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temperature of -74 C and working temperature of -20 C. Rowbotham applied the cannula to three patients with gliomas. Years later in 1961, Irving S. Cooper popularized the widespread use of cryogenic freezing. Cooper helped design a cannula 3 mm in diameter and 20 cm long, containing liquid nitrogen under a pressure of 22 lbs. per square inch. The cannula was insulated except at the tip, where the liquid nitrogen circulated and therefore extracted heat from the surrounding tissue creating an ice ball. The temperature of liquid nitrogen under these conditions is -196 C (-352.8 F) and was able to cool from room temperature to -190 C (-310 F). Selected temperatures could be instantly obtained at the tip inside the cannula and could be adjusted accurately and maintained with relative ease. The tip's outside temperature was continuously monitored by thermocouples.
EFFECTS ON HISTOLOGY & PHYSIOLOGY
The brain undergoes physiologic inhibition in a temperature range of +10 C to -20 C. Between -20 C and -196 C the brain undergoes lethal freezing (6). However, due to the purity of biologic materials, there exists a state below the freezing point of the tissue, about -15 to -20 C, in which no ice crystals form. As the temperatures is decreased below this level there is a natural elevation in energy and therefore in temperature to -2.2 C. This is the latent heat of crystallization and occurs because of the transfer of energy from the cells to form crystals. Consequently, lowering the temperature below the freezing point, above the temperature of crystallization, would produce an area of inhibition. This would allow a temporary lesion in a preselected area of the brain in the conscious, cooperative patient prior to generating a permanent lesion (7). Once the temperature is reduced even further, the extreme cold of cryogenic surgery produces destructive alterations in the tissue by dehydration and toxic concentration of electrolytes, by crystallization with rupture of cellular membranes, by denaturation of liquid protein molecules within the cell membrane, and by thermal shock and vascular stasis (16, 29). Laboratory animal studies by Cooper and Stellar (10) demonstrated that within a few minutes after thawing, the lesion becomes desaturated, dark blue, and retains its sharply delineated margin. The lesions as fashioned by Cooper are spherical but can be cylindrical (23). The shape is directly proportion to the size of the freezing surface and the tip temperature of the cannula. Quigley demonstrated that the cylindrical lesion appears at less than 2 days and after one week becomes irregular in shape with the margins following white matter tracts. They again confirmed the rather impressive demarcation and no surrounding areas of hemorrhage. Microscopically, in an area confined to the lesion, there is hemorrhagic infarction, which becomes organized, demonstrating numerous neutrophils, macrophages, fibroblasts and capillaries. The white matter acquires a spongy appearance. Surrounding the cryolesion, there is a (Continued on page 81)
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