There are several methods available for the measurement of small intestinal transit. The hydrogen breath test relies on metabolism of certain carbohydrates, e.g. lactulose, by microbial flora within the large bowel. The carbohydrate must be one which is not absorbed from the small intestine. The gas generated is detected in the expired air. Using this technique, it is possible to estimate the sum of gastric emptying and small intestinal transit times. This test assumes that the unabsorbed carbohydrate encounters fermentative bacteria only in the colon, however bacterial overgrowth into the small intestine will give erroneously short transit times.
Two-dimensional gamma scintigraphy can be used to measure stomach to caecum transit times, but cannot be used to measure the distance travelled by a unit through segments of the small intestine since it is highly coiled. Transit time through segments of the small intestine has been measured using a perspex capsule containing technetium-99m labelled 'Amberlite' resin16. External markers were placed on the front, back and sides of volunteers who were then imaged from the front, back and side. This enabled the three-dimensional movement of the capsule through the small intestine to be reconstructed and an estimate to be made of the velocity of the unit. After transit through the duodenum, which was too fast to be accurately measured, the capsule moved through the small intestine at between 4.2 and 5.6 cm per minute. There was no difference in transit times for two capsules with different specific gravities (1.0 and 1.6). The transit rate is in close agreement with the velocity of the migrating myoelectric potential down the small intestine (4.7 cm. min-1)17 and that of 1 to 4 cm per minute for chyme2.
Many older textbooks quote small intestinal transit times based on barium X-ray contrast measurements, but barium is not a good model of the intestinal contents.
Measurements of transit from patients with some form of organic disease, such as those with ileostomies, should also be treated with caution.
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