Measurement of Small Bowel Transit

A comparison of methods for evaluation of small-bowel transit is shown in Table 7.2.

Determination of Breath Hydrogen Concentration

Probably the most widely used small-bowel transit test is the determination of breath hydrogen concentration. It measures the time required for the appearance of hydrogen in the breath after ingestion of a substrate metabolized by colonic bacteria.12 Such substrates include lactulose12 and baked beans.13 Lactulose is metabolized by lactobacilli, which in healthy subjects are located in the colon. The test assesses the arrival of the first portion or head of the "meal" in the ascending colon; it thus represents overall oro-cecal transit. To assess small-bowel transit exclusively, lactulose has to be infused into the duodenum by intubation.

Although its simplicity and low cost make the breath hydrogen test attractive, a number of disadvantages render it less useful in pathologic states. Depending on the substrate being used (lactulose or baked beans),this method measures the transit of either liquids or solids. Moreover, results may be greatly influenced by noncolonic bacterial flora in the upper gastrointestinal tract, such as bacteria in the oral cavity,14 as well as small-intestine bacterial overgrowth. However, oral bacteria can be significantly reduced by the use of a mouthwash prior to the test.14 Breath hydrogen may also be altered by smoking or exercise, and by the technique of breath expiration.14 Small-bowel bacterial overgrowth is commonly encountered in patients with motility disorders associated with delayed transit. Since the most proximal bacterial flora that reside in the digestive tract determine the time of the first breath hydrogen peak,bacterial overgrowth in the small bowel may lead to an underestimation of the time taken for the substrate to reach the colon. In some individuals, it may be difficult to identify the true first peak of breath hydrogen,15 necessitating

Table 7.2. Comparison of methods for evaluation of small-bowel transit

Method

Advantages

Disadvantages

Precautions

Breath H2 concentration

Simple, inexpensive

Orocecal transit of head of column

Mouthwash

Influenced by bacterial overgrowth

Avoid cigarettes and exercise

Difficulty identifying "peak"

Standardize substrate in each laboratory

(e.g., liquid lactulose or solid baked

beans

Plasma sulfapyridine

Simple

Multiple blood samples (300mL)

Avoid in those allergic to sulfonamides

concentration after oral

Orocecal transit unless duodenum

salicylazosulfapyridine

intubated

Probably reflects liquid transit

Influenced by bacterial overgrowth

Scintigraphy

Not influenced by bacterial

Requires gamma camera

Radiation exposure

overgrowth

Same radiation with more

99mTc mashed potato

scans Relatively simple

Probably measures liquid transit

Radiation exposure acceptable for clinical

and research studies

,3,I fiber

Measures gastric and small

Very laborious preparation

Excessive radiation

bowel transit of

Long half-life of ,3'I

No longer used

nondigestible solids

'"In-labeled food or inert

Measures gastric and small

Radiation exposure acceptable for clinical

particles, e.g., charcoal

bowel transit of digestible

and research studies

or nondigestible solids

Pellets require laborious regulatory

Easy preparation

oversight

Adapted from von der Ohe M, Camilleri M. Measurement of small bowel and colonic transit; indications and methods. Mayo Clin Proc 1992;67:1169-1179.

approximations or curve fitting of the plotted data (Fig. 7.1). For all of these reasons, this strategy is not used to measure transit in clinical practice at the Mayo Clinic.

Plasma Sulfapyridine After Oral Salicylazosulfapyridine

Another method that depends on bacterial metabolism of a substrate uses the detection of a metabolite in the plasma; oral administration of salicylazosulfapyridine is followed by drawing of multiple blood samples to detect sulfapyri-dine in plasma.16 However, this test requires approximately 300 mL of blood and has the same limitations as the breath hydrogen test in that it provides an overall assessment or orocecal transit unless substrate is administered directly into the duodenum, and its results are greatly influenced by small-bowel bacterial overgrowth.

Small-Intestine Scintigraphy

Several radioisotopes such as technetium-99m,1317 iodine 131,18,19 and indium 11120 have been used as markers to evaluate small-bowel transit. In general, this methodology requires gamma camera equipment and adequate precautions (a pregnancy test in women of child-bearing potential) owing to the exposure to radiation. The exposure should be restricted according to guidelines for clinical and research practice. It is important to note that radiation exposure does not increase as more images are obtained by the gamma camera in research studies. Typical radiation exposures are shown in Table 7.3. Radiolabeled polystyrene ionexchange pellets were extensively used at the Mayo Clinic because they remained nondi-gestible and unaltered in size at the range of pH found in the stomach and small intestine; hence, the same marker evaluated gastric emptying and small-bowel transit.

Small-bowel transit is calculated by subtracting the time taken for a certain amount of the marker (10% or 50% of ingested radioisotope) to empty from the stomach, from the time taken for the same amount of radioisotope to reach the colon. Radioisotope marker studies are not influenced by small-intestine bacterial overgrowth. Complicated deconvolution analyses are rarely used in research or clinical practice.18

Different phases of a meal can be labeled with radioisotopes. A mashed potato meal can be labeled easily with 99mTc sulfur colloid or dieth-ylenetriaminepentaacetic acid (DTPA)13; it measures the gastric transit of triturated solids and does not exhibit the lag time customarily seen with digestible or non-digestible solid markers.13 This is not ideal in the evaluation of gastric emptying in disease states that are known to alter the lag period, such as in gastroparesis.21 However, the small-bowel transit times of solids and liquids are similar in health18 and in disease states19; hence, 99mTc sulfur colloid might be suitable to assess small-bowel transit time, but appears to have not advantage over the use of a liquid meal labeled with 99mTc pertechnetate.17

Fiber labeled with 131I is ingested with a standardized mixed meal and measures gastric emptying and small-bowel transit of nondigestible solids.18 Using 131I-labeled cellulose fiber strands

Figure 7.1. Example of repeated peaks of hydrogen in breath; investigators applied a ninth-order polynomial to identify the hydrogen peak.

(<2mm size), the prolonged mouth-to-cecum transit times observed in patients with gastro-paresis or antral hypomotility and intestinal pseudo-obstruction or dysmotility were shown to result from selective delays in the transit of radiolabeled solids in the stomach or small bowel.21 Radiolabeled fiber is no longer used in Mayo Clinic research studies because this method requires very laborious preparation and is associated with relatively high radiation exposure owing to the long half-life of 131I.

Ion-exchange polystyrene pellets labeled with 99mTc or 111In have been used to evaluate the gastric emptying and small-bowel transit of the solid phase of a meal20 (Fig. 7.2). Scintigraphy also identifies focal accumulation and persistence of isotope in abnormal, hypomotile segments of small bowel (Fig. 7.3). The emptying of the distal ileum into the colon does not occur linearly in healthy individuals (Fig. 7.4), but is

Table 7.3. Radiation exposure organ dose (cGy) per exposure

Intestine

Isotope form

Dose (mCi)

Stomach

Small

Upper large

Lower large

Ovary

Testis

Total body

Bone marrow

He

"'In

0.'

0.'

0.2

0.45

0.75

0.'5

0.0'

0.03

0.04

0.03

99mjc

'.0

0.24

0.24

0.40

0.28

0.08

0.02

0.14

0.'0

HE, effective dose equivalent.

Adapted from von der Ohe M, Camilleri M. Measurement of small bowel and colonic transit; indications and methods. Mayo Clin Proc 1992;67:1169-1179.

Figure 7.2. Dual gamma camera measurement of gastric and small-bowel transit; profiles of transit are shown in the lower half of the figure. Note the normal pattern of intermittent filling of solid radiolabel in boluses separated by periods during which no residue enters the colon.

Figure 7.2. Dual gamma camera measurement of gastric and small-bowel transit; profiles of transit are shown in the lower half of the figure. Note the normal pattern of intermittent filling of solid radiolabel in boluses separated by periods during which no residue enters the colon.

associated with a series of bolus transfers.20 The size and frequency of bolus transfers are significantly impaired in disease processes that affect the smooth muscle of the small bowel, such as progressive systemic sclerosis.22

Gastric Emptying Camera
Figure 7.3. Delayed gastric emptying and focal accumulation of isotope in dilated small-bowel loops.

Because of laborious regulatory oversight, most centers simply radiolabel digestible solid or liquid and follow its transit through both stomach and small bowel, using a region of interest program to evaluate stomach emptying and colonic filling. Scintigraphic small-bowel transit is associated with a large interindi-vidual23 and intraindividual24 variation. Hence its diagnostic validity in clinical practice is limited.

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