mol of H2 to form 1 mol of acetate [18]. This process greatly decreases colonic gas volume. Methanobrevibacter smithii, which uses H2 to reduce CO2 to CH4, is responsible for almost all the CH4 produced in the intestine [19]. Since CH4 production occurs primarily in the left colon whereas H2 is produced primarily in the right colon [20], H2 produced in the left colon may be rapidly converted to CH4. CH4 appears in the breath only when the numbers of methanogenic bacteria reach a critical level, about 108/g dry weight counts [19].
Clinical Problems
Irritable Bowel Syndrome
Intestinal gas is often incriminated as a major cause of irritable bowel syndrome (IBS), particularly when bloating is predominant, but no general relationship between intestinal gas and IBS symptoms has yet been established. Koide et al. [21] found a significantly increased intestinal gas volume score in plain abdominal radiographs in patients with IBS as compared with healthy controls, suggesting that abnormal accumulation of intestinal gas could be a problem in these patients. In contrast, Morken et al. [22] reported that intestinal gas volume is not correlated with abdominal discomfort after lactulose challenge and concluded that intestinal gas may not be the major cause of abdominal discomfort following carbohydrate ingestion in IBS.
In some diseases that favor bacterial proliferation, such as inflammatory bowel diseases, it may be difficult to determine the extent to which clinical deterioration is caused by bacterial overgrowth or the primary intestinal diseases. Actually, many individuals harbor bacterial overgrowth without symptoms. Thus, the clinical features of bacterial overgrowth, closely related to intraluminal gas production, vary greatly in individuals. However, several trials reported the results of H2 breath testing for carbohydrate malabsorption and small intestinal bacterial overgrowth in patients with suspected IBS [23]. Intestinal fermentation products such as gas, short-chain fatty acids and intraluminal acidification could all affect colonic motility, possibly in part resulting in abdominal symptoms.
Functional Dyspepsia
Colonic fermentation influences not only the colonic function, but also the function of the upper digestive tract. Reduced proximal gastric tone after cecal infusion of lactulose and short-chain fatty acids [24] and relaxations of the lower esophageal sphincter [25] suggest that colonic fermentation might impair gastric accommodation to meals and induce gastroesophageal reflux. Several studies have shown that the presence of a large volume of air in the proximal stomach triggers stretch receptors in the gastric cardia and leads to an increase in the rate of transient relaxations of the lower esophageal sphincter. The distribution of gas in the digestive tract is easily detected by plain abdominal radiograph. In clinical practice, plain films of the abdomen are required in several different circumstances. The film of the abdomen with the patient standing erect is obtained to demonstrate fluid levels, a more detailed double-contrast view of the intestinal wall, or free air beneath the diaphragm. The other anteroposterior films with the patient lying on his back may corroborate findings on the erect study mainly to evaluate the distribution of the gas pattern of the abdomen. In the erect position, gas is retained largely in the fundus of the stomach. It has been reported that the volume and distribution of gas within the gut is closely associated with abdominal symptoms [26], whereas it has been unclear whether the gastric bubble on plain films is also linked to any symptom. The prevalence of GERD and dyspeptic symptoms were significantly lower in patients with a dome-type gastric bubble in our study [27]. Even a dome-type gastric bubble in the erect position may be distributed throughout the stomach in the supine position if a volume of gastric bubble is sufficient to fill the entire stomach. Changes in the form of gastric bubble detected on a plain film in the erect position may indicate the functional disorder of upper digestive tracts, such as decreased LES pressure, delayed gastric emptying, or impaired accommodation of the proximal stomach.
Intestinal Obstruction
The patient with colonic obstruction or delayed small intestinal transit may frequently have bacterial overgrowth and increased breath H2 levels because the bacterium can contact with food residues for a longer time. An informative case with ileus after local peritonitis is demonstrated in figure 2 [28]. A 70-year-old woman presented with abdominal pain and distention. She has a past history of surgical treatment for peritonitis before 13 years. Plain abdominal radiograph showed a markedly dilated small bowel, with no dilatation of the colon. Fasting breath H2 concentration was 3 ppm at that time. This low levels of breath H2 before treatment might indicate that unabsorbed food residues did not reach the cecum due to small bowel obstruction. The patient was treated with the use of a gastric tube for drainage of gastric juice. Nutrition was provided intravenously. During the first 4 days, although small bowel gas increased gradually (fig. 1a, b), breath H2 concentrations remain less than 4 ppm (fig. 1). On the fifth day after admission, small bowel gas decreased and a small amount of colonic gas was demonstrated on the plain abdominal radiograph (fig. 1c). The breath H2 concentration at that time increased from 1 to 6 ppm and reduced again to the baseline the next day. This change in breath H2 level may possibly reflect the movement of intraluminal contents from the small intestine to the right colon. This value remained at a low level (1-2 ppm) from hospital days 5 through 8. Since intestinal gas was gradually reduced on radiographic imaging (fig. 1d), a liquid meal was supplied at noon on the ninth hospital day. The breath H2 concentration on the next day increased markedly to 19 ppm despite that she had no abdominal symptoms and intestinal gas was not increased on the plain abdominal radiograph (fig. 1e). This suggests malabsorption of a liquid meal by which unabsorbed carbohydrates reach the colon and is utilized by fermentation, resulting in increased breath H2 levels. Although diets progressed from liquids to solids as tolerated, breath H2 concentrations were decreased to 2 ppm 5 days after supplying meals. The patient did not complain of any abdominal symptoms during a routine progressive diet and intestinal gas concentration decreased continuously (fig. 1f, g). Digestive and absorptive function is considered to be restored gradually. Consecutive breath H2 analysis could provide important information on the movement of intestinal food residues or the presence of malabsorption after meals.
Fig. 1. Clinical course of a patient with ileus. Changes in breath hydrogen concentration and abdominal plain radiographs on representative dates are demonstrated.
