X-Message-Number: 24602 From: "Basie" <> Subject: Can liquid nitrous oxide (N2O) replace LN? Date: Wed, 8 Sep 2004 16:45:31 -0400 Technique In order to achieve hypothermia, liquid refrigerants are forced through a small hole at a pressure range of 750-900 pounds per square inch (psi) (Figge, 1983). This produces a very low temperature at the surface of the probe due to the Joule-Thompson effect. The temperature at the probe tip can range from -65 C to -85 C. Cell death occurs at -20 C to -30 C (Campion, 2000; Boonstra, 1990). Proposed mechanisms for cell destruction include dehydration, crystallization, denaturation of membrane proteins, thermal shock, and vascular stasis (Charles, 1980). The refrigerant most commonly used at present is liquid nitrous oxide (N2O). It is available commercially and is relatively inexpensive. The external temperature of the probe depends on the conductivity of the probe material. Silver and copper are the best materials for use in probe tips because high conductivity produces both a better freezing effect and more effective local cryonecrosis (Charles, 1980). Adequate cryonecrosis of the tissue depends on direct contact of the probe with the lesion. This is best achieved with a water-based lubricant coating an appropriately sized probe. Boonstra and others have shown that the size and shape of the probe can dramatically affect the depth of cryonecrosis (Boonstra, 1990). The cryotip should cover the entire lesion and transformation zone. The 19- and 25-mm mini-cone tips are recommended by Campion. Most authors recommend a freeze-thaw-freeze technique in which the tissue is frozen for a period of several minutes, thawed completely, and then refrozen. Creasman (1973) demonstrated that this method was significantly more efficacious than a single-freeze method. Some authors (Popkin, 1978; Davies, 1981) advocate the use of the double-freeze technique only in patients with CIN3 or CIS. Generally, the freeze-thaw-freeze is divided as follows: a.. Freeze for 3 minutes b.. Thaw for 5 minutes c.. Freeze for 3 minutes The freeze time required depends largely on the ice ball that is generated on the cervix. Campion in the 2000 edition of Practical Gynecologic Oncology recommends that the ice ball extend 7 mm laterally beyond the edge of the probe in order to achieve a 5-mm depth of destruction. This suggests that the operator need not watch the elapsed time but rather the width of the ice ball formation. http://www.emedicine.com/med/topic3337.htm Rate This Message: http://www.cryonet.org/cgi-bin/rate.cgi?msg=24602