Incremental elastic modulus--a challenge to compliance

Compliance and capacity of the normal human rectum--physical considerations and measurement pitfalls

The assessment of parameters which adequately represent rectal and neorectal compliance is complex. Biological properties of the rectum during distension and relaxation show significant departures from in vitro physical compliance measurements; as much dependent upon the viscoelastic charateristics of hollow organ deformation as upon the technique of compliance calculation. This review discusses the pressure/volume characteristics of importance in the rectum during distension from a bioengineering perspective and outlines the disparities of such measurements in living biological systems. Techniques and pitfalls of newer methods to assess rectal wall stiffness (impedance planimetry and barostat measurement) are discussed.

Measurement of mechanical properties of rectal wall

In this paper, a pig's rectum was studied as a model biomaterial and its mechanical behaviors under tensile, compressive, and shear stresses were measured accurately using a multipurpose microtesting system. Based on the stress-strain relations of samples of different orientations, the tangential moduli were calculated through a reverse method combined with self-correlation analysis. The experimental data exhibited pronounced nonlinear and anisotropic characteristics. It was found that the effective compliance in tension along the longitudinal direction was larger than that along the circumferential direction, but smaller than that along the out-of-plane direction.

Rectal compliance in females with obstructed defecation

PURPOSE

This study was designed to investigate whether rectal compliance is altered in females with obstructed defecation.

METHODS

Eighty female patients with obstructed defecation and 60 control subjects were studied. Rectal compliance was measured with an "infinitely compliant" polyethylene bag. This bag was inserted in the rectum and inflated with air to selected pressure plateaus (range, 0-60 mmHg; cumulative steps of 2 mmHg with a duration of ten seconds) using a computer-controlled electromechanical barostat system. Volume changes at the levels of distending pressures were recorded. The distending pressures, needed to evoke first sensation of content in the rectum, earliest urge to defecate, and the maximum tolerable volume were noted.

RESULTS

In all cases, the compliance curve had a characteristic triphasic (S-shaped) form. The mean compliance curve obtained from the patients was identical to that of the controls. However, the course of the compliance curve fell above the normal range (mean + 2 SD) in 14 patients. In ten (71 percent) of these patients, a large rectocele was seen at evacuation proctography. Such a rectocele was observed in only five patients (7.6 percent) with a normal compliance curve (P < 0.001). Eighty percent of the controls experienced earliest urge to defecate during the second phase of the curve. In 75 percent of the patients, this occurred in the third phase. The mean pressure threshold for first sensation, earliest urge to defecate, and maximum tolerable volume were significantly higher in patients compared with control subjects. Ten of the patients experienced no sensation at all in the pressure range between 0 and 60 mmHg.

CONCLUSION

In females with obstructed defecation, the compliance of the rectal wall is normal.

Comparison of methods used for measurement of rectal compliance

INTRODUCTION

Compliance is defined as the change in volume or cross-sectional area divided by the change in pressure. Pressure-volume measurement during distention with a compliant balloon is the most commonly used method for computation of rectal compliance. However, intraindividual and interindividual variations are large, restricting the usefulness of the method. Other methods such as rectal distention by a large, noncompliant bag and rectal impedance planimetry for assessment of pressure-cross-sectional-area relations have been proposed as alternatives owing to the reduction of errors from elongation of the balloon within the rectal lumen. However, in vivo reproducibility of pressure-volume measurement during distention with a compliant balloon, pressure-volume measurement during rectal distention by a large, noncompliant bag, and rectal impedance planimetry have never been compared.

PURPOSE

The aim of this study was to compare in vivo reproducibility of the above-mentioned methods and to study their in vitro reproducibility and validity.

METHODS

Ten healthy volunteers (six men) aged 21-59 years were randomized to either rectal pressure-volume measurement with a compliant balloon or rectal impedance planimetry. After a one-hour rest, the other procedure was performed. After two weeks, both procedures were again performed in the same order. During rectal impedance planimetry the volume of the bag used (maximum volume 450 ml; secured at both ends to the probe) was continuously registered, measuring pressure-volume relations during rectal distention by a large, noncompliant bag. Reproducibility was tested by comparing the difference divided by the mean for each method at eight pressure steps in the range from 5 to 40 cm H2O. Furthermore, the in vitro reproducibility and validity of the three methods were studied using polyvinyl chloride tubes with known cross-sectional areas.

RESULTS

In vivo reproducibility for pressure-volume measurement with a large, noncompliant bag and rectal impedance planimetry was significantly better than for pressure-volume measurement with a compliant balloon (P = 0.005 and P = 0.019, respectively). No statistically significant difference was found between pressure-volume measurement with a large, noncompliant bag and rectal impedance planimetry (P = 0.20). In vitro reproducibility of pressure-volume measurement with a large, noncompliant bag and rectal impedance planimetry was good, but some elongation occurred, reducing the validity of pressure-volume measurement with a large, noncompliant bag. Coiling and elongation of the balloon within the lumen were major sources of error for pressure-volume measurement with a compliant balloon.

CONCLUSION

In vivo and in vitro reproducibility of methods used for measurement of rectal compliance can be improved by restricting the effects of elongation within the lumen either by using a large-volume, noncompliant bag or by rectal impedance planimetry. However, pressure-volume measurement will to some degree depend on the properties of the balloons or bags.

Rectal compliance as a routine measurement: extreme volumes have direct clinical impact and normal volumes exclude rectum as a problem

PURPOSE

The clinical impact of rectal compliance and sensitivity measurement is not clear. The aim of this study was to measure the rectal compliance in different patient groups compared with controls and to establish the clinical effect of rectal compliance.

METHODS

Anorectal function tests were performed in 974 consecutive patients (284 men). Normal values were obtained from 24 controls. Rectal compliance measurement was performed by filling a latex rectal balloon with water at a rate of 60 ml per minute. Volume and intraballoon pressure were measured. Volume and pressure at three sensitivity thresholds were recorded for analysis: first sensation, urge, and maximal toleration. At maximal toleration, the rectal compliance (volume/pressure) was calculated. Proctoscopy, anal manometry, anal mucosal sensitivity, and anal endosonography were also performed as part of our anorectal function tests.

RESULTS

No effect of age or gender was observed in either controls or patients. Patients with fecal incontinence had a higher volume at first sensation and a higher pressure at maximal toleration (P = 0.03), the presence of a sphincter defect or low or normal anal pressures made no difference. Patients with constipation had a larger volume at first sensation and urge (P < 0.0001 and P < 0.01). Patients with a rectocele had a larger volume at first sensation (P = 0.004). Patients with rectal prolapse did not differ from controls; after rectopexy, rectal compliance decreased (P < 0.0003). Patients with inflammatory bowel disease had a lower rectal compliance, most pronounced in active proctitis (P = 0.003). Patients with ileoanal pouches also had a lower compliance (P < 0.0001). In the 17 patients where a maximal toleration volume < 60 ml was found, 11 had complaints of fecal incontinence, and 6 had a stoma. In 31 patients a maximal toleration volume between 60 and 100 ml was found; 12 patients had complaints of fecal incontinence, and 6 had a stoma. Proctitis or pouchitis was the main cause for a small compliance. All 29 patients who had a maximal toleration volume > 500 ml had complaints of constipation. No correlation between rectal and anal mucosal sensitivity was found.

CONCLUSION

Rectal compliance measurement with a latex balloon is easily feasible. In this series of 974 patients, some patient groups showed an abnormal rectal visceral sensitivity and compliance, but there was an overlap with controls. Rectal compliance measurement gave a good clinical impression about the contribution of the rectum to the anorectal problem. Patients with proctitis and pouchitis had the smallest rectal compliance. A maximal toleration volume < 60 ml always led to fecal incontinence, and stomas should be considered for such patients. A maximal toleration volume > 500 ml was only seen in constipated patients, and therapy should be given to prevent further damage to the pelvic floor. Values close to or within the normal range rule out the rectum as an important factor in the anorectal problem of the patient.

Development of a computer-controlled tensiometer for real-time measurements of tension in tubular organs

A computer-controlled tensiometer for studying wall tension in tubular organs has been developed. The system consisted of a probe with an inflatable balloon, an impedance planimeter, pressure transducer and amplifier, a pump with RS232 interface and a PC with dedicated software. Circumferential wall tension was computed in real time from pressure and cross-sectional area measurements (tension measurement mode). Wall tension can be maintained on a preset level or be changed as a step or ramp function by a feedback control of the infusion/withdrawal pump (tension control mode). A software regulator adjusted the volume rate (low volume rate when the computed tension was close to the preset value) to minimize overshoot and oscillation. Validation tests were performed and the technique was applied in the human oesophagus. Volume- and tension-controlled balloon distensions elicited secondary peristalsis of increasing intensity that was decreased significantly by the antimuscarinic agent Hyoscine butyl bromide. In tension control mode Hyoscine butyl bromide caused oesophageal relaxation, i.e. CSA to increase and pressure to decay. Furthermore, pronounced pressure relaxation and tension relaxation were observed during volume-controlled distension after administration of Hyoscine butyl bromide.