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The concept of rectal compliance as a determining factor for rectal function is appealing. However, several important obstacles must be overcome. First, consensus must be reached on what the term “compliance” means; is it a measure of the viscoelastic properties of the rectal wall, or does it refer to the active and passive elements that define distensibility of an innervated, muscular organ? Second, the overlapping effects of rectal compliance and rectal capacity need to be sorted out. Third, agreement is needed as to the correct technique to measure compliance, and standardized equipment and protocols must be adopted. Once these steps have been taken, we will be in a better position to understand exactly how compliance affects rectal behavior, both in health and in disease.

This chapter has highlighted the research approach towards the physiological assessment of distensibility of the rectum using compliant bags, discussing the inherent errors in the determination of compliance as it pertains to biological systems. Because of the anisotropy of the human gut, which is composed of a heterogeneous range of tissues with different deformabilities, the presence of a neurohumoral tone, the nonlinear effects on the stress-strain relationship, and the viscoelastic properties of isolated and repeated distension stresses on the intestine, no model of compliance will define the real relationship of deforming stress and strain. Moreover, this clinical assessment is dependent upon many assumptions concerning gut geometry, directional distension, luminal size, and wall stiffness and thickness that cannot be measured accurately in vivo or that are either wrongly assumed to be constant or are neglected in manometric practice. The use of impedance planimetry for the electrical determination of cross-sectional area during distension has provided a means in the rectum for the determination of a nonlinear incremental elastic modulus that, to some extent, defines rectal wall stiffness in health and disease. It remains to be seen whether this can be used as an objective marker of rectal damage in ulcerative or radiation proctitis, or whether it will assist in the postoperative estimation of the biomechanical properties of constructed neorectal reservoirs.

Although problems with colonic motility and transit are a common cause of patient complaints, accurate assessment of these complaints remains limited by available assessment tools. Transit studies, including radiopaque markers and colonic scintigraphy, are able to provide a gross assessment of transit and are highly useful in the clinical management of the constipated patient. However, many symptomatic individuals will appear “normal” using one or all of these tests. In addition, these tests do not evaluate the underlying mechanisms for abnormal transit. Further information from research studies using measures of colonic pressure and activity hopefully will improve our understanding of both normal and pathologic colonic motility and transit and the clinical role of these specialized investigations in coloproctological practice.