Unconditionally, main parameters characteristic for cryosurgical unit are cryogenic parameters, which ensure the efficiency of its use. The important factor in realizing cryogenic parameters is selection of cryoagent. Representatives of different schools use different cryoagents, physical properties of which determine efficiency and range of application of cryosurgical apparatus. Units where carbon dioxide and nitrogen oxide are used as cryoagents allow achievement, at best, of cryoexposure temperature of minus 69°С and minus 89°С, respectively; therefore their application is very restricted.

Units where gaseous argon is used as cryoagent and they work on Joule-Thompson effect, allow to cool only needle-shaped cryoinstruments (with diameter no more than 3 mm) and to freeze small volumes of biological tissue.

We consider liquefied nitrogen the most effective, available and cheap cryoagent, and cryosurgical equipment where this cryoagent is used, is the most efficient in realization of cryosurgical treatment benefits. However, the use of liquefied nitrogen is necessary but not always sufficient requirement for efficiency of cryosurgical technics. As we said earlier, cryosurgical unit should provide temperature of cryoinstrument effective area, which is in thermal contact with frozen organ about minus 180°С and below. Realization of this requirement is connected with some technical difficulties. Boiling point of liquefied nitrogen under normal conditions is minus 196°С. However, it does not mean that temperature of cryoinstrument effective area, which is in thermal contact with frozen organ, is also minus 196°С. Actual temperature depends on construction of heat exchanger where liquefied cryoagent boils.

There are some traditional approaches to constraction of cryosurgical instruments. From construction of heat exchangers, it is possible to predict temperature of effective area contacted with frozen organ.

fig. 1

Fig. 1 shows option where liquefied cryoagent boils in heat exchange chamber (1), and removable working applicators (2) by means of threaded joint (3) are screwed on heat exchanger. Such construction allows using removable applicators of different shapes.

However, from Fig. 1 we can see that thermal contact of applicator with heat exchanger is made by their mechanical joining that resulted in high thermal resistance in a place of contact and temperature of effective area falls not below minus 100°С (provided that temperature of heat exchanger wall will fall to minus 180°С and below).

fig. 2

The above disadvantage can be removed, if the effective area of applicator at the same time is the wall of heat exchange chamber (Fig. 2).

In this case, thermal resistance between heat exchanger and removable applicator is removed but such design solution does not guarantee refrigerating of effective area to temperature close to the boiling point of liquefied nitrogen.

fig. 3

Fig. 3 shows dependence of heat-transfer coefficient on temperature pressure, or on quantity of heat brought to a surface by boiling of liquefied cryoagent

As we can see this graph, peak heat loss is in the area of transient boiling — from nucleate to film boiling. The designer’s task is to ensure exact regimen of boiling in heat exchange chamber. This requirement can be realized with using in heat exchanger a hollow of pore structure with changed porosity - moderate decrease in pore sizes towards to interior wall of applicator effective area.

As we mentioned above, the important parameter for cryosurgical unit is possibility for precise measurement of temperature of cryoinstrument effective area — temperatures of cryoexposure. Cryosurgeon should know actual temperature of cryoexposure for knowledge of time limits of the cryooperation. Moreover, information about actual temperature will allow application of mathematical modeling of cryoprocesses so that a surgeon could predict real size of freezing area.

Cryosurgical unit should maximally exclude subjective factor from operational process. In other words, doctors should be assured that all given parameters of cryosurgery, cryosurgical unit will provide independently by means of auto control block. In general, cryosurgical operation is important but still a fragment of operational process. This fragment should be combined with general operational pattern. Therefore, the important factors are preparatory activities and they are important to be done promptly and simply without prolongation of operational process. In complex preparation of cryosurgical unit to the work and its immediate activity should not result in loss of the temp of operational process as a whole. It is especially important in case when application of cryosurgical unit during surgery was not planned in advance.

Cryosurgical unit should be economic, i.e. to have high efficiency (to use minimum quantity of liquefied cryoagent for unit time). In other words, liquefied cryoagent should be completely evaporated immediately in hollow of heat exchanger and not to get into atmosphere as liquefied gas.

From our huge own experience in creating and manufacturing of cryosurgical equipment, aswell as using advices of our numerous medical co-authors from different countries, we offer your our concept of making universal automated cryosurgical complex which can be efficiently used in any field of medicine where cryosurgical method of treatment is expedient.