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

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