7. Experimental data on ablation responses in the GI tract

The development of flexible endoscopes has revolutionized the practice of gastroenterology, with the ability to both diagnose and treat a variety of lesions. Many of the current therapeutic approaches to endoscopically amenable lesions involve some form of thermal injury to produce either tissue ablation (for growths) or vascular coagulation (for bleeding). However, heat injury has several drawbacks, even with the use of newer devices such as argon plasma coagulation (APC). Present methods are either cumbersome and expensive (laser) or cannot truly be applied in a non-contact manner (heater probes) and thus require excellent visualization of the target lesion, (can be problematic in patients characterized by brisk bleeding). Further, the time required to complete therapy of multiple lesions in the gut such as arteriovenous malformations is lengthy. Finally, the depth of injury with thermal methods is difficult to control and there is a risk of thermal injury to the intestine. Cryotherapy is a non-contact ablation method that has long been used clinically in the treatment of a wide variety of malignant and premalignant diseases. The relative ease-of-use and unique mechanisms of cellular destruction make cryotherapy particularly attractive for the eradication of dysplastic Barrett’s esophagus. Preliminary data with cryogens liquid nitrogen (LN2) and carbon dioxide (CO2) have shown high efficacy in the reversal of dysplastic Barrett’s mucosa and excellent safety profiles. Intense investigation on cryotherapy ablation of Barrett’s esophagus is ongoing. Currently there are two cryoablative systems for use in gastroenterology. The CryoSpray Ablation System (CSA Medical, Inc., Baltimore, Maryland) uses super-cold cryogen and a multi-layered 7F catheter coated with a special polymer on the outer sheath that is warmed during delivery of the cooling agent to retain its flexibility. With this system, an orogastric suction tube is inserted into the stomach for luminal decompression. A distal port and side holes along the suction tube allow for both gastric and esophageal decompression. Luminal decompression is imperative to prevent perforation since LN2 can expand into 6–8 L of gas during each 20 s of treatment. The endoscopist controls the cryoablation and suction decompression with a foot pedal. The Polar Wand cryotherapy device (GI Supply, Camp Hill, Pennsylvania) was developed utilizing CO2 gas as a cooling agent, employing the Joule–Thompson effect to create massive drop at the distal tip temperature. At flow conditions of 6–8L/min, end effector temperatures of −78 °C can be achieved. The ablation catheter is passed though the working channel and a suction catheter is attached to the tip of the endoscope for decompression. In contrast to LN2 based cryoablation system, the cryocatheter in the accessory channel remains at ambient temperature thus preserving normal endoscopic manoeuvrability. Furthermore, expensive cryogen holding tanks used to contain LN2 is also not necessary when using CO2 based system. Finally, another major technical advantage of this system is the ability to spray the mucosa at will, producing rapid injury of large areas without the need for contact. However, because of the large volume of cryogen gas exiting the catheter, venting is still required.