PARTICIPANTS

Twenty patients with IBS (15 women, five men; age range 20–61 years) and 23 healthy individuals without gastrointestinal symptoms (14 women, nine men; aged 19–28 years) participated in the study after giving written informed consent. Healthy subjects were recruited by public advertising. Patients were recruited from the gastroenterology outpatient clinic and were selected on the presence of IBS diagnostic criteria,6 with no detectable abnormalities in routine testing and no concomitant diseases. Two patients refused to participate in the study. All patients were symptomatic at the time of study. Twelve patients were subjectively constipation predominant, four were diarrhoea predominant, and four alternated between constipation and diarrhoea. All but three patients, each from a different subset, complained of abdominal bloating. Bowel function was not assessed objectively nor was intestinal transit time of solids. The protocol for the study was previously approved by the Institutional Review Board of the Hospital General Vall d'Hebron.

JEJUNAL GAS INFUSION

We used an intestinal polyvinyl tube assembly (3.2 mm OD) that incorporated a gas infusion channel (1.6 mm ID) with multiple side holes over the 2 cm distal segment. A separate gastric polyvinyl tube (3.2 mm OD) was used to measure possible gaseous reflux/swallowing into the stomach.

A gas mixture containing 88% nitrogen, 6.5% carbon dioxide, and 5.5% oxygen bubbled into water for saturation was infused. These proportions were chosen to mimic the partial pressures of gases in venous blood and hence minimise diffusion across the intestinal-blood barrier.7 ,8 A non-absorbable stable gaseous marker, 0.5% sulphurhexafluoride (SF₆), was added to the final mixture.9 Gas was infused continuously into the proximal jejunum at 12 ml/min using a modified volumetric pump (Asid Bonz PP 50-300; Lubratronics, Unterschleissheim, Germany).

MEASUREMENT OF GAS EVACUATION

Intestinal gas evacuation was measured either via an extra-anal cannula placed outside the anus to collect naturally passed flatus5 or via an intrarectal Foley catheter (20 French; Bard, Barcelona, Spain) with the balloon inflated with 5 ml of water (see below). Anal or rectal gas was collected via a leakproof, low resistance collection line using a barostat,10 ,11 and the volume automatically recorded on a paper polygraph (model 6006; Letica, Barcelona, Spain), as previously described.5 Subjects were provided with an event marker to signal every gas expulsion for later correlation with the volume collection recording.

MEASUREMENTS OF ABDOMINAL GIRTH CHANGES

Once subjects were in bed (see below), a non-stretch 48 mm wide belt with a metric tape measure was adjusted around the abdomen over the umbilicus by means of two elastic bands. Girth measurements were taken while the subjects were breathing in a relaxed manner as the average of inspiratory and expiratory determinations over three consecutive respiratory excursions.

PERCEPTION QUESTIONNAIRE

A graded questionnaire was used to measure the intensity and type of sensations perceived, and an anatomical questionnaire to measure the location and extension of the perceived sensations.5 ,12-14 The graded questionnaire, which included four graphic rating scales graded from 0 (no perception) to 6 (pain), was used to specifically score: (a) abdominal pressure/bloating, (b) colicky sensation, (c) stinging sensation, and (d) other types of sensation (to be specified), respectively. The questionnaire included two additional scales for scoring the feeling of difficult gas evacuation from the anus and belching, respectively, which were analysed separately. Participants were asked to score any perceived sensation (one or more perceived simultaneously) on the scales and to tick a box (yes/no) to indicate whether the sensations were identified as their usual clinical complaints.

The anatomical questionnaire incorporated a diagram of the abdomen divided into nine regions corresponding to the epigastrium, periumbilical area, hypogastrium, both hypochondria, flanks, and iliac fossae. Participants were instructed to mark the location—that is, abdominal region(s) or extra-abdominal—where the sensations were perceived.

PROCEDURE

Participants were instructed to follow a diet excluding legumes, vegetables, garlic, onions, nuts, cereals, wholemeal bread, and fizzy drinks during the two days prior to the study. The night before the study they had a light dinner that consisted of meat, fish, eggs, rice, pasta and/or white bread, but avoiding dairy products, salad, fruit, and alcoholic beverages. All medication was discontinued one week before the study. On the day of the study participants were orally intubated after an eight hour fast. The tip of the intestinal tube assembly was positioned 5 cm caudal to the angle of Treitz under fluoroscopic control. The studies were conducted in a quiet room with the subjects placed supine on the bed at a 30° angle. A 30 minute equilibration period was allowed before starting intestinal gas infusion and collection. During the study period intestinal gas was continuously infused and gas evacuation recorded; a sample of gas evacuated (flatus) for every 30 minute period was stored in metallised bags (Gas collection 750 ml; QuinTron, Milwaukee, Wisconsin, USA) for later analysis. Perception and abdominal girth were measured at 10 minute intervals.

EXPERIMENTAL DESIGN

OUTCOME MEASURES

In each subject, the volume of gas retained at different time periods within the gut was calculated as the difference between the volume of gas infused and the volume of gas recovered. Over the same time periods, SF₆ concentration in the gas evacuated was measured by infrared absorbance after determination of standard curves.15 Based on SF₆ recovery (volume of gas collected×SF₆ concentration) the volume of exogenous gas recovered was calculated. This value was subtracted from the total volume of gas evacuated to calculate the volume of endogenous gas evacuated.

Perception during the studies was measured by the score in the graded questionnaire. When more than one sensation was simultaneously scored, only the maximum score, instead of the cumulative score, was computed for comparisons. In each subject, the number of times each sensation was scored was counted to calculate the frequency (as per cent distribution) of each specific sensation. In the anatomical questionnaire, the percentage of stimuli referred to over more than one abdominal region was calculated.

Changes in abdominal girth throughout the study were referred to measurements taken at the beginning of the study before gas infusion.

STATISTICAL ANALYSIS

Mean (SEM) values of the parameters were calculated in each group of subjects. Mean gas retention, perception scores, and changes in abdominal girth in IBS patients were compared with those of healthy controls using the Mann-Whitney U test. Within each group, parameters during anal and rectal gas collection were compared using the paired Wilcoxon signed rank test. Contingency tables of both groups were analysed by Fisher's exact test. Correlations of paired data were examined by linear regression analysis.

VALIDATION STUDIES

GAS EVACUATION

Most healthy subjects adequately handled the 12 ml/min gas load, and gas evacuation rates paralleled gas infusion rates with no substantial intra-abdominal gas retention (fig 1). As in previous studies,5 only a small proportion of healthy subjects presented progressive gas retention during gas infusion and only in those did retention exceed 400 ml after a two hour infusion period. Hence an upper limit of gas retention of 400 ml was chosen to define gas retainers. In IBS patients, gas retention at the end of the two hour period was significantly greater than in healthy controls (667 (104) ml v 273 (56) ml; p<0.01) (fig 1). The proportion of gas retainers, defined as those with progressive gas retention exceeding 400 ml after two hours, was significantly higher among IBS patients than healthy subjects (p<0.01) (figs 2, 3).

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Figure 1

Evacuation of intestinal gas in patients with irritable bowel syndrome (IBS) (n=20) and healthy subjects (n=20). Gas was infused into the intestine at a constant rate (represented by the broken line) for two hours and collected via an anal cannula. Note that IBS patients expelled a significantly lower volume of gas. Values are means (SEM). **p<0.01.

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Figure 2

Individual perception scores and gas retention after a two hour intestinal gas infusion in patients with irritable bowel syndrome (IBS) and healthy subjects. Perception of abdominal symptoms was scored on a seven point scale (0–6). Gas retained is volume infused minus volume evacuated. Broken lines represent the upper limits for perception (score 3—that is, mild to moderate) and gas retention (400 ml, see text) in healthy subjects. Only 4/20 healthy subjects progressively retained gas exceeding 400 ml. In contrast, most IBS patients (18/20) retained gas (>400 ml) and/or developed moderate to severe symptoms (>3 perception score).

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Figure 3

Individual abdominal distension and gas retention after a two hour intestinal gas infusion in patients with irritable bowel syndrome (IBS) and healthy subjects. Gas retained is volume infused minus volume evacuated. Note that in contrast with healthy subjects, most IBS patients (17/20) retained gas (>400 ml) and/or developed abdominal distension (>3 mm girth increment).

The correlation between reported and recorded flatus evacuation was excellent in all participants (99 (1)% of the reported evacuations were recorded; pooled data). Reduced evacuation in IBS patients was associated with a decreased number of flatus passages (12 (2) in IBSv 21 (2) in healthy subjects; p<0.01) but mean gas volume per flatus was similar in both groups (52 (7) ml in IBS and 61 (6) ml in healthy subjects). Mean gas retention was 727 (70) ml in constipation predominant IBS patients, 572 (70) ml in diarrhoea predominant, and 583 (32) ml in patients alternating between constipation and diarrhoea; these values were not significantly different. Gastric gas reflux was detected in only three IBS patients who failed to tolerate the procedure, just immediately before vomiting and interruption of the gas infusion (see below).

INTENSITY OF PERCEPTION

In healthy subjects, the intestinal gas load induced relatively low abdominal perception and no subject perceived a score above 3 (average score 2.0 (0.2)) (fig 2). IBS patients reported higher perception (average score 3.5 (0.4); p<0.05v healthy controls). Furthermore, more than half of IBS patients reported perception scores above 3, seven experienced frank discomfort, and the study was discontinued in three patients after 50–90 minutes of infusion because of abdominal pain, nausea, and vomiting. The correlation between gas retention and abdominal perception was fair (r=0.42; p<0.01). Thus some IBS patients developed symptoms with retained gas volumes that were well tolerated by healthy subjects. Considering the combination of gas retention and perception, 18 of 20 IBS patients fell outside the normal range (p<0.01 v healthy subjects) (fig 2).

NATURE OF SYMPTOMS

During intestinal gas infusion, mild abdominal symptoms reported by healthy subjects consisted of colicky sensation, pressure/bloating, and stinging sensation (64 (6)%, 22 (6)%, and 13 (5)%, respectively). Patients with IBS reported a similar symptom pattern, albeit more severe. Eleven of 20 IBS patients frequently (on 71 (11)% of occasions) recognised the perceived sensations as their usual clinical symptoms. Eight IBS patients with gas retention exceeding 400 ml described feelings of difficult gas evacuation. Only three IBS patients who were unable to complete the study presented belching just before interruption.

Both healthy subjects and IBS patients referred their symptoms to the abdominal midline (27 (5)% of symptoms were referred to the epigastrium, 50 (6)% to the periumbilical area, and 48 (6)% to the hypogastrium; pooled data for healthy and IBS patients). The extent of the referral area tended to be greater in IBS patients but the differences detected did not reach statistical significance (65 (6)% of sensations were perceived over more than one area in IBS patientsv 51 (7)% in controls).

ABDOMINAL DISTENSION

The belt used for girth measurements remained in place and permitted automated measurements over time. Girth measurements showed good reproducibility in consecutive determinations (coefficient of variation <1%). During infusion of gas, abdominal girth changed by 1 (1) mm in healthy subjects and by 7 (1) mm in IBS patients (p<0.01). Fifteen IBS patients in contrast with only two healthy subjects (p<0.01) developed abdominal distension greater than 3 mm during intestinal gas infusion (fig 3). The correlation between gas retention and abdominal distension was relatively high (r=0.66; p<0.001).

ROLE OF THE ANUS

Gas retention decreased during the second part of the study with rectal collection, and this effect was similar in healthy subjects and IBS patients: at the end of the two hour period, gas retention had decreased by 364 (82) ml and 364 (113) ml, respectively. Only two IBS patients evacuated large volumes of gas after rectal intubation (>600 ml in 10 minutes) with improved symptom perception. However, during the period of the study with intrarectal cannula, IBS patients still presented significantly more gas retention than healthy subjects (302 (127) ml v −110 (127) ml, respectively; p<0.05). After two hours of rectal collection, IBS patients reported a mean perception score of 3.5 (0.5) compared with 2.0 (0.4) in healthy subjects; girth increment was 7 (3) mm in IBS patients compared with −1 (1) mm in healthy subjects (p<0.05 for both parameters). Pooling retention (<400 ml), perception (<score 3), and distension (<3 mm), 13 of 17 IBS patients but only two of 10 healthy subjects fell outside the normal range during intrarectal gas collection (p<0.01).

ENDOGENOUS VERSUS EXOGENOUS GAS EVACUATION

The amount of endogenous gas evacuated during the 240 minute gas infusion period was similar in healthy subjects and IBS patients. Exogenous gas, labelled with SF₆, started to be evacuated after a time lag, the concentration in the external collection increased thereafter and then plateaued from 180 minutes onwards. The increase in exogenous gas concentration was significantly slower in IBS patients than in healthy subjects: for example, at one hour the concentration was 2 (1)% in IBS patients compared with 19 (4)% in healthy subjects (p<0.05). Based on the SF₆ concentration in the last two sample measurements (after steady state had been reached) the volume of endogenous gas produced during the study was calculated. This value was then subtracted from the total volume of endogenous gas evacuated to obtain the volume of endogenous gas present in the intestine at the beginning of the experiment. This volume did not differ significantly in IBS patients (287 (41) ml) compared with healthy subjects (199 (43) ml).