No statistical differences were detectable between the BT-positive and BT-negative study patients as determined by the symptoms questionnaire. The mean quality-of-life scores for the BT-positive study patients compared with BT-negative patients were 69.6 (±15.0) and 69.4 (±8.0) for the vitality dimension (P = 0.97), 74.6 (±16.0) and 69.2 (±14.5) for the digestion dimension (P = 0.37), and 81.5 (±18.7) and 83.3 (±16.2) for the eating habits dimension (P = 0.79).
The use of low-dose (Monday, Wednesday, Friday) azithromycin as an anti-inflammatory was associated with increased odds of a positive BT. The use of daily laxatives and inhaled ipratropium was associated with decreased odds of a positive BT. Other medications, patient characteristics, and laboratory values examined did not reach significance. See Table 1 for summarized results.
In this study, most patients were diagnosed as having SBBO on the basis of a high fasting hydrogen level. This phenomenon has been noted in CF patients in previous studies (7,8,19). This high level presumably is due to low-level fermentation of endogenous carbohydrates such as glycoproteins by gut microflora in the upper small bowel (20). Fasting hydrogen levels in healthy children average 7.1 ± 5.0 ppm (21). High fasting levels of hydrogen are associated with bacterial overgrowth (14,16) and with the presence of fermentable carbohydrates in the colon (22). The cutoff value for defining a high fasting hydrogen varies considerably between studies (14,19,22,23). We chose a hydrogen level of 15 ppm as our cutoff on the basis of the curve patterns of hydrogen excretion in our study and control groups and our review of the literature. It is important to note that our comparison control group was an at-risk group with a high prevalence of positive BTs of 20% rather than normal controls with no risk factors and presumably a lower prevalence of SBBO.
When fasting hydrogen levels in BTs are interpreted, other factors such as slow transit of complex carbohydrates through the intestine must be considered. In 1 study of CF patients, 6 of 7 with fasting hydrogen levels >20 ppm after a 12-h fast decreased their fasting hydrogen to <20 ppm after prolonging the fast to up to 23 h (19). Other motility studies in patients with CF also showed prolonged intestinal transit, with a mean transit time of 136 versus 88 min for control subjects (24). Another possible cause for high fasting hydrogen levels may be the presence of malabsorbed simple carbohydrate in the colon. We did not assess this in our study, but prior studies have shown that pancreatic enzyme supplementation results in normal digestion of starch (25).
BTs demonstrate an early peak in hydrogen excretion following an oral sugar load to diagnose SBBO. We chose glucose rather than lactulose as our carbohydrate substrate because of the more accurate interpretation of the glucose BT (26). With a glucose BT, any rise in hydrogen is abnormal. With a lactulose BT, the early rise in SBBO must be distinguished from the late colonic rise, which may be difficult when intestinal motility is disordered as in CF. The rise is early because the abnormally high number of bacteria present in the proximal small intestine ferment the glucose more rapidly than it can be absorbed. In this study, only a small number of CF study patients showed a rise in hydrogen after the glucose challenge. The hydrogen peak may have been blunted by the effects of abnormal ileal pH. Low colonic pH has been shown to cause false-negative lactose and lactulose hydrogen BTs because the pH is outside the optimal level for the bacteria to ferment the sugar (27,28). Because pancreatic-insufficient patients also lack pancreatic bicarbonate secretion, gastric acid is inadequately neutralized, and ileal pH is significantly lower in patients with CF (29,30). Another potential cause of a failure to note a postglucose rise in hydrogen levels in CF patients with SBBO may be unusually rapid uptake of glucose from the small intestine in patients with CF. Rapid uptake, shown in in vivo and in vitro studies, would limit the availability of glucose for fermentation by bacteria (31,32). Blunting of the early rise in hydrogen also may be seen in the setting of delayed gastric emptying. Although gastric emptying was not assessed in this study, patients with diabetes requiring insulin therapy were excluded because of concerns about gastroparesis in this subgroup.
With regard to predisposing factors for SBBO in pancreatic-insufficient CF patients, 1 factor may be the low level of pancreatic secretions. Pancreatic secretions have intrinsic antibacterial activity and participate in the control of intestinal flora. The antibacterial activity is independent of digestive enzyme activity and is thought to be due to the activity of a polypeptide present in the secretions (33,34). Supporting evidence comes from studies showing that pancreatic juice from patients with chronic pancreatitis has reduced in vitro bacterial-killing properties (35). In rats, bacterial counts in the cecum increased after obstruction of the pancreatic duct (36). Pancreatic-insufficient CF patients take pancreatic enzyme replacement therapy to supply exogenous digestive activity, but undoubtedly beneficial substances in native pancreatic secretions are not being replaced by the currently prescribed porcine-derived enzymes.
The abnormally thick intestinal mucus layer noted in CF may also predispose these patients to SBBO. Increased production of luminal mucus is described as a possibly protective response to bacterial overgrowth in rats (37). However, the nature of mucus in CF is altered, and the thickened layer, rather than being protective, may contribute to bacterial overgrowth by blocking natural defenses. In the CFTR knockout mouse intestine, mucus filled the crypts and the area between the villi; this effect was not seen in the wild-type mice (10). The luminal mucus was filled with bacteria in the CF mice only. Staining showed that Paneth cell lysozymes appeared to be trapped in the crypts of the CF mouse intestine. These lysozymes are normally released into the gut to participate in the defense against bacteria. In addition, the stasis resulting from poor motility in CF may contribute to SBBO by impairing the clearance of mucus, which provides a reservoir for bacteria.
We found no statistically significant difference in body mass index or transaminase or hematocrit levels between BT-positive and BT-negative patients, although such differences had been noted in previous studies (38–41). However, our study results suggest that laxative use may have a protective effect (polyethylene glycol PEG 3350 or docusate sodium). This may be due to the shortened transit time that results from laxative use. We also noticed a protective role of inhaled ipratropium (Atrovent, Boehringer Ingelheim Pharmaceuticals, Ridgefield, CT). This finding was unexpected because this product is used for its respiratory anticholinergic effects. In the GI tract the anticholinergic effects may be expected to cause smooth muscle relaxation and stasis, favoring SBBO. However, the protective role may be mediated through its effects on goblet cells because anticholinergics decrease goblet cell secretion and therefore mucus production (42). Finally, the use of low-dose azithromycin as an anti-inflammatory was associated with a positive BT. This broad-spectrum macrolide antibiotic may inhibit the growth of beneficial commensal bacteria in the gut and allow proliferation of resistant bacteria.
Despite studies supporting the role of proton pump inhibitor use in gastric bacterial overgrowth, we found no correlation between the use of a gastric acid suppressor and a positive BT in CF patients. Some physicians believe SBBO in CF is related to swallowed bacteria-laden respiratory secretions rather than overgrowth of colonic-type flora. The role of gastric acid would be more pertinent to overgrowth with respiratory flora as a result of failure of gastric bacterial killing (43). The lack of association noted in this study favors overgrowth with colonic-type flora, and mouse studies have shown a predominance of Enterobacteriacae in the CFTR knockout mouse intestine (10).
This pilot study demonstrates a high prevalence of positive BTs among CF patients. The findings suggest a high prevalence of SBBO. Multiple confounding variables should be considered in attempts to interpret the results of BTs in this patient population. Historically, the gold standard for the diagnosis of SBBO has been culture of jejunal fluid. However, this technique excluded analysis of unculturable bacteria and may be surpassed by bacterial DNA analysis in the future. Either way, this study needs to be confirmed by analysis of jejunal samples in a comparable population. Although the CF population has many risk factors for SBBO, the clinical significance is uncertain, especially among those who respond well to pancreatic enzyme replacement therapy. However, certain medications were seen to confer an additional risk for or apparent protection from a positive BT. The use of acid-suppressing medications had no effect. This small pilot study is not conclusive but points to the dearth of clinical research in intestinal issues of CF patients. Some medications used in CF maybe beneficial for the lungs but have a deleterious effect on the GI tract, whereas others may improve GI health. As CF patients enjoy improved pulmonary health, intestinal issues deserve closer investigation.
The authors wish to thank Mariska Anderson, LRSA1, the staff of the LPCH CF Center at Stanford, and Robert C. De Lisle, PhD, for his helpful comments.
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