, thus allowing a more reliable and faster freeze.

CONCLUSION

The simple measure of introducing a moratorium period between filling the dewar with liquid nitrogen and its use in surgery allows partial evaporation of the nitrogen and enhances cryoprobe performance. This protocol modification may reduce the chance of inconsistent probe performance thus making liver cryosurgery more reliable.