Direct cell injury
As the CoolLoop catheter rapidly absorbs heat from the tissue, ice crystals form within the cells. This leads to immediate cell death by causing physical damage to intracellular organelle membranes and the plasma membrane.
However, rapid tissue cooling only occurs nearby the catheter (center of the ablation zone). The further the distance from the apparatus, the slower is the cooling rate. Slow cooling rates do not produce intracellular but extracellular ice crystals resulting in a higher tonicity of the extracellular space. Osmotic tension draws free intracellular water from cells and increases the intracellular solute concentration, causing damage to cytoplasmatic proteins and destabilization of the cell membrane.
During thawing, the ice within the extracellular space melts and the extracellular environment becomes hypotonic; a fluid shift occurs, leading to cell swelling and/or bursting. Additionally, the influx of water into the cells results in the growth and fusion of intracellular ice crystals (-20° to -25°C), a phenomenon known as recrystallization. In tissues with closely packed cells the large crystals are disruptive to cell membranes.
Cells (more) remote to the catheter (periphery of the ablation zone) that are not immediately killed by intracellular ice or osmotic stress may nevertheless die by programmed cell death (apoptosis). In otherwise intact cells, mitochondria that have been damaged during freezing may activate caspases in the cytoplasm which leads to the cleavage of various proteins. During 8-12h after ablation cells begin to shrink, their genome becomes fragmented and blebs are formed on the membranes.
Vascular stasis emerges as a consequence of the impaired blood supply after thawing. In the course of freezing ice crystals are damaging the endothelia of blood vessels. After thawing thrombocytes, neutrophile granulocytes and macrophages migrate through the newly formed gaps in the vascular walls, leading to ischemia followed by coagulation necrosis.
The consecutive processes of freezing and thawing outlined above are known as the “freeze-thaw cycle”. To amplify the disruptive effects on tissues, two of these freeze-thaw cycles are performed at a time.