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In
addition to measuring whole gut or colonic transit time, small
bowel transit time can be measured by the lactulose-hydrogen
breath test (LHBT), which indicates the ability to metabolize
carbohydrates (Metz
et al., 1976). The test relies on the fact that lactulose
is rapidly broken down by the colonic flora, with a resultant
release of hydrogen which can be detected in the breath. It should
be noted, however, that this ability is lacking in about 5% of
the population (Bond
and Levitt, 1975). Samples of breath can be aspirated from
a Haldane tube and then kept for several hours in sealed syringes
before being passed through a hydrogen analyser, without loss
of accuracy.
Small
bowel transit time has been used as a proxy for whole gut transit
time in studies of the ability of opioid antagonists to reverse
opioid-induced intestinal delay (Basilisco
et al., 1987; Yuan
et al., 1997). The small bowel transit time is much shorter
(1 to 3 hours) than the colonic transit time, which means that
the process of measurement is also far more compact and less arduous,
but one measurement cannot be extrapolated from the other. Small
bowel transit time clearly cannot give information about the performance
of different segments of the gut, and whole gut (i.e. principally
colonic) transit time is the more frequently used measure in trials
relating to constipation.
Indirect
measurement of transit time
All methods
of direct measurement of intestinal transit time require attendance
at a unit possessing specialized apparatus, and make significant
demands on those participating. In measuring an entity that is
important as a symptom it is inappropriate to place undue burdens
on patients. An alternative to direct measurement is to use a
readily observable correlate.
The
consistency of the stools as revealed by their shape was first
proposed to be a reflection of gut transit time 80 years ago (Burnett,
1921). Several scales of stool form, relying on either photographs
or descriptions, have been produced since (Cowgill,
1933; Davies
et al., 1986; O'Donnell,
Virjee and Heaton, 1990). Stool form can be estimated reliably
by patients themselves (O'Donnell,
Virjee and Heaton, 1990) and correlates well with transit
time in both volunteers (Davies
et al., 1986) and palliative care patients (r=0.83, p<0.001)
(Sykes, 1990) (see Figures
4.2 and 4.3).
Stool
consistency can also be measured directly by two methods that
have been shown to correlate highly with transit time and stool
form (Davies
et al., 1986; Sykes, 1990),
however, either measure is unattractive to perform:
- Use of
a penetrometer, a device widely used in the oil and food industries
to provide an objective measure of the density of fluid or
semi-solid substances (Exton-Smith,
Bendall and Kent, 1975), or
- Expressed
as the water content of the stool, which can be arrived at
by drying samples of the feces.
Figure
4.2 Stool From Scale
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Courtesy
of Dr John Yiannakou
Figure
4.3 Form Transit Time Correlation
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| A
graph of the relationship between stool form and intestinal
transit time measured by the single marker dose technique
using three sizes of marker. The correlation coefficient,
r, is 0.83 (p<0.001) (compare Figure
4.1). (Sykes, 1990,
with permission) |
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