# The Merck Manual of Diagnosis & Therapy - Gas



## JeanG (Oct 20, 1999)

The URL for this article is: http://www.merck.com/pubs/mmanual/section3/chapter32/32b.htm Gas(Eructation; Flatulence)Physiology Gas is present in the gut as a result of swallowed air, production in the lumen, or diffusion from the blood into the lumen.Normally, air is swallowed in small amounts (aerophagia) during eating and drinking, but some people unconsciously swallow air repeatedly while eating and at other times, especially when anxious. Most swallowed air is subsequently eructated (belched); only a small amount passes into the small bowel. The quantity of air passed is apparently influenced by position: The esophagus empties into the posterior cephalad aspect of the stomach. In an upright person, air rises above the liquid contents of the stomach, comes in contact with the gastroesophageal junction, and is readily belched. In a supine person, air trapped below the stomach fluid tends to be propelled into the duodenum. Excessive salivation may also increase aerophagia and may be associated with various GI disorders (gastroesophageal reflux disease), ill-fitting dentures, certain medications, or nausea of any cause. Belching may be associated with use of antacids. Attributing the relief of ulcer symptoms to belching rather than to antacids, the person may continue to belch to relieve distress.Gas is produced in the lumen by several mechanisms. Bacterial metabolism yields significant volumes of hydrogen (H2), methane (CH4), and CO2. Nearly all H2 is produced by bacterial metabolism of ingested fermentable materials (carbohydrates, amino acids) in the colon and therefore is negligible after a prolonged fast or after a meal that is completely absorbed in the small bowel. Poorly understood factors (eg, differences in colonic flora and motility) may also account for variations in gas production. Normal persons incompletely absorb carbohydrates in certain common foods. The normally indigestible polysaccharides in fruits and vegetables (eg, fiber, raffinose) may also cause excess gas.H2 is produced in large quantities after ingestion of certain fruits and vegetables containing indigestible carbohydrates (eg, baked beans) and in patients with malabsorption syndromes. In patients with disaccharidase deficiencies (most commonly lactase deficiency), large amounts of disaccharides pass into the colon and are fermented to H2 (see Carbohydrate Intolerance in Ch. 30). Celiac disease, tropical sprue, pancreatic insufficiency, and other causes of carbohydrate malabsorption should also be considered in cases of excess colonic gas.CH4 is produced by bacterial metabolism of endogenous substances in the colon; the production rate is only minimally influenced by food ingestion. Some people consistently excrete large quantities of CH4; others, little or none. The tendency to produce large quantities is familial, appearing during infancy and persisting for life.CO2 may also be produced by bacterial metabolism, but a more important source is the reaction of HCO3 and H2 ions, in which 22.4 mL of CO2 is released for each mEq of HCO3. H2 ions may be derived endogenously from gastric hydrochloric acid or exogenously from the fatty acids released during digestion of fats, the latter sometimes representing several hundred milliequivalents of H2 ion. Theoretically, up to 4 L of CO2 may be released into the duodenum after ingestion of a meal. The acid products released by bacterial fermentation of unabsorbed carbohydrates in the colon may also react with HCO3 to produce CO2. Although bloating occasionally occurs, the rapid diffusion of CO2 into the blood prevents intolerable distention.Gas diffuses between the lumen and the blood in a direction dependent on the difference in partial pressure. The production of H2, CO2, and CH4 may reduce the partial pressure of N in the lumen to far below that in the blood, which may account for much of the N in the lumen.Gas is eliminated by belching, diffusion from the lumen into the blood with ultimate excretion by the lungs, bacterial catabolism, and passage through the anus (flatus, farting).Symptoms, Signs, and Diagnosis Excessive intestinal gas is commonly thought to cause abdominal pain, bloating, distention, belching, or excessively voluminous or noxious flatus. However, excessive gas has not been clearly linked to these complaints; it is likely that many symptoms are incorrectly attributed to "too much gas." In most normal persons, 1 L/h of gas can be infused into the gut with a minimum of symptoms, whereas persons with symptoms related to gas often cannot tolerate smaller quantities. Similarly, retrograde colonic distention by balloon inflation or during colonoscopy often elicits severe discomfort in patients with IBS but minimal symptoms in others. Thus, the basic abnormality in persons with gas-related problems may be a hypersensitive intestine. Altered motility may contribute further to symptoms; gas may be the inciting agent or have no role in their pathogenesis.Repeated belching indicates aerophagia. Some persons with this problem can readily produce a series of belches on command. When aerophagia is suspected, patient education and behavior modification should be undertaken rather than extensive medical evaluation and drug therapy.In the splenic flexure syndrome, swallowed air becomes trapped in the splenic flexure and may cause diffuse abdominal distention. Left upper quadrant fullness and pressure radiating to the left side of the chest may result. There is increased tympany in the extreme left lateral aspect of the upper abdomen. Relief occurs with defecation or passage of flatus.Infantile colic is a syndrome presumably caused by crampy abdominal pain. Colicky infants appear to pass an excessive amount of gas. However, studies show no increase in H2 production or increase in mouth-to-cecum transit times in colicky infants. Hence, the cause of this syndrome remains unclear.In flatulence, the quantity and frequency of gas passage shows great variability. As with stool frequency, persons who complain of flatulence often have a misconception of what is normal. In a study of eight normal men aged 25 to 35 yr, the average number of gas passages was 13 ï¿½ 4/day with an upper limit of 21/day, which overlapped with many persons who complained of excess flatus. Hence, objectively recording flatus frequency (using a diary kept by the patient) should be the first step in evaluating a complaint of excessive flatulence.Despite the flammable nature of the H2 and CH4 in flatulence, working near open flames is not hazardous. Children have been known to make a game of expelling gas over a match flame. However, gas explosion, rarely with fatal outcome, has been reported during jejunal and colonic surgery and even when diathermy was used during endoscopic procedures in poorly evacuated patients.Because symptoms of excessive gas are nonspecific and commonly overlap with IBS (see above) and with organic disease, a detailed history is essential to guide the extent of medical evaluation. Long-standing symptoms in a young person who is otherwise well and has not lost weight are unlikely to be caused by serious organic disease. The older person, especially with the onset of new symptoms, merits more thorough examination before excessive gas, real or imagined, is treated. It is not uncommon for patients with eating disorders (anorexia nervosa, bulimia) to misperceive and be particularly stressed by symptoms such as bloating and belching. Clinicians should explore for the possible presence of an eating disorder in patients, particularly young women with these symptoms.Treatment Belching, bloating, and distention are difficult to relieve because most complaints are caused by unconscious aerophagia or by exaggerated sensitivity to normal amounts of gas. An attempt must be made to reduce aerophagia. Aerophagia may be caused by excessive salivation, so the patient should avoid habits such as gum chewing or smoking. Upper GI tract diseases (eg, peptic ulcer) that may cause reflex hypersalivation and disorders that may cause nausea and reflex salivation should be treated. Carbonated beverages or antacids should be eliminated if associated with belching. Foods containing unabsorbable carbohydrates should be avoided. Dairy products should be excluded from the diet of lactose-intolerant patients.The mechanism of repeated belching should be explained and demonstrated. When aerophagia is troublesome, biofeedback and relaxation therapy can retrain the patients to swallow and chew more effectively and break the cycle of aerophagia-discomfort-belch-relief.Few well-controlled studies have demonstrated clear-cut benefit from any drug. Simethicone, an agent that breaks up small gas bubbles, has been incorporated into several preparations, and various anticholinergic drugs have also been used, all with variable results. Some persons with dyspepsia and postprandial upper abdominal fullness have benefited from antacids. Cisapride (10 to 20 mg 30 min before meals--Caution: Risk of serious drug interactions) can facilitate gastric emptying and increase lower esophageal sphincter pressure. Complaints of excessive flatus are treated with similar measures to try to minimize the volume of gas in the gut. Roughage (eg, bran, psyllium seed) may be added to the diet to try to increase colonic transit; however, in some patients, worsening of symptoms may result. Activated charcoal can sometimes help reduce gas and the unpleasant odor created by hydrogen sulfide produced in the bowel. However, its tendency to stain clothing and the oral mucosa makes it somewhat awkward to use.In general, functional bloating, distention, and flatus run an intermittent, chronic course that is only partially relieved by therapy. Reassurance that these problems are not detrimental to health is important.


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