Carbon dioxide (CO2) has been used instead of atmospheric air for insufflation of the colon for colonoscopy as well as for barium enema examination because it has been found to decrease patient discomfort. CO2 is readily resorbed through the colonic wall because of a steep diffusion gradient and it is then exhaled from the lungs. One hundred patients were randomized to undergo colonoscopy with insufflation with either air or CO2. Post-procedural pain was reported in 45% and 31% of patients receiving air at 1 h and 6 h, respectively, after colonoscopy compared with 7% and 9%, respectively in subjects insufflated with CO2 (Sumanac et al. 2002). In a study of 142 subjects, approximately half of the patients received room air and the other half received CO2 to distend the colon for barium enema. Patients who received CO2 were found to have a reduced incidence of both immediate and delayed pain, from 31% to 12.5% and from 12.9% to 4.2% respectively (Robson et al. 1993). In another study of 151 patients undergoing barium enema, 86 received room air and 65 received CO2 for colonic insufflation. Almost one-third of patients who received room air experienced pain versus only 11% of patients who underwent colonic distension with CO2. Whereas none of the CO2 patients reported severe pain, five patients who received room air reported significant pain (Coblentz et al. 1985). In a comparative study, 105 patients undergoing barium enema received either manually administered air, CO2, or a 50/50 mixture of the two gases. No difference in mucosal coating was found. Patients who received CO2 had significantly less immediate and delayed pain than those who received air and less delayed pain than those insufflated with the 50/50 mixture. It was also found that air provided better distension than the other two gases although the difference did not attain statistical significance (Holemans et al. 1998). Another study identified less optimal colonic distension with manually administered CO2 than with room air in 100 patients referred for barium enema. It was concluded that poor distension could lead to diagnostic errors and thus outweigh any advantages in patient acceptability when using CO2 as an insufflation agent (Scullion et al. 1995).
More recently, CO2 has also been used to distend the colon for CT colonography. The retrograde administration of CO2 may be performed either manually, similar to retrograde air insufflation, or electronically using a specific commercially available mechanical device developed for CT colonog-raphy. Although manual administration of CO2 may lead to suboptimal bowel distension as described above, our experience shows more reliable and consistent optimal bowel distension with the use of electronic CO2 insufflation for CT colonography, which maintains a constant infusion of CO2 into the colon up to a certain preset pressure. For colonoscopy, a pressure maintained at 35 mmHg with a CO2 flow rate of 1 l/min has been proven safe (Phaosawasdi et al. 1986). For CO2 administration during CT colo-nography, the maximum pressure setting allowed using the mechanical device is 25 mmHg. With a fixed flow rate of 3 l/min, the pressure is set at about 15 mmHg to start with and then slowly increased to a maximum of 25 mmHg depending upon patient tolerance. CO2 instillation is continued in the supine and prone positions until completion of the scan. The total amount of CO2 insufflated during CT colonography is typically about 4 l due to the relatively short procedural time. This amount is far less than during an average laparoscopic procedure of approximately 2 h using CO2 flow rates of 5--15 l/min with a total CO2 consumption of approximately 40 l (Taskin et al. 1998). No complications have been reported in the literature to date for intracolonic CO2 insufflation.
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