CRYOSURGICAL SYSTEMS
Cryosurgical is the application of extreme cold to tissues and is a useful technique for the destruction of tissues.
It is characterized by minimal bleeding, or pain, in the post-operative period.
Cryosurgical procedures are well established in many specialties including:
• General surgery,
• Dermatology,
• Neurology, and
• Urology.
The first practical equipment for the control and maintenance of extreme low temperature in surgery was produced in 1962.
The apparatus was not very sophisticated and used liquid nitrogen to achieve low temperature in the region of – 196°C.
This very low temperature was subsequently found to exceed requirements.
A more simple apparatus which used nitrous oxide or carbon dioxide to achieve temperature in the region of – 70°C.
The design principles established, are still used in modern cryosurgical equipment which employs high-pressure, non-siphon cylinders of N2O, or CO2.
- Cryosurgical systems consist of a flexible tubing, connecting the gas flow control unit, to a cryoprobe or working tip, this unit comprises of two concentric tubes.
- The inner tube delivers nitrous oxide or carbon dioxide at pressures between 4,000 and 6,000 kpa to a narrow orifice in the end.
- The sudden expansion of this gas, through the Joule-Thompson orifice, produces a rapid drop in temperature of the probe surface, forming an ice ball.
- The expanded gas is then returned at atmospheric pressure along the outer tube.
- Some cooling along the shaft of the probe occurs, which can be a disadvantage, for example in neurosurgery.
- To overcome this, manufactures designed a probe which employs a reversed gas flow.
- This design enables the incoming gas, to be carried to the probe tip, via the outer of the two concentric tubes; it is allowed to expand through an annular orifice, being finally returned through the central tube.
- The incoming gas acts as an insulating barrier and the cooling is confined to the probe tip.
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