Biological effects of low temperature
In the past couple of decades, the use of cryosurgery for the treatment of tumours, i.e. destroying abnormal lesions while protecting surrounding tissues, has spread. The biological effects of cryosurgery can be divided into 2 types: early/direct effects and delayed effects. M.O. Maiwand recently explained how to maximize the effectiveness of low-temperature surgery. The direct effects of low temperature start at temperatures around -10°C. At this temperature, ice crystal formation is slow as the cell membranes and contents protect it, but cell destruction occurs if this temperature is maintained for sufficient time. With further cooling, ice crystals start to form, initially in the extracellular spaces, causing the water in the cell to withdraw and leading to cell death. If the temperature is lowered even more, to around -30°C, ice crystals will form within the cell. This is usually lethal. Quick freezing will cause this process, as there is no time for the water in the cell to withdraw. Thawing will cause further damage to the cell, as small ice crystals will cluster together to form large, more destructive crystals that can rupture the cell membrane. The delayed effects, also known as vascular effects, follow with a brief period of vasoconstriction. Thawing will then cause vasodilatation and increased vascular permeability leading to oedema and damage causing the occlusion of small vessels and stopping the blood supply, inevitably leading to cell death. Thus, in order to maximize the effectiveness of cryosurgery, the lowest temperature possible must be used, the tissue must be cooled as rapidly as possible and thawed as slowly as possible. More than one freeze-thaw cycle should be used and the cryosensitivity of the tissue is important.