Abstract: A method and apparatus are described for controlling the rate of supply of a liquid cryogen, for example, for cryosurgical use. The flow rate of the liquid cryogen is controlled by controlling variably the pressure propellant gas acting directly or indirectly on the liquid cryogen. A means for controlling the propellant gas pressure may comprise a source of pressurised propellant gas, and control valve means coupled between the source and the liquid cryogen reservoir chamber for controlling the pressure supplied to the reservoir chamber. A plurality of reservoir chambers may be driven by a common pressure source. When the pressure source is a pressure raising system, the cryogen reservoir chambers may be contained within the vessel of the pressure raising system. A feedback control arrangement is also described for controlling the cryogen flow rate in response to a measured characteristic associated with, or affected by, the cryogen flow rate.

Abstract: Apparatus is described for feeding a cryosurgical device by passing a liquid cryogen (e.g. liquid nitrogen) through the device to produce cooling by vaporization, and to enable thawing of the device by passing a heated inert thaw gas (e.g. nitrogen gas) through the device when the liquid cryogen is turned off. The thaw gas is passed through the device in the same direction of flow as the liquid cryogen. The apparatus can supply a plurality of cryosurgical devices, and provide independent control of cryogen and thaw for each probe. Novel valve arrangements are also disclosed for controlling the supply of liquid cryogen and the supply of the thaw gas.

Abstract: A method of thawing of cryosurgical apparatus, and in particular a cryosurgical probe, subsequent to a freezing process involves the interuption of the cryogen supply to the probe followed by the introduction of a warmed inert gas. The gas is circulated within the probe cryogen circulation system to affect thawing of the apparatus so that the probe may easily be withdrawn from the body. The gas, which may be nitrogen, is preferably passed through a heat exchange arrangement before being introduced into the probe.