HISTORY-DEPENDENT MECHANICAL-BEHAVIOR OF GUINEA-PIG SMALL-INTESTINE

Measurement of pressure-volume relations is a commonly used technique to elucidate small Intestinal stiffness. There is a lack of data on th e relation between stiffness and history-dependent mechanical properti es of the gastrointestinal tract. We aimed to distinguish between pass ive properties of the tissue that depend on the time-history of load ( viscoelastic effects) Versus those that depend on the maximum previous road (strain softening effects). Ten repeated pressure-volume relatio ns were measured at each peak pressure level in six isolated, passive guinea-pig jejuni in vitro during balloon inflation and deflation cycl es. With inflation to a new higher peak pressure (ranging from 3 to 15 mm Hg), the pressure-volume relation became less stiff, particularly in the low pressure range, without a significant change in unloaded je junal volume. We computed the jejunal normalized volume change as a fu nction of the integrated volume-time history and maximum volume. Analy sis of covariance revealed significant dependence of the normalized vo lume change on the volume-time history (P<0.001) and the maximum volum e history (P<0.001). Multiple Linear regression analysis showed that a pproximately 90% of the history dependence could be attributed to the maxi mum volume. Most softening (loss of stiffness) happens in the low pressure range of the curve (0-3 mm Hg). We adopted the Johnson and B eatty strain softening theory and computed the volume amplification fa ctor. This factor was shown to be a linear function of the normalized peak volume (r(2)>0.999). Since strain softening effects were signific antly greater than viscoelastic effects, we conclude that history-depe ndent changes in jejunal stiffness are more likely to involve alterati ons to elastic rather than viscous structures in the tissue. These eff ects must be taken into account when performing balloon distension stu dies in the gastrointestinal tract for studying physiological and path ophysiological problems in which loading conditions are altered, e.g., mechanoreceptor studies in normal intestine and in acute and chronic obstruction, in order to have an accurate description of the biomechan ics. (C) 1998 Biomedical Engineering Society.