Cryoablation

Cryoablation is a process that uses cold energy (cryo) to kill tissue (ablation).

Cryoablation is used in a variety of clinical applications using hollow needles (cryoprobes) through which cooled (from a peripheral freezing unit), thermally conductive, gases and fluids are circulated. Cryoprobes are inserted into or placed adjacent to tissue which is determined to be diseased in such a way that ablation will provide correction yielding benefit to the patient. When the probes are in place, the cryogenic freezing unit removes heat ("cools") from the tip of the probe and by extension from the surrounding tissues.

Ablation occurs in tissue that has been frozen by at least three mechanisms: (1) formation of ice crystals within cells thereby disrupting membranes, and interrupting cellular metabolism among other processes; (2) coagulation of blood thereby interrupting bloodflow to the tissue in turn causing ischemia and cell death; and (3) induction of apoptosis, the so-called programmed cell death cascade.

The most common application of cryoablation is to ablate solid tumors found in the lung, liver, breast, kidney and prostate gland. The use in prostate and renal cryoablation are the most common. Although sometimes applied through laparoscopic or open surgical approaches, most often cryoablation is performed percutaneously (through the skin and into the target tissue containing the tumor).

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Prostate cryoablation

Prostate cryoablation is highly effective but also can result in impotence. Prostate cryoablation is used in three patient categories: (1) as primary therapy in patients for whom sexual function is less important or who are poor candidates for radical retroperitoneal prostatectomy (RRP, surgical removal of the prostate); (2) as salvage therapy in patients who have failed brachytherapy (the use of implanted radioactive "seeds" placed within the prostate) or external beam radiation therapy (EBRT); and (3) focal therapy for smaller, discrete tumors in younger patients.

Renal cryoablation is highly effective in smaller tumors found in the kidney and adrenal gland. The results are excellent and spare surrounding tissues from ablation.

Cardiac cryoablation

Another specific type of cryoablation is used to restore normal electrical conduction by freezing tissue or heart pathways that interfere with the normal distribution of the heart’s electrical impulses. Cryoablation is used in two types of intervention for the treatment of arrhythmias: (1) catheter-based procedures and (2) surgical operations.

A catheter is a very thin tube that is inserted in a vein in the patient’s leg and threaded to the heart where it delivers energy to treat the patient’s arrhythmia. In surgical procedures, a flexible probe is used directly on an exposed heart to apply the energy that interrupts the arrhythmia. By cooling the tip of a cryoablation catheter (cardiology) or probe (heart surgery) to sub-zero temperatures, the cells in the heart responsible for conducting the arrhythmia are altered so that they no longer conduct electrical impulses.

Catheter-based procedures

  Different catheter-based ablation techniques may be used and they generally fall into two categories: (1) cold-based procedures where tissue cooling is used to treat the arrhythmia and (2) heat-based procedures where high temperature is used to alter the abnormal conductive tissue in the heart.

Cold temperatures are used in cryoablation to chill or freeze cells that conduct abnormal heart rhythms. The catheter removes heat from the tissue to cool it to temperatures as low as -75°C. The use of freezing neutralizes these malfunctioning cells so that they can no longer interfere with the normal conduction and distribution of the electrical impulse that controls the heart’s rhythm. The patient does not feel the cold as only the active tip reaches this temperature.

Site testing

Cryotherapy is able to produce a temporary electrical block by cooling down the tissue believed to be conducting the arrhythmia. This allows the physician to make sure this is the right site before permanently disabling it. The ability to test a site in this way is referred to as site testing or cryomapping.

When ablating tissue near the AV node (a special conduction center that carries electrical impulses from the atria to the ventricles), there is a risk of producing heart block - that is, normal conduction from the atria cannot be transmitted to the ventricles. Freezing tissue near the AV node is less likely to provoke irreversible heart block than ablating it with heat.

Surgical procedures

  As in catheter-based procedures, techniques using heating or cooling temperatures may be used to treat arrhythmias during heart surgery. Techniques also exist where incisions are used in the open heart to interrupt abnormal electrical conduction (Maze procedure). Cryosurgery involves the use of freezing techniques for the treatment of arrhythmias during surgery.

A physician may recommend cryosurgery being used during the course of heart surgery as a secondary procedure to treat any arrhythmia that was present or that may appear during the primary openchest procedure. The most common heart operations in which cryosurgery may be used in this way are mitral valve repairs and coronary artery bypass grafting. During the procedure, a flexible cryoprobe is placed on or around the heart and delivers cold energy that disables tissue responsible for conducting the arrhythmia.

References

Liapi E, Geschwind JF. J Clin Oncol. Transcatheter and ablative therapeutic approaches for solid malignancies. 2007 Mar 10;25(8):978-86.

Rukstalis, D and KatzHandbook, A Handbook of Urologic Cryoablation, hardcover - Feb 13, 2007.

Jensen-Urstad, et al., Cryoablation of AV Nodal Reentrytachycardia, PACE, Vol. 29 May 2006, pp 487.

Doll, et al., Intraoperative Cryoablation with a new Argon Probe, Ann Thorac Surg 2004;77:1460–2.